Sythesis Of P-Co3O₄/n-TiO₂ Nanoparticles For Overall Water Splitting

The sharp increase of the global energy consumption makes efficient utilization of solar energy more urgent. The solar energy irradiating the surface of the Earth exceeds the current global human energy consumption by roughly four orders of magnitude. Hydrogen is a new green energy, with the advantages of high mass-energy ratio, clean, efficient transport and convenient storage. Although the content of hydrogen is very rich in the universe, direct hydrogen resources are not available on Earth. Overall water splitting under sunlight has received much attention for production of renewable hydrogen from water on a large scale. Photocatalyst TiO2 has been studied extensively on water splitting, since the discovery of the Honda–Fujishima effect in the early 1970 s. However, TiO2 can not be irradiated by visible light because of its wide band gap(3.2eV). The band gap of the Co3O4 nanomaterials is very close to the energy of visible light, so the Co3O4 nanomaterials can be excited by sunlight. But the typical p type Co3O4 semiconductor can not be use on overall water splitting. Therefore, Co3O4-TiO2 nanocomposite is designed in this work. The whole research is concluded as follow:(1) A new structure of Co3O4-TiO2 nanocomposite is designed. The p-n herejunction is introduced to prove the efficiency of photocatalytic water splitting.A new structure of Co3O4-TiO2 nanocomposite with p-n herejunction is designed in this part. The p type Co3O4 semiconductor nanomaterials who can be excited by visible light and n type TiO2 semiconductor nanomaterials are chosen to form the p-n herejunction. The different band gap positions of the semiconductors make electrons transform from the conduction band of Co3O4 to that of TiO2, while the holes transform from valence band of TiO2 to that of Co3O4. The photo excited carriers transform along the inside electronic field, so the combination possibility of the electrons and holes is reduced hugely. The visible light excited water splitting rate is proved in this way.(2) The Co3O4 semiconductor nanosphere was prepared via a brand new method, and the best parameters of this method were comformed. The Co3O4-TiO2 composite with p-n herejunction was synthesized based on the Co3O4 nanosphere.A new method, which is named carbon assisted method, was used to obtain the Co3O4 semiconductor nanoparticles. Commercial degreasing cotton was used as the reactant, which was heated after absorb the Co2+ solution. The components of the nanomaterial were confirmed using X-ray diffraction(XRD) and X-ray photoelectron spectroscopy(XPS) measurement. The morphology of the Co3O4(~50nm) nanoparticles was measured by transmission electron microscopy(TEM). The reaction process was comfirmed by Fourier transform infrared spectroscopy(FTIR). The results show that the best parameters is that the reactants are commercial degreasing cotton and Co(NO3)2, the reaction time is 2h, and the heating time is 600℃. The Co3O4-TiO2 composite prepared via Sol-Gel method and carbon assisted method was measured by XRD, SEM, TEM and HRTEM. Characterization results show that the Co3O4-TiO2 composite with p-n herejunction is obtained via Sol-Gel method.(3)The photocatalytic ability of Co3O4 and Co3O4-TiO2 nanomaterials are demonstrated by characterization results. Nanocomposite p-Co3O4/n-TiO2 photocatalyst was prepared. The experimental platform was set up, the water splitting ability of the Co3O4 and Co3O4-TiO2 nanomaterials was illustrated then.The specific surface areas of the Co3O4 and mesoporous structure Co3O4-TiO2 are 17.7m2/g and 39.64m2/g respectively, which is computed from the results N2 physisorption at 77 K by using the BET(Brunauer-Emmet-Teller) and BJH(Barrett-Joyner-Halenda). The Co3O4 and Co3O4-TiO2 nanomaterials schematic of sunlight photocatalytic water splitting was surmised through the results of UV-VIS spectra,(Ahv)2-hv curve and valence-band XPS spectra of TiO2 and Co3O4 nanoparticles. The experimental platform was set up, and the the photocatalytic activity of the p-Co3O4/n-TiO2 nanocomposite was researched via sunlight irradiating overall water splitting reaction. The results of the Co3O4 and Co3O4-TiO2 water splitting reaction show that the Co3O4 can only be used as photocatalytic hydrogen evolution and the evolution rate of H2 is 6.5μmol h-1 g-1, while the Co3O4-TiO2 can be used on overall water splitting reaction, the optimal evolution rate of H2 and O2 are 8.16μmol h-1 g-1and 4.0μmol h-1 g-1, and the ratio of the evolution rate of H2 and O2 is 2: 1. The photocatalytic ability is improved by the p-n herejunction.