| Cuprous oxide (Cu2O) is a kind of typical p type semiconductor material with a cubic crystal structure, and the band gap is 2.17 eV at room temperature. Cuprous oxide with specific photoelectric properties and electrochemical properties has been used in many fields such as superconductor, hydrogen, solar cells and electrochromic device due to the quantum size effect of nano-sized cuprous oxide. The raw materials of cuprous oxide are rich in nature and cuprous oxide has good chemical stability, high photocatalytic efficiency. Therefore it is regarded as a potential material in the photo catalytic field due to its low cost and non-toxic. Zinc oxide (ZnO) has attracted researchers’ attention because of the formation of a variety of nanostructures, such as nanowires, nanobelts, nanotubes, clusters, nanometer fibers and nanometer cone. Perpendicular to the substrate of ZnO nanorods (NRs) and nanorods arrays with unique performance deserve special attention. Highly ordered ZnO nanostructure is expected to enhance the performance of various technical important equipment, such as short wavelength laser, electroluminescent devices, sensors, photocatalytic system and the third generation solar cells. The length of ZnO nanorods, diameter and density are important parameters that influence the efficiency of solar cells. The research work based on Cu2O, ZnO, and both composite films can be described as follows.(I) Cuprous oxide powders are prepared using simple hydrothermal technique under different reaction temperature and structure, surface morphology, chemical composition and optical properties are discussed in detail. XRD analysis show that Cu2+ in the glucose solution can not be completely restored into Cu+ under low reaction temperature (70 ℃) resulting in many impurities in products. And when the reaction temperature is too high (130 ℃). reduction of Cu2+ in the solution can been reduced to Cu+ and then to Cu with the corresponding low product purity. Researches on morphology and particle size of Cu2O particles show that Cu2O crystal increases with the increase of reaction temperature. In addition, cuprous oxide show obvious agglomeration phenomenon under high reaction temperature and morphology varied from the spherical to cube. XPS analysis shows the surface stoichiometric ratio of Cu: O of the sample deposited at 110 ℃ is found to be 2.3:1.(Ⅱ) Cuprous oxide (Cu2O) thin films were prepared by using electrodeposition technique at different applied potentials (-0.1,-0.3,-0.5,-0.7, and -0.9 V) and were annealed in vacuum at a temperature of 100℃ for 1 h. Micro structure and optical properties of these films have been investigated by X-ray diffractometer (XRD), field-emission scanning electron microscope (SEM), UV-visible spectrophotometer, and fluorescence spectrophotometer. XRD measurement shows the existence of Cu2O with cubic structure and the peak of Cu only at -0.5 V. The intensity of Cu2O peaks decrease with increasing the deposition potential. Peaks corresponding to the Cu2O disappear when deposited at -0.9 V. This may be due to quicker growth of Cu2O particles and worse crystallization at higher applied potential. SEM images reveal that the applied potential has significant influence on the surface morphology. The morphology of Cu2O films turns octahedral into cubic and agglomerate as the applied potential becomes more cathodic. The films deposited at -0.1 V,-0.3 V, and -0.5 V are formed by regular, wellfaceted, polyhedral crystallites. The films change from octahedral to cubic and then to agglomerate as the applied potential becomes more cathodic. It shows that co-deposition of agglomerate Cu with cubic structure Cu2O when deposited at -0.5 V, which is in good agreement with the observation confirmed by XRD spectra. The films deposited at -0.7 V and -0.9 V exhibit a granular spherical morphology, and the average diameter of the grains tends to be approximately 50 nm. UV-vis analysis shows that Cu2O film deposited at -0.5 V with the strongest absorption, which is due to the resonance absorption of metal copper particles. Band gap values of the films vary from 1.83 to 2.03 eV. The emission at 603 nm (2.06 eV) of FL spectra can be caused by near band-edge emission from free exciton recombination.(Ⅲ) Cu2O thin films were deposited on FTO substrates at different applied potentials by electrodeposition method. XRD measurement shows Cu2O thin films are obtained only with the applied potential below -0.2 V while Cu/Cu2O thin films are possible at-0.3 V and above with respect to the SCE. XRD results have revealed that the in situ transformation of Cu2O to Cu happened with increased deposition potential and the content of Cu2O and Cu could be tuned by controlling the deposition potential. SEM images reveal that the applied potential has a significant influence on the morphology of the as-deposited films. The films grown on FTO substrates are uniform and polycrystalline. The surface morphology of the samples depends on the deposition potential The Cu2O thin films deposited at-0.1 and-0.2 V exhibit hollow four frustum of a pyramid structure the average grain size of Cu2O is about 1 μ m. It exhibit co-deposition of Cu and Cu2O when deposited at -0.3 and -0.4 V. The copper particles are embedded in the Cu2O films. Small grains of Cu distributed over the FTO substrate as the applied potential becomes more cathodic. The surface stoichio metric ratio of Cu:O of the films deposited at -0.3 and -0.6 V is found to be 5.9:1 and 14.2:1, respectively, which is accordance with XRD measurement. Band gap values of the films vary from 2.35 to 1.63 eV, which is due to the co-deposition of Cu and Cu2O as the applied potential becomes more cathodic.(IV) Cu2O thin films were deposited on ITO substrates at different bath temperature and KCl concentration by electrodeposition method. XRD measurement shows pure Cu2O films are obtained at 40-60℃ bath temperature. A characteristic cubic copper peak appears by introducing KC1 into the electrolyte. SEM images reveal that the morphology of the as-deposited films was depended on bath temperature and KC1 concentration. The Raman peaks of 218,412 and 625 cm-1 are red-shifted with KCl concentration increasing. The average absorption coefficient of the Cu2O thin films deposited at 10 mM KCl are larger than those at 2 and 5mM KCl due to the specific structure of Cu2O thin films.(V) The Cu2O-modified ZnO nanorods are prepared by electrodeposition method on ITO substrates. SEM graphs show that each hexagonal nanorod has a diameter of about 200 nm and the length is about 1 μm The nanorods are gradient and uniformly disperse on the ITO substrates. It can be observed that cubic structure Cu2O particles embedded in the interspaces of the ZnO nanorods and the amounts of the Cu2O particles increase obviously when the deposition time increases. It can also be found that as the Cu2O deposition time increases, the diameter and length of the nanorods decreased, which can be affected by electrolyte corrosion during the Cu2O deposition process. The Cu2O (111) peak (2θ=36.50°) is very close to the ZnO (101) peak (2θ=36.25°), and they are overlapped in the pattern. The intensities of the Cu2O characteristic peaks increase with the Cu2O electrodeposition time for increased amounts of the Cu2O nanoparticles. The characteristic peaks of Cu2O electrodeposited for 1 min can barely be detected, and this can be ascribed to an insufficient amount. The peaks of Cu2O particles are relatively weaker due to the shorter deposition time compared with ZnO nanorods. UV-vis spectra show that an absorption edge at 390 nm for the ZnO nanorods was observed. The absorption edges of the Cu2O-modified ZnO nanorods show an obvious redshift compared with pure ZnO nanorods and exhibit a broad absorption band in the UV region, which originates from the combinational effect of the narrow bandgap of Cu2O (approximately 2.17 eV) and wide bandgap of ZnO (approximately 3.37 eV). The absorbance in the visible light range increases with the increase of the deposition time of Cu2O. The introduction of Cu2O particles in ZnO nanorods extends the absorption edge to the visible light range, which is very important in making full use of sunlight. For Cu2O-modified ZnO nanorods, when the Cu2O deposition time increases from 1 to 10 min, the corresponding bandgaps of Cu2O particles are 2.43,2.38, and 2.30 eV, respectively. In addition, the bandgaps of ZnO nanorods shift from 3.22 to 2.75 eV, which is also consistent with previous SEM and XRD results. In addition, a wide emission with a peak at 600 nm was detected and regarded as defectrelated emissions of ZnO nanorods. The existence of Cu2O particles has little impact on peak position of ZnO nanorods. The emissions at 380 and 600 nm were diminished when Cu2O particles were deposited on the ZnO nanorods, which may originate from the random multiple scattering in such structure. All the Cu2O-modified ZnO nanorods have strong degradation ability of MO than the pure ZnO nanorods. With increasing Cu2O electrodeposition time. the degradation abilities of the Cu2O-modified ZnO nanorods enhanced. The reason is that Cu2O has higher degradation ability than ZnO. Meanwhile, the amount of Cu2O particles on the ZnO nanorods increases when increasing the Cu2O electrodeposition time.(VI) Microstructure, surface topography and optical properties of Zno.7Cu0.3O nanorods were investigated by XRD, SEM and PL spectra. XRD spectrum is composed of four peaks lcated at 2θ=34.42°,36.25°,47.54°, and 62.86° corresponding to (002), (101), (102), and (103) planes of ZnO with hexagonal phase (JCPDS 36-1451) and one peak at 2θ=33.50°orresponding to (009) plane of Zn(OH)2 (JCPDS 20-1435). The calculated texture coefficients of TC(002), TC(101), TC(102) and TC(103) are 2.90,0.14,0.47 and 0.49, respectively. It can be seen that the TC value of (002) plane of the Zn0.7Cu0.3O nanorods is much larger than 1. The results indicate that the nanorods grown along c-axis are perpendicular to the substrate surface. The sample consists of sparse, bending and agglomerating nanorods. Many spaces between the nanorods are filled with air. The formation mechanism of bending and agglo merating nanorods may be due to centripetal surface tension for the evaporation and coagulation processes of the water film on the surface of the sample. A broad and strong visible emission band in the range of 500-850 nm and a weak ultraviolet emission band centered at 376 nm have been observed. The ultraviolet emission is attributed to the band edge excitonic recombination. It can be seen that the intensity of visible emission bands becomes weaker with increasing temperature. This result can be attributed to the decrease of ratio of radiative and nonradiative photoelectrons (photoholes) and/or the prolonging of radiative photoelectrons (photoholes) lifetime and/or the shortening of non-radiative photoelectrons (photoholes) lifetime.With increasing the temperature, more electrons in conduction band first relax to lower zinc interstitial (Zni) defect levels nonradiatively, and then recombine with holes at oxygen vacancy (Vo) defect levels, which results in the increase of relative intensity of green emission. but the decrease of relative intensity of yellow emission. |
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