Graphene Quantum Dots Supported on Fe-based Metal-Organic Frameworks for Efficient Photocatalytic CO2 Reduction

Photocatalytic reduction of CO2 to valuable chemicals is an essential but still remains challenging. Metal-organic frameworks (MOFs) featuring high special surface area, large CO2 adsorption uptakes, adjustable structures and function, have become a kind of promising porous materials for photocatalytic CO2 reduction. However, MOFs often suffer from problems like short light harvesting range, rapid recombination of photogenerated carriers, resulting in lower activity. Here, graphene quantum dots (GQD) were supported on the Fe-based nano-sized MOFs, NH2-MIL-88B(Fe), via electrostatic self-assembly strategy. GQDs were prepared by electrolysis of graphite rod in pure water firstly, and then centrifuged to remove the large species. Transmission electron microscope (TEM) reveals that ultrafine GQDs with 3 nm were obtained. Atomic force microscope (AFM) further demonstrates that the thickness of GQDs is around 0.34-1.5 nm (1-4 stacked layers). The MOFs, NH2-MIL- 88B(Fe), were synthesized with traditional solvothermal method, with a nano spindle shape of 250 nmx40 nm. The amino groups on MOFs provide strong electrostatic force with the carboxylic groups on GQDs, making the composite very stable and efficient electron transfer. High resolution transmission electron microscope (TEM) reveals that the nano MOFs were surrounded by tiny GQDs firmly. The bandgap of composite was determined by solid ultraviolet visible diffuse reflectance spectroscopy (UV-Vis DRS) and Mott-Schottky measurement, which indicate that it is thermodynamically appropriate for photocatalytic CO2 reduction. Photocurrent experiments further demonstrate the composite is beneficial for the photogenerated electron-hole separation. Thus, the resulting GQD/NH2-MIL-88B(Fe) composite showed much enhanced CO production rate (4 times) compared with the parent NH2-MIL-88B(Fe), reaching 590 mu mol/g under 10 h visible light irradiation with triethanolamine (TEOA) as sacrificial agent. The hugely improved photoreduction activity benefits from both the high CO2 adsorption of MOFs and the enhanced separation of photogenerated electrons and holes. This work provides an avenue for preparation of MOFs based materials with high CO2 photoreduction activity.