| Photocatalytic H2 production from water splitting is of great importance in the research of the global energy and environment issues. As a kind of visible light response photocatalysts for hydrogen production, cadmium sulfide (CdS) is a good candidate catalyst for photocatalytic water splitting, however, it has two main inherent shortcomings, low QE and photocorrosion. With the presence of noble metal co-catalysts, the photocatalytic activity of CdS increases prominently. But the high cost of noble metals restricts its practical application. This thesis focuses on reducing the dosage of Pt and improving the photocatalytic activity for CdS, there are two routes:1) adopting non-noble metal Ni as co-catalyst to increase the photocatalytic activity through controlling the size and crystallinity of Ni nanoparticles.2) using Pt-based bi-metal Pt@M(M=Pd, Ru) as co-catalyst to explore their synergistic catalytic effect on the H2 evolution rate of this system. The following is the details:1. The preparation of Ni co-catalyst and its influence on the H2 evolution of CdS under visible lightDecrease the size of Ni co-catalyst:Ni nanoparticles were prepared via chemical reduction of NiCb by NaBBH4 in the presence of polyvinlylpyrolidone (PVP), and loaded on the surface of CdS by photo-induced electrons while water splitting reaction occurred simultaneously. Resultant Ni/CdS was characterized by high-resolution transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, ultraviolet-visible light diffuse reflectance spectrometry, and photoluminescence spectrometry. It was found that as-prepared Ni nanoparticles are about 3 nm, and preferentially deposited on (100), (002), and (101) crystal planes of CdS. Meanwhile, loading nickel decreases the photoluminescence intensity of CdS, which means nickel functions as the trapper of photo-generated electrons. Therefore, nano-Ni/CdS photocatalyst with a Ni loading of 2.5% possesses the best visible-light catalytic activity for water splitting-hydrogen evolution and provides a hydrogen production rate of up to 9.050 mmol·h-1·g-1, while it exhibits stabilized activity towards H2 evolution as well.Increase the crystallinity of Ni co-catalyst:Ni nanoparticles were prepared via chemical reduction of aqueous NiCl2·6H2O by N2H4·H2O, and loaded on the surface of CdS by photo-induced electrons as water splitting reaction was occurring. Resultant CdS modified with Ni nanoparticles (denoted as Ni/CdS) was characterized by transmission electron microscopy, X-ray diffraction, UV-vis diffuse reflectance spectrometry, and photoluminescence spectrometry, and its photocatalytic performance for water splitting under visible light irradiation producing hydrogen was evaluated with a 300 W Xe lamp as the light source(??420 run). It was found that as-obtained Ni nanoparticles with an average size of about 10 nm have face centered cubic structure, and they are preferentially deposited on the (100), (002), and (101) crystal planes of CdS nanorods to afford Ni/CdS photocatalyst. Besides, as-prepared Ni/CdS photocatalyst has a surface area of 28.8 m2/g (determined by BET method), higher than that of CdS nanorods, which indicates that Ni nanoparticles is beneficial to increasing the surface area of CdS nanorods. Moreover, as-prepared Ni/CdS photocatalyst shows absorption traits in the visible light region. In addition, and its photoluminescence peak intensity is lower than that of CdS, which means that Ni nanoparticles function as the trappers of photo-generated electrons to quench the photoluminescence of CdS. More importantly, although pristine CdS exhibits no activity for hydrogen production from water splitting under visible light irradiation, Ni/CdS photocatalyst with a Ni content of 4%(mass fraction) provides a hydrogen production rate of 25.848 mmol/(h-g) (QE=26.8%, ?=420 nm) from water splitting of (NH4)2SO3 aqueous solution under the same testing condition and it retains a high stability and activity even after 20 h of water splitting. This demonstrates that Ni/CdS could be a promising candidate photocatalyst for visible light water splitting yielding hydrogen.2. The preparation of Pt@M(M=Pd, Ru) and their influence on the H2 evolution of CdS under visible lightBi-metal nano-particles of Pt@Pd and Pt@Ru were prepared by the successive two-step chemical reduction method, and then deposited on the surface of CdS by photo-reduction. The synergistic catalysis of bi-metals on CdS in photocatalytic water splitting reaction CdS was studied. Resultant Pt@M/CdS(M=Pd, Ru) was characterized by X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, ultraviolet-visible light diffuse reflectance spectrometry and time-resolved fluorescence spectrometry. XPS results verify the core-shell structure of bi-metals, TEM images show the average size of Pt@M/CdS(M=Pd, Ru) particles is about 10 nm. The as-prepared Pt@M/CdS(M=Pd, Ru) photocatalyst shows an enhanced absorption in the visible light region. The photocatalytic performance of M/CdS(M=Pt, Pd,Ru) and Pt@M/CdS(M=Pd, Ru) for water splitting under visible light irradiation producing hydrogen was evaluated with a 300 W Xe lamp as the light source (??420 nm). The results imply the synergistic catalysis of bi-metals improves the photocatalytic activity significantly, the optimal ratio of Pt:M (M=Pd, Ru) is 7:3, and corresponding rates of hydrogen evolution are 26.9 mmol·h-1·g-1 and 18.4 mmol·h-1·g-1 respectively, which is much better than the monometallic counterparts. Time-resolved fluorescence spectrometry confirms the loading of bi-metal prolongs the lifetime of the photogenerated electrons and holes, thus resulting the high photocatalytic activity. |
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