Over the last decade, significant efforts have been devoted to the design and applications of functional porous materials owing to their outstanding properties such as high surface area, excellent accessibility to active sites, enhanced mass transport and diffusion, and unique light management ability. The porosity, structure, morphology and compositions of these functional porous materials are key to their high performances in all kinds of applications. My thesis research is directed at understanding and development of unique functional porous materials and study their applications in the hydrogen evolution catalysis and carbon-based perovskite solar cell.

Firstly, I report a novel space-confined method to fabricate sandwiched porous MoS₂/reduced graphene oxide (RGO) composites, in wh...[ Read more ]

Over the last decade, significant efforts have been devoted to the design and applications of functional porous materials owing to their outstanding properties such as high surface area, excellent accessibility to active sites, enhanced mass transport and diffusion, and unique light management ability. The porosity, structure, morphology and compositions of these functional porous materials are key to their high performances in all kinds of applications. My thesis research is directed at understanding and development of unique functional porous materials and study their applications in the hydrogen evolution catalysis and carbon-based perovskite solar cell.

Firstly, I report a novel space-confined method to fabricate sandwiched porous MoS₂/reduced graphene oxide (RGO) composites, in which the nanosize of the MoS₂ nanosheets can be effectively controlled by leveraging the confinement effect within the porous two-dimensional graphene layers. Moreover, the presence of the three-dimensional porous RGO substrate guarantee fast electron transfer between external circuit and electrode and the porous structure can efficiently facilitate electrolyte and ion diffusion. Significantly, the resulting sandwiched porous MoS₂/RGO₂ composite shows excellent catalytic activity of hydrogen evolution reaction (HER) with small onset overpotential of 140 mV, a large cathodic current density (23 mA cm^-2 at 200 mV), and small Tafel slope of 41 mV per decade. Further experiments on the effect of oxidation degree of graphene and the exposed active site density on the HER performance of the sandwiched porous MoS₂/RGO composites show that there is an optimum condition for the catalytic activity of HER due to a balance between the numbers of exposed active sites of MoS₂ and the internal conductive channels provided by graphene.

Secondly, I develop a generalized “immobilized crystallization in silica nanoreactor” (ICSR) strategy for the synthesis of an extensive series of well-defined and high-quality metal oxide mesoporous single crystals (TiO₂, SnO₂, and CeO₂ MSCs) with varying morphologies, sizes, and phases. Close-packed colloidal SiO₂ is used as a nanoreactor, a peculiar reaction medium in which immobilized nucleation and crystallization have been systematically studied. High hydrophilicity of residual Si-OH groups facilitates surface adsorption and pore filling of precursor solution, leading to spontaneous nucleation and subsequent crystal growth in the reactor template. The silica template merely serves a faithful negative replication without interfering in the crystallization process but with an added advantage of avoiding crystal aggregation without the need for surfactant. The universality of the ICSR strategy is demonstrated by synthesizing MSCs of different materials. The great value of the as-obtained MSCs is exemplified by a case study on the conspicuous photocatalytic activities of the TiO₂ MSCs. The resultant TiO₂ MSCs exhibit remarkable photocatalytic performance on hydrogen evolution and degradation of methyl orange due to their increased surface area, single-crystal nature, and the exposure of reactive crystal facets coupled with the three-dimensionally connected mesoporous architecture. Overall, this work has provided insights and strategies for the rational fabrication of MSC materials and opened unprecedented opportunities for studying their structure-property relationship.

Thirdly, I design a macroporous TiO₂ nanobowl (NB) array with pore size, bowl size and film thickness being easily controllable by a sol-gel process and polystyrene (PS) template diameter. Based on the TiO₂ NB array, I fabricated carbon-based perovskite solar cells (C-PSCs) to investigate the impact of TiO₂ NB nanostructures on the performance of the as-obtained C-PSC devices. As expected, the TiO₂ NB based devices show higher power conversion efficiency (PCE) than that of the planar counterpart, mainly due to the enhanced light absorption arising from the NB assisted light management, the improved pore-filling of high quality perovskite crystals and the increased interface contacts for rapid electron extraction and fast charge transport. Leveraging these advantages of the novel TiO₂ NB film, the 220 nm-PS templated TiO₂ NB based devices performed the best on both light absorption capability and charge extraction, and achieved a PCE up to 12.02% with good stability, which is 37% higher than that of the planar counterpart. These results point to a viable and convenient route toward fabrication of porous TiO₂ nanostructures for high performance PSCs.

Lastly, I utilize a boron (B)-doped porous multi-walled carbon nanotubes (MWNTs) network as the counter electrode of C-PSCs, wherein the porous MWNTs network provides efficient porous channels for perovskite precursor solution permeation fluently and following high quality of perovskite crystal growth. This method can efficiently improve the interface between carbon electrode and perovskite crystals. Moreover, the B-doped MWNTs significantly improve the rapid hole extraction and transfer by lowering their Fermi energy (F_E) and increasing the number of conduction carriers. In addition, thin insulating aluminum oxide (Al₂O₃) layer coated on the mesoporous TiO₂ film can act as physical barrier for substantially avoiding contact between MWNTs and meso-TiO₂ as well as decreasing the charge recombination. The use of the B-doped porous MWNTs electrode together with the optimized interface engineering leads to remarkable improved performance of C-PSCs, showing high FF and long-term stability.

In my thesis, I developed different functional porous materials, including sandwiched porous MoS₂/RGO composite, metal oxide MSCs, macroporous TiO₂ NB film and porous B-doped MWNTs film, which exhibit outstanding performances in hydrogen evolution catalysis and C-PSCs owing to their unique porous structures and superior material properties. Overall, the study and development of unique functional porous materials for high performance applications open unprecedented opportunities for studying their structure-dependent-performance relationship.