Roles of Cu-based Cocatalysts in the Photocatalytic Systems for Hydrogen Evolution from Water

【摘要】：Hydrogen production from water splitting and environmental pollution treatment are topics of increasing attention in nowaday society. Many works have been done in the above fields. Therein, photocatalytic systems using semiconductors as photocatalysts have been thought to be promising because of low cost, chemical stability, and non-toxicity. However, the progress in the extensive applications of these photocatalytic systems has been impeded by several deficiencies, such as rapid recombination rate of electron/hole pairs before migrating to the surface of a semiconductor to carry out photocatalytic reactions, poor activity under visible light irradiation due to large band-gap energy and so on. Therefore, the development of highly efficient photocatalytic systems is an important subject from the practical point of view. In the present work, we fabricated the photocatalytic system containing SrTiO3 and CuO cocatalyst, the photocatalytic system containing SrTiO3 and Cu cocatalyst, and the photocatalytic system containing Eosin Y, multiwalled carbon nanotubes and CuO cocatalyst, respectively. Meanwhile, the photocatalytic hydrogen evolution from water over these systems was investigated using TEOA or CH3OH as a sacrificial reagent. The role of CuO played in these photocatalytic systems was further discussed using cyclic voltammetry and electrochemical impedance spectroscopy. In addition, TiO2 nanoparticles doped with Si were prepared using a hydrothermal method. The photocatalytic acitivity for degradation of methyl orange (MO) and photoelectrochemical behavior was explored. Furthermore, the relationship between the photocurrent and photocatalytic activity of Si-doped TiO2 nanoparticles was studied. In the photocatalytic system containing SrTiO3 and CuO cocatalyst, the CuO cocatalyst was loaded on SrTiO3 nanoparticles using an impregnation method followed by heat-treatment. The obtained samples were characterized with powder X-ray diffraction (XRD), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS) and nitrogen adsorption. The results showed that CuO was homogeneously loaded on the surface of cubic SrTiO3 nanoparticles. In addition, it can be observed that CuO-loaded SrTiO3 nanoparticles possess high activity for the photocatalytic hydrogen evolution from water under ultraviolet irradiation. After irradiation for 48 h, the rate and quantum yield (ΦH2) of H2 evolution over 1.5% CuO-SrTiO3 are up to 2.31 mmol·h-1·g-1 and 1.19% (in the range of 200-400 nm), respectively. On the contrary, H2 can be hardly producted using pure SrTiO3 as a photocatalyst. Therefore, CuO plays an important role in the enhancement of photocatalytic activity of SrTiO3. CuO is an efficient cocatalyst in this photocatalytic system and acts as a charge transferring site and/or active site in the photocatalytic process. For the photocatalytic system containing SrTiO3 and Cu cocatalyst, the Cu cocatalyst was loaded on the surface of SrTiO3 nanoparticles using a photo-deposition method. This photocatalytic system was characterized with XRD, TEM, energy dispersion X-ray analysis (EDX) and nitrogen adsorption. The photocatalytic acitivity of Cu-loaded SrTiO3 for hydrogen evolution from methanol aqueous solution was studied under ultraviolet irradiation. It can be found that Cu-loaded SrTiO3 nanoparticles display very high photocatalytic activity for hydrogen evolution, which is comparable with that of SrTiO3 loaded with Pt. The quantum yield of hydrogen evolution (ΦH2) of 0.5 wt.% Cu-SrTiO3 is calculated to be 1.35% after 48 h irradiation. These results indicate that the Cu loaded is an efficient alternative to Pt as a cocatalyst. The Cu can facilitate the separation of photo-induced electron/hole pairs and serves as the active site of hydrogen evolution. Furthermore, the effects of loading amount of Cu, reaction temperature, type and concentration of electron donors, and dosage of the photocatalyst on the photocatalytic hydrogen evolution were studied in detail. Besides, we have developed a visible light active photocatalytic system for hydrogen evolution from water using Eosin Y, CuO and multiwalled carbon nanotubes (MWNTs). The CuO-loaded MWNTs (CuO-MWNTs) were prepared using the impregnation method followed by heat treatment. The results of XRD and TEM indicate that CuO is homogeneously loaded on the surface of MWNTs. The photocatalytic system containing Eosin Y, CuO and MWNTs is an efficient photocatalyst under visible irradiation. In order to further understand the role of CuO played in the photocatalytic system, the effect of CuO loaded on the electrochemical properties of MWNTs was investigated by using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). It can be observed that after CuO is loaded on MWNTs, the electron conductivity of MWNTs is enhanced and CuO-MWNTs exhibit fast electron transfer rate. We presume that the loading of CuO on MWNTs can make electrons aggregate at the interface between CuO and MWNTs, implying an enhancement of the electron transfer between CuO on MWNTs. These results further confirm that CuO may be as a charge transferring site in the photocatalytic system. MWNTs act as an efficient electron transfer channel besides supporting material. The Si-doped TiO2 nanoparticles exhibit a smaller size and better dispersity than the undoped ones. In addition, the Si-doped TiO2 nanoparticles display much higher photocatalytic activity under ultraviolet light irradiation than that of commercial TiO2 nanoparticles (Degussa P25). The results of linear sweep voltammetry show that doping of Si in TiO2 has both a positive effect and a negative effect on the photocurrent of TiO2. The photocurrent value of the 15 mol.% Si-doped TiO2 (15% Si-TiO2) electrode (54.9μA) is highest in the Si-doped TiO2 electrodes and much higher than that of the pure TiO2 electrode (16.7μA), indicating that more photogenerated electrons on the 15% Si-TiO2 electrode flow to cathode, and more photogenerated holes participate in the oxidation reaction under the same conditions. Therefore, it can be deduced that the enhanced photocatalytic activity of 15% Si-TiO2 comes from the easy transfer and separation of photogenerated electrons and holes because of doping of Si; and the photocurrent may be an auxiliary parameter to explain the photocatalytic behavior of a photocatalyst.