| Energy and environment are two significant global issues to the humanity in 21st century. The exhaustion of fossil fuels (such as coal, oil and natural gas) and the global environmental contamination caused by fossil fuels encourage all the countries in the word to develop a novel, environmental-friendly and renewable energy resource. Hydrogen has been considered as an attractive and ideal candidate for the energy carrier of the future because of its high combustion energy and free environmental pollution. Because solar energy is greatly abundant, clean and especially renewable, it will play an important role in the development of new energy sources. Since Fujishima and Honda firstly reported the photoelectrochemical water splitting on a TiO2 electrode, sunlight induced the photocatalytic reactions are widely identified as one of the most promising routes for converting solar energy to hydrogen energy. In view of this, direct photocatalytic production of hydrogen via water splitting reaction over various kinds of oxide semiconductors has received much attention to develop the sustainable source of hydrogen energy. In this work, Ni(OH)2 and CuO facilitate the separation of photogenerated charge carriers and thus enhance the photocatalytic H2-production activity of TiO2.Ni(OH)2 cluster-modified TiO2 (Ni(OH)2/TiO2) nanocomposite photocatalysts were fabrication by a simple precipitation method using Degussa P25 TiO2 powder (P25) as support and Ni(NO3)2 as precursor. The effect of Ni(OH)2 cluster loading content on the photocatalytic hydrogen production rates of the as-prepared samples in methanol aqueous solution was investigated. The results showed that the photocatalytic H2-production activity of TiO2 was significantly enhanced by loading Ni(OH)2 clusters. The optimal Ni(OH)2 loading content was found to be 0.23 mol%, giving H2-production rate of 3056μmolh-1g-1 with quantum efficiency (QE) of 12.4%, exceeding that on pure TiO2 by more than 223 times. This high photocatalytic H2-production activity is due to the deposition of Ni(OH)2 clusters on the surface of TiO2. The enhanced mechanism is because the potential of Ni2+/Ni (Ni2++2e-=Ni, E°=-0.23 V) is slightly lower than conduction band (CB) (-0.26 V) of anatase TiO2, meanwhile higher than the reduction potential of H+/H2 (2H++2e-= H2, E°=-0.00 V), which favors the electron transfer CB of TiO2 to Ni(OH)2 and the reduction of partial Ni2+to Ni0. The function of Ni0 is to help the charge separation and to act as co-catalyst for water reduction, thus enhancing the photocatalytic H2-production activity.Efficient hydrogen production and decomposition of glycerol were achieved on CuO-modified titania (CuO-TiO2) photocatalysts in glycerol aqueous solutions. CuO clusters were deposited on the titania surface by impregnation of Degussa P25 TiO2 powder (P25) with copper nitrate followed by calcination. The resulting CuO-TiO2 composite photocatalysts were characterized by X-ray diffraction (XRD), UV-visible spectrophotometry, X-ray photoelectron spectroscopy (XPS), N2 adsorption-desorption, transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. The low-power ultraviolet light emitting diodes (UV-LED) were used as the light source for photocatalytic H2-production reaction. A detailed study of CuO effect on the photocatalytic H2-production rates showed that CuO clusters can act as an effective co-catalyst enhancing photocatalytic activity of TiO2. The optimal CuO content was found to be 1.3 wt%, giving H2-production rate of 2061μol h-1 g-1 (corresponding to the apparent quantum efficiency (QE) of 13.4% at 365 nm), which exceeded the rate on pure TiO2 by more than 129 times. The quantum size effect of CuO clusters is deemed to alter its energy levels of the conduction and valence band edges in the CuO-TiO2 semiconductor systems, which favors the electron transfer and enhances the photocatalytic activity. This work shows not only the possibility of using CuO clusters as a substitute for noble metals in the photocatalytic H2 production but also demonstrates a new way for enhancing hydrogen production activity by quantum size effect.
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