| Since the photoelelctrochemical splitting of water into H2 and O2 on titaniatitanium oxide (TiO2) electrode was reported in 1972, a great number of attempts on water photosplitting over visible-light responsive semiconductors have been made, with a view to constructing solar energy conversion system to H2 fuel from water in the opinion of the practical application. It is well known that WO3 and BiVO4 is stable and can photo-oxidize water into O2 under visible-light irradiation. Few studies focus on selected-controlled synthesis of WO3 and BiVO4 which are unique morphology, specific crystalline phase, strong light absorption and high activity of photocatalytic O2 evolution. CdS has been extensively studied because of its excellent water photosplitting, property in that its bandgap (ca.2.3 eV). Nevertheless, CdS is prone to photocorrosion during the photochemical reaction because CdS itself is oxidized by the photogenerated holes, which obstructs the large-scale application of the photocatalytic H2 production. Syntheses of CdS nanoparticles in some hetero-matrices have also attracted more research interests as the relative matrix can effectively suppress the photocorrosion of CdS during the photochemical reaction. Cellulose films and multi-walled carbon nanotubes (MWCNT) are quite suitable as host matrixes for the embedment of CdS nano-particles. Though, as far, CdS/MWCNT and CdS/cellulose had not been reported in literature.Hence, in the present paper, microspheric and lamellar BiV04, nanorods WO3, CdS/MWCNT and CdS/cellulose were prepared. Moreover, the effects of reaction conditions and compound proportion on morphology, crystalline phase, light absorption and photacatalytic activity of the obtained samples were also primarily discussed, which is preliminarily reveals internal cause of high catalytic activity and stability of photocatalyst. The mechanism of photoelectrochemical behavior, electron transfer and recombination of BiVO4 electrodes was systematically studied by electrochemical methods. The detail researches and the conclusions are as follows: 1. Monoclinic BiVO4 nanoparticles were prepared through a homogeneous precipitation process. Photocatalytic O2 evolution efficiencies over the obtained BiVO4 nanoparticles under visible-light (λ> 420nm) irradiation were investigated comparatively by using AgNO3 and Fe(NO3)3 as sacrificial reagent, respectively. It was found that BiVO4/Fe(NO3)3 system is more promising in the opinion of practical application because of long-period photocatalytic activity and conveniently reactivate. However, AgNO3 was a more effective sacrificial reagent for the photocatalytic O2 evolution over the BiVO4 in short-period irradiation in comparison with Fe(NO3)3 attributed to loaded Ag clusters on the surface of BiVO4.2. Microspheric and lamellar BiVO4 powders were selectively prepared through a hydrothermal process. Microspheric BiV04 of high symmetry with particle sizes in the range of 7-12μm can be derived from a relative low hydrothermal temperature (≤160℃), and possess a mixed crystal consisting of tetragonal and monoclinic phases; whereas dissymmetry lamellar BiVO4 with pure monoclinic phase can be obtained at a higher hydrothermal temperature (200℃). Their photocatalytic activities for O2 evolution were investigated by using Fe(NO3)3 as a sacrificial reagent under visible-light irradiation, and the lamellar BiVO4 shows a better photoactivity than the microspheric product due to its pure monoclinic crystal phase.3. Nanorods hexagonal WO3 powders were prepared through a simple hydrothermal process by using Na2SO4 as a direction reagent of crystal plane. Nanorods hexagonal WO3 with a diameter of 30-150nm and a length of 0.5μm-5μm can be obtained with 0.25M Na2SO4 at 160℃for 24h. When Na+cation concentration is low (≤0.5M), the coverage of Na+cation at (200) crystal plane of h-WO3 would restrict the further growth of (200) crystal plane, and induce WO3 growth along (001) crystal plane. But once Na+cation concentration is too high (≥1M), Na2SO4 might obstruct the crystal phase transformation from a mixed crystal to pure h-WO3. Their photocatalytic activities for O2 evolution were investigated by using Fe(NO3)3 as a sacrificial reagent under visible-light irradiation, and nanorods hexagonal WO3 shows a high photoactivity due to it’s good dispersibility and high length-diameter Ratio. 4. CdS nanoparticles were immobilized in porous regenerated cellulose (RC) films with different pore sizes via in situ a precipitation method. The mean pore sizes of the porous RC films can be modulated from about 20nm to 57 nm by adjusting the concentration of cellulose solution, and the porous structures within RC film act as reacting sites to lead to the embedment of CdS nanoparticles with mean particle diameter of about 8 nm. High photocatalytic H2 production efficiency of about 1.323 mmol g-1 h-1 under visible-light irradiation (λ≥420nm) has been attained over the Pt-loaded CdS/RC nanocomposite film synthesized by 4.5% cellulose solution due to the efficient light absorption, fast carriers transfer and photochemical reaction between the loaded CdS nanoparticles and electrolyte interfaces. In a long-period visible-light irradiation, the present CdS/RC nanocomposite film shows excellent fixity and photostability in comparison with a nanoparticle suspension system, indicating its promising in the practical application, and the present synthesis strategy could be a general method for other semiconductor photocatalyst/porous cellulose films.5. CdS/MWCNT composites were prepared through in situ adsorption and hydrothermal process. Moreover, the effects of compound proportion on morphology, crystalline phase and light absorption were also discussed. The carboxyls on the surface of CNT would help the achievement of direct chemical bonding between CNT and CdS, which result in synergistic effect of CNT and CdS. MWCNT would contribute to red-shifts in the absorption band for the CdS/MWCNT composites. Compared with CdS, most of CdS/MWCNT composites show higher photocatalytic activity. In a long-period visible-light irradiation, high photocatalytic H2 production efficiency of about 2.019 mmol has been attained over 10% CdS/MWCNT composite, which is 2.4 times higher than that of CdS. A high-energy photon excites an electron from the valence band to the conduction band of CdS photogenerated electrons formed in the space-charge regions are transferred into theMWCNTs, and holes remain on the CdS to take part in redox reactions. This results in the reduced recombination of photogenerated carriers and increase photocatalytic activity of CdS/MWCNT composites.6. The mechanism of photoelectrochemical behaviors, electron transfer and recombination of BiV04 electrodes was systematically studied by electrochemical methods. The effect of calcination temperature and film thickness of BiVO4 electrodes on the transient photocurrent, monochromatic incident photon-to-electron conversion efficiency and flat-band is also discussed. Calcination temperature can significantly influence the photoelectrochemical characteristics of electrodes. When temperature is low (≤500℃), the photoelectrochemical activity is enhanced with temperature. But once is temperature too high (>500℃), the photoelectrochemical activity is decreased due to the remarked increase of bulk defeats of electrodes. The transient photocurrent spectra show high visible-light photon-to-electron conversion efficiency of BiVO4 electrodes. The band gap of BiVO4 electrodes is 2.36 eV calculated with IPCE and hv. And the flat-band potential (Efb) of semiconductor electrodes measured by Mott-Schottky analysis is-0.46 V (vs Ag/AgCl). The above conclusion is reference of optimization of the BiVO4 photocatalytic system. |
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