Fabrication Of Reduced Titania/layered Double Hydroxide Hybrid Materials Towards Photoelectrochemical Water Oxidation

Photocatalytic (PC) or photoelectrocatalytic (PEC) water splitting has received considerable attention owing to its key role in converting solar energy and water resource into a green, efficient and storable chemical energy in the development of new energy. Up to now, titanium oxide is still regarded as one of the promising and irreplaceable photocatalysts and photoanode materials; moreover, the reduced titania (i.e., Ti3+ self-doped titania, TiO2-x) exhibits significantly improved PEC activity owing to an enhanced donor density with higher electronic conductivity. However, it is found that the transport or transfer of photoexcited carriers in reduced titania is insufficient, and the oxygen vacancy resulting from Ti3+ doping may become a new recombination site for electron-hole pair within the bulk. Therefore, how to further improve the PEC water splitting behavior of reduced titania so as to obtain high preformance photoanode materials remains a big challenge.Layered double hydroxides (LDHs) has attracted a broad attention as a photocatalyst or cocatalyst since it can facilitate the electron-hole separation and improve interface oxygen evolution reaction (OER) kinetics in water splitting. Therefore, in this dissertation, a series of reduced titania/LDHs hybrid photoanodes were prepared, and the influence of synergestic effect on the PEC property for water oxidation was revealed.We studied the construction rule for the reduced titania/LDHs hybrid material based on DFT calculations, explored the preparation methods, and investigated the intrinsic electric property of reduced titania as well as the synergestic effect between these two components. The key factors that affect the PEC water oxidation were studied, and the reaction mechanism within the reduced titania/LDHs hybrid photoanode was discussed and revealed. The details are summarized briefly as follows:1. The band structure, band gap and band edge placement ofpristine reduced titanias, Mn-doped reduced titania, Fe-doped reduced titania and several kinds of LDHs have been systematically investigated by the density functional theory (DFT) with Hubbard correction. Firstly, for the metal-doped reduced titania, the band gap shows a somewhat decrease compared with titania and undoped reduced titania. The conductive band minimum (CBM) is mainly comprised of Ti 3d; while the valence band maximum (VBM) is mainly comprised of O 2p for reduced titanias, Mn 3d and O 2p for Mn-doped reduced titania, and Fe 3d for Fe-doped reduced titania, respectively. Secondly, the VBM of LDHs mainly includes O 2p;while the composition of CBM is rather different from each other. Thirdly,these materials mentioned above possess driving force towards water oxidation according to their band edge placements for OER. Furthermore,the band match between the reduced titania and LDHs indicates that it is feasible to construct reduced titania/LDHs hybrid photoanode for PEC water oxidation.2. Several kinds of doping reduced titania were prepared via hydrothermal method, and their PC or PEC performances for water oxidation were studied. Furthermore, hybrid photoanodes fabricated by modifying LDHs on the surface of reduced titania were systematially evaluated. Firstly, we prepared the reduced titanias self-doped with different amount of Ti3+ and measured their PEC performance for water oxidation. It is found that the PEC performance enhances gradually along with the increase of Ti3+ content, and a 95% improvement is obtained for the reduced titania relative to common titania. Secondly, Mn-doped and Fe-doped reduced titania were synthesized, and their PC and PEC performances for water oxidation were tested. The results show that the oxygen yield and oxygen production rate of Mn-doped reduced titania increase by 93.6% and 61.7% compared with undoped sample, and an increase of 54.1% and 28.7% are observed for Fe-doped reduced titania.The current density of Mn-doped and Fe-doped samples at 1.0 V vs. RHE bias increases by 32% and 21%, respectively. This is ascribed to the integrated effect including the increased donor density, decreased impedance and improved interface reaction kinetic. Finally, these undoped and doped reduced titania samples are respectively modified with CoAl-LDH and CoCr-LDH, and their PEC performances for water oxidation are found to decrease. The main reason is that the LDHs modified on the surface of reduced titania dramatically increases the impedance of photoanode owing to the poor conductivity of LDHs,resulting in the recombination of electron-hole between reduced titania and LDHs. Therefore, how to facilitate the electron transport between photocatalyst and cocatalyst is a key issue to enhance the PEC preformance.3. According to the problem mentioned above, a sophisticated fabrication method was employed. The reduced titania photoanode was firstly prepared via an in situ growth method on Ti foil substrate, and then different kinds of LDHs was respectively decorated onto the surface of reduced titania by electrochemical deposition to construct TiO2-x/LDHs hybrid photoanode with core-shell structure. The influences of LDHs type and content on PEC water oxidation performance were thoroughly studied.It is found that the onset potential decreases by at least 30 mV for both reduced titania/Cr-containing LDHs and reduced titania/Co-containing LDHs, but increases by 30 mV for reduced titania/Fe-containing LDHs compared with pristine reduced titania photoanode. Moreover, the PEC performance of these photoanode materials shows the following sequence:TiO2-x/NiCr-LDH > TiO2-x/CoCr-LDH > TiO2-x/ZnCr-LDH > TiO2-x/CoAl-LDH > TiO2-x/ZnFe-LDH > TiO2-x/CoFe-LDH > TiO2-x/ZnCo-LDH. The TiO2-x/NiCr-LDH hybrid photoanode exhibits the optimal PEC water oxidation behavior, with an onset potential of 0.17 V and a current of 0.98 mA·cm-2. The machnism of the enhanced property, based on an experiment-calculation combination study, was revealed. The synergistic effect between the reduced titania and LDHs plays a key role: a good match of band structure between the reduced titania core and LDHs shell facilitates the electron-hole separation and accelerates the migration of holes from reduced titania to LDHs, followed by the water oxidation reaction catalyzed by LDH shell.