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Designing and screening two dimensional covalent organic frameworks for effective water splitting

Periodic Reporting for period 1 - 2D-COF-WS (Designing and screening two dimensional covalent organic frameworks for effective water splitting)

Reporting period: 2017-04-01 to 2019-03-31

Hydrogen fuel production has gained increased attention as oil and other nonrenewable fuels become increasingly depleted and expensive. Methods such as photocatalytic water splitting are being investigated to produce hydrogen, a clean-burning fuel. Water splitting holds particular promise since it utilizes water, an inexpensive renewable resource. Photocatalytic water splitting has the simplicity of using a powder in solution and sunlight to produce H2 and O2 from water and can provide a clean, renewable energy, without producing greenhouse gases or having many adverse effects on the atmosphere. Due to the high surface area and abundant active sites, 2D materials have shown high potentials for photocatalytic water splitting. A photocatalyst dedicated for water splitting requires some prerequisites for its electronic structure: the conduction band minimum (CBM) should be more positive than the potential of H2 evolution (H+/H2, -4.44 eV vs. vacuum) and the valence band maximum (VBM) more negative than the potential of O2 evolution (O2/H2O, -5.67 eV vs. vacuum) reaction, respectively. This condition has excluded most 2D semiconductors from being possible unassisted photocatalysts. Therefore, exploring stable 2D semiconductors with appropriate electronic and optical properties is demandable for developing effective photocatalytic water splitting.
Among 2D structures, 2D covalent organic frameworks (COFs) are of specific promise for water splitting under solar irradiation. This is because 2D COFs possess porous structures with large surface area, high porosity and favourable open space for water oxidation and reduction and the ordered π systems can facilitate effective charge transports in the stacking direction. Meanwhile, the structural configurations and intrinsic properties of COFs can be experimentally controlled by using different organic building units and synthesizing approaches. However, many 2D COFs suffer from poor stabilities in aqueous solution. The structural, electronic and optical properties of 2D COFs can vary significantly with different compositions and lattices, which will determine the final photocatalytic performance of 2D COFs. Therefore, design and screen viable 2D COFs for effective water splitting requires in-depth fundamental research.
The overall objective of this project is to design and screen potential 2D semiconductors that made of 2D COFs with extroadinary properties and utilize them for photocatalytic water splitting.
By performing DFT calculations, we proposed new 2D materials, namely PdPX (X=S or Se) and investigated the possibilities of using them for water splitting. we have shown that 2D PdPX possess good stabilities and appropriate electronic and optical properties for solar driven water splitting. On basis of the investigation of PdPX for water splitting, we further explored the potentials of using Pd3P2S8 monolayer and bilayer to catalyze water splitting and studied the specific oxygen reduction process on surface of the catalyst. Our computations indicated that this 2D material can provide enough driven force for the oxygen reduction reaction, thus is an attractive candidate of photocatalyst. These two works have been published as academic papers (Chem. Eur. J. 2017, 23, 13612-13616; J. Mater. Chem. A, 2018, 6, 23495-23501) and the results are well disseminated to the academic communities. By accomplishing these two works, a mature research process in exploring the photocatalytic water splitting performance of 2D materials has been set up.
On the basis of the experimental knowledge of covalent organic frameworks in literature, we have designed several COFs built of hetero-triangulenes forming an intriguing Kagome lattice in two dimensions and systematically explored the structures and properties of them. According to our calculations, these 2D COFs show general electronic properties of hexgaonal and kagome lattices, while the band structures can be further determined by the hetero atoms. The resulting 2D kagome polymers have a characteristic electronic structure with a Dirac band sandwiched by two flat bands and are either Dirac semimetals (C center), or single-band semiconductors(B or N centers). Our investigations provide insights in designing 2D COFs with specific electronic properties and these design principles guide to screen desirable 2D COFs in experiments. This part of research has been published as an academic paper on an influential journal of chemistry (J. Am. Chem. Soc. 2019, 141, 2, 743-747).
We have further explored the possibilities of using the designed 2D COFs of D3 for water splitting. Extensive computations have been done to examine the band structures, optical properties and the specific oxygen reduction processes on surface of 2D COFs. According to our investigations, several 2D COFs can provide appropriate band edge alignment and pronounced light adsorption for water splitting. By comparing the front orbitals of the building monomers and the band edge of the corresponding 2D COFs, we further found the principle in designing possible photocatalysts of 2D COFs. This part of work has been mostly accomplished and will be submitted as an academic paper and disseminated to the academic communities soon.
Understanding the intrinsic properties and photocatalytic properties of 2D COFs from molecular perspectives aid in screening promising catalysts for water splitting and promote the further development of photocatalysis. As a fundamental research, our work provide deep insights into the water splitting research and several promising 2D semiconductors have been proposed to be pormising in photocatalyzing water splitting. These results will also motivate the development of hydrogen production and help relieve the energy crisis problem and environmental pollutions. Therefore, the implementation of this work is of positive socio-economic impact to Europe and the world.