| In order to improve the photocatalytic activity of zinc oxide (ZnO) as a photocatalyst which existing higher photo-generated electron-hole recombination rate and low photocatalytic efficiency problem, the present work prepared different metal ion-doped ZnO(Me:ZnO) samples by selecting and screening metal ions, using low-temperature solution method. The crystal structure, morphology and optical properties of samples were respectively characterized by X-ray diffraction(XRD), field emission scanning electron microscopy(FESEM), X-ray energy spectrometer(EDS), ultraviolet and visible spectrophotometer(UV-vis), photolum-inescence spectroscopy(PL), and so on. And their photocatalytic activity towards degradation of methyl orange(MO) were tested under ultraviolet irradiation. The researches about the effect of different metal ions and their amounts, and process parameters, such as addition amount of NaOH, reaction time and heat treatment on the rule and mechanism of the photocatalytic activity in Me:ZnO samples have been systemically done here. The results are as follow.Fe:ZnO nano-sheets with different contents of Fe(the molar ratio Fe:Zn 0.01-0.03) are successfully synthesized under the environment of 80℃ aqueous solution then drying, controlling the Fe:ZnO morphology by adjusting the proportion of sodium hydroxide(NaOH), in which the as-prepared samples are well crystallized showing the average grain size about 28nm, and present a hexagonal wurtzite structure without other anomalous peaks related to Fe metal clusters or Fe oxides. The main diffraction peaks of Fe:ZnO samples shift slightly to higher value. With the increase of Fe doping amount, there is a red shift in UV absorption spectra, and the calculated value of band gap are narrowing for Fe:ZnO samples. Fluorescence spectra shows that Fe iron doped ZnO(Fe:ZnO) samples don’t lead to form a new luminescence center, and an appropriate Fe concentration of Fe:ZnO sample improves the photocatalytic activity of ZnO as it decreases the recombination rate of free charge carriers. The optimum parameter for Fe:ZnO samples is achieved that the molar ratio of Zn2+to NaOH is 1:5, the reaction temperature is 80℃ with reaction time 24h, and the 1.5mol% Fe doped, in which it exhibits the best photocatalytic activity towards degradation of MO under UV lamp for 5 hours. And its final degradation rate of MO(20mg/L) is 60.4%, which is improved by 43.8% over pure ZnO. Moreover, the photocatalytic activities of Fe:ZnO samples are reduced by heat treatment owing to the agglomeration.Cu:ZnO nano-sheets with different contents of Cu(the molar ratio Cu:Zn 0.005～0.02) are also successfully synthesized under the environment of 80℃ aqueous solution and then drying. The XRD shows that the as-prepared samples are also well crystallized with the average grain size about 30nm, and present hexagonal wurtzite structure without other impurity peaks, too. The main diffraction peaks of Cu:ZnO samples shift slightly to higher value as same as the Fe:ZnO samples. The band gap of Cu:ZnO samples decreases slightly and the intensity of UV-vis absorption enhance when compared with pure ZnO. It is interesting that two new light-emitting phenomena at 600nm and 750nm are brought by Cu doping observed by PL, which is quite different from the Fe:ZnO samples. And doping small amount Cu ions decreases the recombination rate of free charge carriers. The photocatalystic experiment for degradation of MO indicates that there is an optimal preparation condition for Cu:ZnO photocatalyst, that is, molar ratio of Zn2+/NaOH 1:5, reaction temperature at 80℃ for 24h, and 0.5mol% Cu doped. The highest degradation rate of MO(20mg/L) for Cu:ZnO photocatalyst is 71.10%, which is improved by 69.3% over pure ZnO when it is irradiated under UV lamp for 5 hours. |
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