Preparation And Photocatalytic Property Of Nanosized Cu₂O And CdS With Visible Light Response

Recently, the exploration of novel, highly efficient photocatalysts under visible light (VL) covering the main band of solar light has become one of the most important research areas for environmental pollution control and renewable energy sources. As a p-type semiconductor with small band gap (Eg = 2.0 eV), cuprous oxide (Cu₂O) is an important photocatalyst with VL response. Ever since it was reported to be able to produce H₂ from water splitting under VL, Cu₂O has been regarded as one of the most potential photocatalysts. Until now, there are many publications about Cu₂O as the photocatalyst for the photodegradation of organic dyes under VL. However, the effect of morphology, structure on the photocatalytic activity of Cu₂O under VL irradiation has not been studied in detail. Cadmium sulfide (CdS) with a band gap (E_g = 2.4 eV) close to Cu₂O is another important photocatalyst able to respond to VL. Due to the enough negative potential of its conduction band, CdS has been regarded as an effective photocatalyst for H₂ evolution from water spitting under VL. In addition, there are also many reports about the application of CdS to photodegradation of organic pollutant. However, the poor quantum efficiency and photocorrosion inhibits the practical utility of CdS in photodegradation of pollutants generally.In this thesis, Cu₂O and CdS have been considered as two main objects..We not only studied the effect of morphology and structure on the photocatalytic activity of Cu₂O nanomaterials, but also investigated the positive effect of multiwalled carbon nanotubes (MWCNTs) as a carrier on the adsorption and photocatalytic activity of nanoscaled CdS under VL for the first time. There are three main parts for this thesis. Firstly, we prepared Cu₂O nanomaterials with different morphology and structure through polyol method and their photocatalytic activity under VL irradiation was explored. Secondly, Cu₂O/MWCNTs and CdS/MWCNTs composites were synthesized by the same method. The photocatalytic activity of the CdS/MWCNTs was studied under VL. Thirdly, Cu₂O films composed of two-dimensional (2D) Cu₂O nanosheets were deposited onto Cu foils by polyol method, and the prepared Cu₂O films display an interesting photoelectrochemical property. The detailed contents are as follows:1. Cu₂O with different morphology and structure has been synthesized by polyol method with copper acetate as a precursor and diethylene glycol (DEG) as both solvent and reducing agent under different reaction conditions. The samples have been characterized by XRD, SEM, TEM, and DRS. The results include:(1) Because H₂O improves the preferential crystal growth along the [111] direction and make it far exceed that of [100], {111} faces shrink and the cubic Cu₂O bound by {100} surfaces was formed when more distilled water put into the polyol system. In addition, the amount of water in the system influences the supersaturation of Cu₂O so that the addition time of the distilled water should have a great effect on the size of the prepared Cu₂O nanocubes. Two kinds of Cu₂O nanocubes are obtained with different sizes of-500 nm and -150 nm.(2) Solvent can adsorb onto the surfaces of the Cu₂O solid probablly through a chemical interaction. So, if there is no water added, the concentration of copper acetate in the DEG is the most important factor influencing the morphology of the prepared Cu₂O. If the concentration of copper acetate is 2.25 mmol/50 ml DEG, Cu₂O anisotropic nanorods are obtained. Otherwise, if that decreases to 2.25 mmol/100 ml DEG, big Cu₂O nanoparticles with the size of 100 nm are prepared.(3) It is well known that the presence of CH₃CONH₂ leads to oriented assembling of the small Cu₂O crystal with each other and finally formation of a single-crystal structure. So, when there is CH₃CONH₂ in the system, Cu₂O single crystal nanorods will be obtained in a relatively short reaction time. With the increase of reaction time, the rose like Cu₂O nanoflowers composed of single crystal nanosheets can be synthesized. Herein, these two kinds of Cu₂O display certain quantum size effect.2. The activity of the prepared nanosized Cu₂O was evaluated by the photocatalytic degradation of organic dyes under VL. Firstly, the decrease of the size of Cu₂O nanocube benefits for improving the photocatalytic activity. Secondly, Cu₂O with single crystal has a higher photocatalytic activity compared to multi crystal structure. Thirdly, Cu₂O nanoflowers have the highest photocatalytic activity among all the Cu₂O samples with different morphology. Finally, a little of N in Cu₂O not only benefits for the improvement of the photocatalytic activity but also makes the sample stable. The prepared Cu₂O single crystal nanorods and nanoflowers are stable both in air and during photocatalytic process.3. Homogeneous Cu₂O nanoparticles with the size of 8-10 nm are deposited on multiwalled carbon nanotubes (MWCNTs) by the polyol method. With the change of the reaction conditions, Cu₂O nanoparticles on the surface of MWCNTs can be leafage-like or big spherical particle. HRTEM images indicate that all the leafage-like and big spherical particles are assembled by small Cu₂O particles with size of about 2-5 nm. Tentative mechanism is proposed for the formation of Cu₂O nanoparticles with different morphologies on the surface MWCNTs.4. Cadmium sulfide (CdS) nanoparticles dotted on the surface of MWCNTs have been synthesized by the polyol method. The results indicate that CdS nanoparticles with diameter of 5-8 nm are thickly and uniformly coated on the surface of the MWCNTs. The photodegradation of azo dye using these materials was evaluated by the degradation of Brilliant Red X-3B under VL. The coated nanotubes show higher photocatalytic activity than both CdS alone and a CdS/activitaed carbon sample. In addition, there is an optimum content of MWCNTs. The presence of MWCNTs can also hamper the photocorosion of CdS. The mechanism for the enhancement of MWCNTs on the adsorption and photocatalytic property of CdS is investigated for the first time in this thesis.5. Cu₂O films on the Cu foils with 2D nanosheets structure are synthesized by the polyol method with acetamide as an additive and Cu(CH₃COO)₂ as the precursor. It is demonstrated that the prepared Cu₂O nanosheets, which are single crystal with ultrathin thickness (25 nm) grown on the Cu substrates, are aligned. The addition of acetamide, as well as the stirring speed during reaction is important for the growth of the aligned 2D Cu₂O nanosheets on the substrates. Moreover, the prepared film exhibits interesting photoelectrochemical property under VL with the presence of oxygen compared to the Cu₂O microparticle films.6. Based on the experiment results above, we can conclude that the polyol method is an excellent method for the preparation of nanosized Cu₂O with different morphology and structure. The morphology and structure are two important factors influencing the photocatalytic activity of Cu₂O under VL. In addition, MWCNTs as a substrate has the function to improve the adsorption and photocatalytic activity of CdS. So, MWCNTs may be an excellent support for photocatalysts to improve their photocatalytic activity. At last, the synthesized Cu₂O film with interest structure-2D aligned nanosheets structure displayed a higher photoelectrochemical activity compared to Cu₂O film composed of particles, which indicates that this film as an electrode will have great potential for photoelectrodecomposition of organic pollutant and solar cells.