Morphology-dependent antibacterial activities of Cu2O

Cubic and octahedral Cu₂O microcrystals were synthesized by the reduction of a copper-ligand complex solution with glucose under microwave irradiation using ethylenediaminetetraacetic acid (EDTA) and N,N,N′,N′-tetramethyl ethylenediamine (TMEDA) as ligands. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) confirmed that the surfaces of the cubic and octahedral of Cu₂O microcrystals had {100} and {111} lattice planes. The antibacterial activity of the Cu₂O microcrystals against E. coli was examined using optical density (OD) methods. The antibacterial activity of the cubic Cu₂O crystals was superior to that of the octahedral Cu₂O crystals. The mechanism of the specific morphology-controlled synthesis of Cu₂O and their morphology-dependent antibacterial activity are discussed.     

Solution-phase synthesis of Cu2O cubes using CuO as a precursor

We describe a facile route for the synthesis of Cu₂O cubes using CuO as the precursor and hydrazine hydrate as the reducing agent. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations indicate that the Cu₂O products are composed of cubes with an edge length of 0.7-1.2 μm. This method is surfactant-free, and well-dispersed Cu₂O cubes can be obtained through this route. The presence of small amounts of ions in reaction solution has a great influence on the morphology of Cu₂O products.     

Fabrication of TiO2/Au nanorod arrays employing a positive sacrificial ZnO template and their electrochromic property

A novel method to fabricate large scale TiO₂/Au nanorod array using a positive sacrificial ZnO template has been developed. This method includes a two-step process, (1) preparation of ZnO/Au nanorod array by a simple low-temperature hydrothermal process, and (2) preparation of TiO₂/Au nanorod array by electrochemically induced sol-gel process. The TiO₂/Au nanorod array has showed a reversible electrochromism in lithium-ion-containing organic electrolyte. The coloration and bleaching throughout a visible range can be switched on and off within a few seconds.     

Influence of Cu2O surface treatment on the photovoltaic properties of Al-doped ZnO/Cu2O solar cells

Low cost Al-doped ZnO (AZO)/Cu₂O Schottky barrier solar cells with a high conversion efficiency of 2.19% were fabricated by depositing a transparent conducting AZO thin film on high quality Cu₂O sheets prepared by thermally oxidizing copper sheets. To achieve efficiencies higher than 2%, it is necessary to form the AZO thin film at a low deposition temperature using a low-damage deposition method, i.e., at room temperature by a pulsed laser deposition. In addition, the obtained efficiency could be enhanced with a surface treatment of the Cu₂O sheets, such as by applying a Pd-Sn catalyst layer as a coating or a rapid thermal annealing treatment at approximately 500 °C in air.     

Fabrication of carbon-modified TiO2 nanotube arrays and their photocatalytic activity

TiO₂ nanotube (TN) arrays were fabricated by an anodic oxidation process. Through a heat treatment of the as-fabricated TN arrays under a continuous Ar and acetylene flux, carbon-modified TN (C-TN) arrays were obtained. The as-fabricated catalysts were characterized by FE-SEM, HRTEM, XPS, Raman and UV-Vis spectra. Moreover, photocatalytic activity of the C-TN arrays was evaluated through the photodegradation of aqueous methyl blue. The experiments demonstrated that the C-TN arrays display an excellent photocatalytic activity. Under sunlight irradiation, the C-TN arrays are able to almost completely decompose the methylene blue pollutant of 1 × 10^− 5 M within 300 min.     

Low-temperature hydrothermal synthesis of flower-like ZnO microstructure and nanorod array on nanoporous TiO2 film

A novel ZnO architecture, with flower-like microstructure on the top layer and nanorod arrays on the bottom layer, was hydrothermally synthesized on the Fluorine-doped SnO₂ (FTO) conducting glass pre-coated with nanoporous TiO₂ film. The as-prepared architecture was characterized with Field-emission scanning electron microscopy (FE-SEM) and X-ray diffractometer (XRD). Dye-sensitized solar cell studies showed that the power conversion efficiency (η) was 1.26% for this novel ZnO architecture-covered TiO₂ electrode.     

Optoelectronic properties of sputter-deposited Cu2O-Ag-Cu2O treated with rapid thermal annealing

Cu₂O and two types of Cu₂O-Ag-Cu₂O (CAC) multilayered thin films were deposited on glass substrates using DC-magnetron sputtering. For CAC films, the mass thickness of Ag layer was controlled at 3 nm. After deposition, some of these films were annealed using a rapid thermal annealing (RTA) system at 650 °C, in order to create embedded Ag particles. AC films were used to study the clustering effect of Ag in Ar atmosphere, as well as for forming the 2nd type of CAC film by covering another Cu₂O layer on the annealed AC structure. A UV-VIS-NIR photometer, a Hall measurement system, and a I-V measurement system were used to characterize the optical and electrical properties of these films with and without RTA. The results show that 2-dimensional Ag layer can transform into many individual particles due to its high surface tension at annealing temperature, no matter when the annealing was carried out. For CAC films, without annealing, the optical transmission and the resistivity are decreased with the inserted Ag layer. After annealing, both the transmission and resistivity are increased, possibly due to the clustering effect of Ag layer. Most importantly, it is found that the embedded Ag particles can increase the light absorption in the NIR-IR region, which can increase photo-induced current.     

Mechanisms of carrier generation and transport in Ni-doped Cu2O

The density and mobility of hole carriers in Ni-doped and undoped cuprous oxide (Cu₂O) films prepared by pulsed laser deposition (PLD) from Ni-doped and undoped CuO targets, respectively, were measured in order to examine the mechanisms of carrier generation and transport in doped films. The temperature dependence of the carrier density of the films revealed that regardless of the Ni content, the activation energies of the acceptor level of the films are 0.22-0.25 eV. The temperature dependence of the mobility of the films changed from −0.58 to ∼0 by doping with Ni. These results evidenced that hole carriers in Ni-doped Cu₂O as well as in undoped Cu₂O were generated by Cu vacancies and were primarily scattered by neutral impurity scattering centers. X-ray diffraction (XRD) measurements of the films showed that the mass fraction of Cu₂O in the films decreased with increasing Ni content, while that of CuO increased. It was also found that the reduction process of CuO to Cu₂O was suppressed by the Ni doping.     

Effect of annealing on the electrodeposited Cu2O films for photoelectrochemical hydrogen generation

Cu₂O films were electrodeposited on stainless steel substrates followed by Ar annealing for photoelectrochemical hydrogen generation. Plating variables including time and pH for the plating bath were explored to obtain desirable film qualities. X-ray diffraction (XRD) patterns indicated that the as-deposited Cu₂O films exhibited preferred orientations in (200) and (111) planes from the plating bath of pH 9 and pH 11, respectively. Images from scanning electron microscope (SEM) revealed pyramid-like grains in 1 µm size for the Cu₂O films from pH 9 plating bath and large plate-like grains in 3-8 µm size from pH 11 plating bath. Identical results from SEM and XRD were obtained from the Cu₂O films at longer plating time. After annealing at 350 °C for 30 and 60 min, the Cu₂O phase was nicely maintained but SEM images demonstrated coarser grains. Photoelectrochemical activity for H₂ generation was obtained on the Cu₂O films before and after annealing by recording relevant photoelectrochemical currents at − 0.3 V in 0.5 M aqueous Na₂SO₄ solution. For the Cu₂O films from both baths, substantial increments in photoelectrochemical current were observed for the annealed samples as opposed to as-deposited ones. The largest photoelectrochemical current was obtained at 0.143 mA/cm² from the Cu₂O film of pH 9 plating bath with 60 min annealing, which exhibited a 560% increase over the as-deposited sample. We attributed the enhanced photoelectrochemical current to the improved crystallinity and reduced defects for the annealed Cu₂O films.     

Effects of H2O partial pressure on the crystallinity of ZnO thin film and electrical characteristics of film bulk acoustic wave resonators

We have improved electrical characteristics of a film bulk acoustic wave (BAW) resonator that features the injection of H₂O gas into a process chamber. The preferred crystallinity of piezoelectric ZnO film was obtained by RF sputtering at a high H₂O partial pressure 1.5×10⁻⁴ Pa. The effective electromechanical coupling coefficient () of the BAW resonator remarkably goes up from 1.8% to 4.7% for which the corresponding H₂O partial pressures are 2.7×10⁻⁵ and 1.5×10⁻⁴ Pa. Injection of H₂O during the deposition process contributes to the improvement of crystallinity of ZnO thin film and the electrical characteristics of the BAW resonator.     

Synthesis of Cu2O crystals by galvanic deposition technique

A simple galvanic deposition technique has been developed to demonstrate the deposition of cuprous oxide (Cu₂O) on transparent conducting oxide substrate for the first time. The result shows that the morphologies of galvanically obtained Cu₂O crystals are mainly dependent on the nature of anions in aqueous solution. The presence of NO₃⁻ ions tends to increase the stability of (111) planes of Cu₂O cubes and makes the crystals develop a fraction of (111) planes at the corners of the Cu₂O cubes, thus resulting in the formation of truncated octahedral Cu₂O crystals.     

Repeatable synthesis of Cu2O nanorods by a simple and novel reduction route

Cu₂O nanorods were synthesized by reducing bamboo leaf-shaped Cu(OH)₂ with sodium hypophosphite (NaH₂PO₂) in an H₂O/ethylene glycol (EG) mixing solution. The Cu(OH)₂ was prepared by adding an alkaline solution to an aqueous solution containing CuSO₄ and NaH₂PO₂ at room temperature. The optimum temperature range for the reduction of the Cu(OH)₂ to Cu₂O nanorods was 55-70 °C. The products were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and high-resolution transmission electron microscopy. The result showed the prepared Cu₂O nanorods were uniform and had diameters of 10-20 nm and lengths of 150-200 nm. The synthesis is simple, inexpensive, and highly repeatable.     

Fabrication and photocatalytic activity of YMn2O5 nanoparticles

In this work, a polyacrylamide gel route is introduced to synthesize YMn₂O₅ nanoparticles. It is demonstrated that high-quality YMn₂O₅ nanoparticles with a uniform size and spherical shape can be prepared using different chelating agents. However, the average particle size of the products is found to have a dependence on the choice of the chelating agent. The sample prepared using citric acid as the chelating agent has an average particle size of ~ 45 nm, while the sample prepared by using the chelating agent EDTA has a particle size centered around 70 nm. The optical energy bandgap of the citric acid- and EDTA-resulted samples is obtained, from optical absorption measurements, to be 1.21 and 1.17 eV, respectively. The photocatalytic experiments reveal that the as-prepared YMn₂O₅ nanoparticles exhibit an interesting photocatalytic activity for oxidative decomposition of methyl red under ultraviolet and visible-light irradiation.     

Shape controlled synthesis and characterization of Cu2O nanostructures assisted by composite surfactants system

A simple methodology has been demonstrated to synthesize various nanocrystalline Cu₂O materials assisted by composite surfactant system, SDS and Tween 80 using the polyol method. Glycolaldehyde prepared in situ by heating ethylene glycol solvent at 160 °C for 2 h, was utilized as the reducing agent. The relative ratio of the two surfactants was manipulated to achieve different Cu₂O morphologies, e.g. nanocrystalline Cu₂O flowers, hollow spheres consisting of holes and ring type structure. The FT-IR spectroscopy confirmed that the SDS and Tween 80 moieties were indeed present on the surface as capping agents in order to stabilize the surface nanocrystallites by the co-ordinative interactions between the oxygen atoms of Tween 80 and SDS and the Cu atoms at the surface of the synthesized Cu₂O particles. These oxygen atoms eventually encourage the oxidation of the surface Cu atoms to form a thin CuO layer, presence of which on the surface was corroborated by the XPS measurements. Sputtering of the samples was also carried out to remove the surface CuO thin layer and expose the inner Cu₂O. These nanomaterials were then investigated for their potential applications in photocatalytic degradation of Rhodamine B dye.     

Reactive sputtering of precursors for Cu2ZnSnS4 thin film solar cells

The quaternary semiconductor Cu₂ZnSnS₄ (CZTS) is a possible In-free replacement for Cu(In,Ga)Se₂. Here we present reactive sputtering with the possibility to obtain homogeneous CZTS-precursors with tunable composition and a stoichiometric quantity of sulfur. The precursors can be rapidly annealed to create large grained films to be used in solar cells. The reactive sputtering process is flexible, and morphology, stress and metal and sulfur contents were varied by changing the H₂S/Ar-flow ratio, pressure and substrate temperature. A process curve for the reactive sputtering from CuSn and Zn targets is presented. The Zn-target is shown to switch to compound mode earlier and faster compared to the CuSn-target. The precursors containing a stoichiometric amount of sulfur exhibit columnar grains, have a crystal structure best matching ZnS and give a broad peak, best matching CZTS, in Raman scattering. In comparing process gas flows it is shown that the sulfur content is strongly dependent on the H₂S partial pressure but the total pressures compared in this study have little effect on the precursor properties. Increasing the substrate temperature changes the film composition due to the high vapor pressures of Zn, SnS and S. High substrate temperatures also give slightly denser and increasingly oriented films. The precursors are under compressive stress, which is reduced with higher deposition temperatures.     

Multi-stage evaporation of Cu2ZnSnS4 thin films

Multi-stage evaporation is a well-established method for the controlled growth of chalcopyrite thin films. To apply this technique to the deposition of Cu₂ZnSnS₄ thin films we investigated two different stage sequences: (A) using Cu₂SnS₃ as precursor to react with Zn-S and (B) using ZnS as precursor to react with Cu-Sn-S. Both Cu₂SnS₃ and ZnS are structurally related to Cu₂ZnSnS₄. In case (A) the formation of copper tin sulphide in the first stage was realized by depositing Mo/SnS_x/CuS (1 < x < 2) and subsequent annealing. In the second stage ZnS was evaporated in excess at different substrate temperatures. We assign a significant drop of ZnS incorporation at elevated temperatures to a decrease of ZnS surface adhesion, which indicates a self-limited process with solely reactive adsorption of ZnS at high temperatures. In case (B) firstly ZnS was deposited at a substrate temperature of 150 °C. In the second stage Cu, Sn and S were evaporated simultaneously at varying substrate temperatures. At temperatures above 400 °C we find a strong decrease of Sn-incorporation and also a Zn-loss in the layers. The re-evaporation of elemental Zn has to be assumed. XRD measurements after KCN-etch on the layers prepared at 380 °C show for both sample types clearly kesterite, though an additional share of ZnS and Cu₂SnS₃ can not be excluded. SEM micrographs reveal that films of sample type B are denser and have larger crystallites than for sample type A, where the porous morphology of the tin sulphide precursor is still observable. Solar cells of these absorbers reached conversion efficiencies of 1.1% and open circuit voltages of up to 500 mV.     

Cu2ZnSnS4 thin film solar cells from electroplated precursors: Novel low-cost perspective

Thin-film solar cells based on Cu₂ZnSnS₄ (CZTS) absorbers were fabricated successfully by solid-state reaction in H₂S atmosphere of electrodeposited Cu-Zn-Sn precursors. These ternary alloys were deposited in one step from a cyanide-free alkaline electrolyte containing Cu(II), Zn(II) and Sn(IV) metal salts on Mo-coated glass substrates. The solar cell was completed by a chemical bath-deposited CdS buffer layer and a sputtered i-ZnO/ZnO:Al bilayer. The best solar cell performance was obtained with Cu-poor samples. A total area (0.5 cm²) efficiency of 3.4% is achieved (V_oc = 563 mV, j_sc = 14.8 mA/cm², FF = 41%) with a maximum external quantum efficiency (EQE) of 80%. The estimated band-gap energy from the external quantum efficiency (EQE) measurements is about 1.54 eV. Electron backscatter-diffraction maps of cross-section samples revealed CZTS grain sizes of up to 10 µm. Elemental distribution maps of the CZTS absorber show Zn-rich precipitates, probably ZnS, and a Zn-poor region, presumably Cu₂SnS₃, close to the interface Mo/CZTS.     

Tuning the electronic structure of the transparent conducting oxide Cu2O

The electronic structure of Cu₂O is important for its application as a p-type transparent conducting oxide (TCO). To be useful as a TCO, a material needs to show enhanced transparency in the visible range (band gap > 3 eV) as well as good conduction properties. While Cu₂O has too small a band gap, alloys of Cu₂O and Al₂O₃ or Cu₂O and alkaline earth oxides are known to display enhanced transparency, with little degradation of electrical properties. It is of interest to consider how to dope Cu₂O p-type, e.g. Cu vacancies (oxidation) or cationic dopants. We present a study of the electronic structure and effective hole masses of stoichiometric and oxidised Cu₂O and study metal cation doping, using density functional theory (DFT), to analyse p-type doping scenarios. We show that formation of a Cu vacancy is relatively facile, introducing delocalised hole states, with a light hole present. Substitutional cation doping with Al and Au/Ag is found to decrease the band gap but maintains a light hole effective mass necessary for p-type conduction.     

Effects of Ag contents and deposition temperatures on the electrical and optical behaviors of Ag-doped Cu2O thin films

Cu₂O-Ag thin films were co-deposited by reactive sputtering on glass substrates. During deposition, Ag contents and deposited temperatures were varied. After deposition, a UV-VIS-NIR photometer and a Hall measurement system were used to characterize the optical and electrical properties of these films. The results showed that the Cu₂O-Ag thin films had a decreased optical transmittance with the increase of Ag contents. The resistivity was also decreased, which is most likely due to the formation of Ag phase. Through the measurement of photo-induced conductivity, it was found that, when Ag concentration was at 4 at.%, the film had the highest increase in conductivity under light irradiation. This is due to the co-existence of Ag₂O and Cu₂O. However, when deposited at a temperature higher than room temperature, the photo-induced conductivity of this film became less obvious, apparently due to the dissociation of Ag₂O. The results of Photoluminescence (PL) measurement confirmed that the Cu₂O-Ag(4 at.%) sample might produce more electron-hole pairs than other samples, which caused the increase of conductivity.     

Hot-injection synthesis and characterization of quaternary Cu2ZnSnSe4 nanocrystals

Cu₂ZnSnSe₄ (CZTSe) is one of promising materials in the use of absorber layers of solar cells. It contains earth-abundant elements of zinc and tin, a near-optimal direct band gap of ∼ 1.5 eV, as well as a large absorption coefficient ∼ 10⁴ cm-1. The CZTSe nanocrystals in oleylamine (OLA) was successfully prepared via hot-injection method. The characterization of its structure, composition, morphology and absorption spectra were done using powder X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and UV-vis absorption spectra. The results revealed that the monodispersed nanocrystals were single phase polycrystalline within the range of 15-20 nm. Optical measurements showed a direct band gap of 1.52 eV, which was optimal for low cost solar cells. The capping property of OLA was also demonstrated by examining Fourier Transform Infrared Spectroscopy (FTIR) feature peaks of CZTSe and OLA, respectively.