Interface engineering towards transition metal based nanocomposites for water splitting
Date of Issue2017-12-18
School of Chemical and Biomedical Engineering
Hydrogen (H2) is a promising energy source to replace fossil fuels and the key to solving current energy and environment problems. Hydrogen production from water splitting via photocatalysis or electrolysis is considered to be an economical and environmentally friendly approach to convert clean energy into chemical fuels. The main difficulty of water splitting is the lack of low-cost, stable and efficient catalysts. The works presented in this thesis are focused on the development of highly efficient noble metal free catalysts for both hydrogen evolution and oxygen evolution reactions via photocatalytic and electrocatalytic water splitting. In these multi-step processes, more than one component are generally required to accomplish light absorption and charge separation (for photocatalytic reaction), charge carrier transportation and surface redox reaction. The overall efficiency of the process is strongly affected by the interplay among the components as well as the interfacial properties. Therefore, the overall aim of this thesis works is to assemble appropriate functional components with engineered interfaces to achieve remarkably enhanced photocatalytic and electrocatalytic performances for water splitting. In the first part of the research works, MoP particulates were synthesized and dispersed into nanosized particles using probe sonicator. The MoP nanoparticle as a co-catalyst exhibits 4 times of improved photocatalytic hydrogen evolution reaction (HER) activity compared to the bulk form due to the small particle size with increased surface area and better integration with the semiconductor light absorber, CdS quantum dots (QDs). The nanosized dimension of CdS QDs facilitate the charge migration from bulk to surface where holes are consumed by lactic acid. More importantly, the good dispersion of CdS QDs in solution allows them to be trapped in the cluster of MoP nanoparticles. An intimate interface between CdS QDs and MoP is thus formed, which is favorable for the efficient charge transfer from CdS QDs to MoP. Besides, the metallic property and good HER activity of MoP lead to efficient and stable H2 evolution. Next, to further reduce the particle size of metal phosphides and improve the interface between light absorber and co-catalysts, metal oxide (ZnO) was introduced as a low-cost metal oxide support to disperse and stabilize CoP on its surface. The interface between metal oxides and CdS QDs is formed via electrostatic interactions since ZnO is positively charged whereas CdS QDs is negatively charged. Besides, the band structure alignment between ZnO and CdS QDs facilitate the charge transfer from CdS QDs to ZnO, which was further transferred to CoP. The excellent HER activity of CoP and the engineered interface result in the highly efficient and stable H2 production under visible light irradiation. Apparent quantum efficiency of this system can reach as high as 66% at 420 nm and no activity loss is observed for this system after 144 h photocatalytic reaction. The third part of the research work is focused on the development of a noble metal free HER electrocatalyst that has an activity close to that of Pt. CoNA/PDA (NA: Nitrilotriacetic acid; PDA: Polydopamine) core/shell nanowires were first synthesized by coating PDA on the surface of CoNA nanowires (NWs). N, P co-doped carbon nanotube is obtained through phosphidation of CoNA/PDA NWs with subsequent pyrolysis in N2 atmosphere. CoNA NWs decomposed to Co nanoparticles wrapped by several graphene layers. CoP is formed at the cobalt/carbon interface. After activation, the wrapped nanoparticles become accessible and less stable Co nanoparticles are removed by acidic solution. The CoP nanoparticles stabilized by NCoP bonding are exposed which exhibit a high HER activity and stability. Lastly, research efforts of this thesis work were also spent to tackle the other half of the water splitting reaction, oxygen evolution reaction (OER), since the sluggish kinetics of OER is the bottleneck of the overall performance of water splitting. In this part of the work, a promising OER catalyst, Ni-Fe layered double hydroxide (LDH) was chosen and its intrinsic high OER activity was harnessed by blending ultra-fine NiFe-LDH nanocrystals with conductive carbon. The NiFe-LDH/C hybrid was fabricated by a novel one-pot solvothermal method using molecule precursors of metal cations and organic ligand. The resultant NiFe-LDH/C nanosheet consists of poorly crystalized NiFe-LDH (< 5 nm) interconnected with N doped carbon nanodomains. The in situ formation of both components leads to a self-confined growth and fine blending of NiFe-LDH nanocrystals and carbon domains. Such a unique structure results in improved electrical conductivity, increased active sites and enhanced electrochemical active surface area. In addition, the strong interaction between metal centers and carbon leads to the local electronic structure modification of metal centers. These factors contribute together to the development of a highly efficient and stable NiFe-LDH based OER catalyst. In summary, the research efforts in this thesis were spent on designing efficient and noble metal free photocatalysts and electrocatalysts for water splitting reactions. In particular, engineering suitable interfaces is a key focus. Detailed materials characterization and structural analyses were carried out to understand the key factors contributing to the high performances of the catalysts. Through such efforts, several promising transition-metal based catalysts have been developed with high efficiency for HER and OER reactions. It is believed that the findings from this work would contribute to the advancement of the energy research field and the development of practical catalysts for water splitting utilizing solar energy directly or electricity from clean energy.