Hong Wang,Sha Zhu,Zhanshuo Dai,Xinze Li,Taigang Zhou
Selective synthesis of large diameter single-walled carbon nanotubes (SWCNTs) is desired for a variety of applications, but most catalysts can only synthesize SWCNTs with a diameter less than 2 nm. Here, a low cost and green catalyst (Co/RH-SiO 2 ) was prepared by using rice husk-derived SiO 2 as the catalyst support, which selectively synthesized large SWCNTs (d t > 2 nm) with a percentage of 70.7 % among all tubes grown at the high growth temperature of 950 ℃. The carbon yield produced on this catalyst containing 1 wt% Co was 4.49 wt%. In contrast, commercial SiO 2 supported Co catalyst (Co/Cab-SiO 2 ) synthesized smaller SWCNTs with a broad diameter distribution under the same growth condition. The detailed characterizations of catalysts showed that Co/RH-SiO 2 catalyst is unique with the abundant small mesopores (2–4 nm) and reducible Co hydrosilicate and surface Co silicates highly dispersed in the pore structures. The reduction of Co hydrosilicate and surface Co silicates facilitates the formation of large Co clusters, which are stabilized by irreducible Co silicates and small mesopores, leading to the growth of large-diameter SWCNTs. Results in this study provide useful insights to develop low cost and green catalysts from biomass waste for the selective growth of SWCNTs.
Changwang Shao,Guowei Zhou,Zhaocheng Li,Yue Wu,Delan Xu,Bin Sun
Large-diameter tube-like mesoporous TiO 2 nanotubes were synthesized via homogeneous precipitation from aqueous solutions containing titanium oxysulfate (TiOSO 4 ) using urea. The cationic surfactant cetyltrimethylammonium bromide was used as a structure-directing agent. Prepared samples were characterized by X-ray diffraction, transmission electron microscopy (TEM), field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, and N 2 adsorption–desorption isotherms. Experimental results indicated that the samples exhibit tube-like structures with lengths of approximately 6 μm to 8 μm, large tube diameters of approximately 500 nm to 600 nm, and a wall thickness of approximately 450 nm. TEM images and N 2 adsorption–desorption isotherms showed that the tube walls of the samples have a uniform mesoporous structure. A possible mechanism for the formation of tube-like mesoporous TiO 2 was proposed. The photocatalytic activities of the large-diameter tube-like mesoporous TiO 2 were assessed via the photocatalytic decomposition of papermaking wastewater under ultraviolet light irradiation. The effects of different parameters such as UV light irradiation, catalyst dosage and solution pH on photocatalytic activities of tube-like mesoporous TiO 2 were studied. The as-synthesized tube-like TiO 2 showed higher photocatalytic activities than Degussa P25. MTS3 exhibited the highest photocatalytic activity. The chemical oxygen demand and chroma percent degradations of papermaking wastewater were approximately 73.0% and 99.5%, respectively, at MTS3 dosage of 1.0 g/L and pH value of 3.0 after 12 h of photodegradation.
Aspheric thin-walled crucibles with large diameters are widely used for photovoltaics and semiconductor industries, while they have hard-brittle and aspheric characteristics. Nevertheless, these crucibles strictly require atomic surfaces and high material removal rate (MRR). This is a challenge for chemical mechanical polishing (CMP). To break this challenge, a novel green CMP was developed for fused silica, containing ceria, lanthana, hydrogen peroxide, diethylene triaminepentaacetic acid pentasodium, carboxymethyl cellulose sodium and guanidine carbonate. After CMP, surface roughness Ra of 0.115 is obtained at a scanning area of 5 × 5 μm 2 measured by atomic force microscopy, and MRR is 563 nm/min. In such an atomic surface, the MRR is the highest compared with those published previously. Transmission electron microscopy confirms that the thickness of damaged layer after CMP is only 2.63 nm. A novel approach of robot floating polishing is proposed for aspheric crucibles with a diameter of 810 mm and a height of 580 mm. Using the developed CMP, surface roughness Sa of 0.178 nm on inner surface of an aspheric thin-walled crucible is achieved under a measurement area of 50 × 50 μm 2 . X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance reveal that the content of Ce 3+ increased from 28.54 to 43.76 %. Through Raman, XPS and Fourier transform infrared spectroscopy, CMP mechanisms are elucidated. Densified damaged layer on the surface of fused silica was induced by the transformation of surface structure from six-membered to four- or three-membered network. Ce-OH and Si-OH were formed between fused silica and ceria abrasives, forming Ce-O-Si by condensation reaction of dehydration. The developed green CMP and approach provide new pathway to polish and manufacture atomic surfaces for aspheric thin-walled crucibles with large diameters.
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