Controlled Deposition Of Noble Metal And Its Effect On CdS’s Activity For Water Splitting Under Visible Light

This thesis focuses on the controlled preparation and deposition of Pt nano-particles, and bimetallicnano-particles of Pt-Pd and Pt-Ru as well. Noble metal particles was prepared by chemical reduction, andthen loaded on the surface of CdS by photo-reduction. The structure of noble metal particles and itseffects on the hydrogen evolution by CdS were explored. The following is the details:1. Nano-rod CdS was prepared by solvothermal method using thiourea as sulfur source. The CdSsynthesized within120²00oC exhibited absorption under533⁵57nm. The micro-morphology andstructure of as-prepared CdS were investigated using XRD, HRTEM, and ED, based on which wesuggest that nano-rod CdS may have a preferential orientation growth along the c-axis. The most exposedcrystal plane, which subsequently involved in the water splitting reaction, is （002）. The experiments ofwater splitting were performed with a300W Xe light as resources （>420nm）, and the preparationconditions of solvothermal reaction were systematically studied. According to the experimental results,the CdS synthesized at160oC has a high photo-activity with a hydrogen evolution rate at34mmol/h/gCat..The as-prepared CdS possess a large BET surface area of36.7292m²/g and pore volume of0.088496cm³/g. Therefore, the high activity might be due to its high crystallinity and large surface area.2. Nano-Pt was prepared by chemical reduction, and loaded on CdS by photo-induced electrons atthe same time when water splitting reaction was occurring. HRTEM showed that the particle size ofso-obtained Pt is2⁵nm. XPS revealed that Nano-Pt prepared by chemical reduction was a complex ofPt⁰and Pt²⁺, and after the water splitting reaction, most of thePt²⁺。reduced to Pt⁰. The CdSused as photocatalyst was synthesized by solvothermal method, and is nano-rod liked, with its length of 1²m and diameter of40⁶0nm. XRD indicated that as-synthesized CdS was a single crystal ofhexagonal structure, with high purity. UV–Vis diffuse reflectance spectrum of CdS nanoparticlesinterpreted its trait in visible light adsorption, and PL spectrum indicated that loading Pt decreased the PLpeaks intensity, which means Pt functions as the trapper of photo-generated electrons. The effects of thepreparation conditions of Pt were also studied in this paper. The experiments of water splitting wereperformed with a300W Xe light as resources （>420nm）. According to the experimental results,polyvinlylpyrolidone （PVP） has effect on the preparation of Pt, and thus significant effect on thehydrogen production rate. In addition, the loading amount of Pt has an optimum value,1.25%（wt%）,and the hydrogen production rate is high up to34mmol/h/gCat..3. Bi-metal nano-particles of Pt-Pd and Ru-Pt were prepared by two-step chemical reduction methodrespectively, and then introduced on the surface of commercial CdS （>99.999%） by photo-reduction.The co-catalysis of bi-metals on the decomposition of water by CdS was studied.The composition and structure of Pt, Pd binary metal nano-particles were analyzed by EDS, XPSand other means. XPS shows that Pt are mainly of Pt⁰in binary metal and there are amounts of Pd0andsmall number of Pd²⁺.EDS and XPS show that Pd/Pt on the surface are significantly greater than3/7,which preliminary means the form Pt-nuclear and Pd-shell structure has been obtained. Based above all,the structure of the photocatalyst is assumed as Pt-Pd-Pd_xCd_1-xS.The experiments of water splitting wereperformed with a300W Xe light as resources （>420nm） with1M （NH₄）₂SO₃solution as hole cleaningagent. Results showed that Pt, Pd binary metallic nano-particles on the visible to CdS are more favorablefor H₂generation than single metals When Pt for core, Pd for package, and quality of Pd and Pt is7/3,the hydrogen production rate is maximum to26mmol/h/gcat.. Pd_xCd_1-xS with compound semiconductorstructures improve the efficiency of photochemical transformation. Pt, Ru metal nanoparticles are characterized by XPS. XPS showed that Pt in binary metal are mainlyPt⁰and Pt²⁺, and Ru is so little amount that the signal is weak, which is because they are completelyovershadowed by the1s diffraction peaks of C contaminants on the surface.After the samples etched withargon, Pt in binary metal are mainly Pt⁰, and Ru are mainly Ru⁰. EDS and XPS analysis of surfaceelements showed that Pt/Ru on the surface is apparently higher than7/3, which preliminary meansRu-nuclear and Pt shell structure is formed. Based above all, the structure of the photocatalyst is assumedas Ru-Pt-CdS.Effects of Pt, Ru binary metal nanoparticles on CdS for water-splitting reaction has beenstudied with the same evaluation system. Results showed that the Pt, Ru binary metal nanoparticles havesignificantly increased the hydrogen production activity of CdS than single precious metal depositsbecause of its synergy effect, of which, the hydrogen production activity of the catalyst in which Ru is asthe nuclear and its nanocrystalline was packaged with Pt, is relatively stable, reaching14.74mmol/h/gCat..