Design of Two-Dimensional Materials for Environmental and Energy Applications by Density Functional Theory

二維材料在環境與能源應用的理論設計

Student thesis: Doctoral Thesis

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Award date8 Aug 2019

Abstract

With rising concern about pollution, global warming and energy crisis, tremendous academic efforts have been devoted to develop new and green functional materials for environmental and energy issues. Due to their extraordinary physical and chemical properties, two-dimensional (2D) materials have been regarded as one of the most promising candidates for new functional materials for environmental applications such as water treatment, gas pollution sensing,renewable energy conversion and generation. In designing for new functional 2D materials in target applications, theoretical simulation such as density functional theory (DFT) is a versatile and powerful tool. Benefited from the exponential increase in computing power in recent decades, DFT has greatly promoted computational materials science so that it is not only achievable to design new functional materials with specific properties for targeted applications but also assessable to deeper insight into the fundamental properties of materials and reaction mechanisms. In this thesis, the feasibility of some experimentally available 2D materials for environmental and energy applications are explored by DFT.

Firstly,graphene functionalized by doping was theoretically designed for toxic gas sensor. Accordingly, the adsorption of various gas molecules on pristine graphene and Ga-doped graphene was investigated by DFT. In addition, the effect of external electric field on the adsorption mechanism was studied. It was found that toxic gas NO2 showed stronger adsorption energies on Ga-doped graphene and the affinity can be improved by electric field. This study has provided a feasible gas sensor designed for the detection of NO2 toxic gases,and an effective approach to tune the adsorption of NO2 by the external electric field.

As a further design of graphene-based materials for environmental and energy applications, the feasibility of transition metal-decorated N-doped graphene for hydrogen storage was systematically investigated by DFT. The biaxial strain was proposed to be a reversible switch for hydrogen storage and release. Our computational results suggested that Co-decorated N-doped graphene is a highly promising material for hydrogen gas storage with good thermal stability and excellent gravimetric density. Additionally, the adsorption of H2 is sensitive to the biaxial tensile strain. The storage capacity can be effectively improved to 6.0 wt%, by applying 10% strain for releasing adsorbed hydrogen. Thus,our work demonstrated that Co-decorated N-doped graphene is highly feasible for hydrogen storage with excellent gravimetric density and tunability by biaxial strain at room temperature and low pressure.

Then,a new 2D material, phosphorene, was designed as hydrogen purification membranes by DFT. The mechanisms of H2, CO2, N2, CO, and CH4 penetrating through self-passivated porous phosphorene membranes with different pore sizes were systematically explored. By applying an external electric field perpendicular to the porous phosphorene membrane, the diffusion of CO2 and Nthrough the pores was remarkably suppressed due to the polarizability of these molecules, whereas the energy barrier and permeance of H2 passing though the membrane was virtually unaffected. Thus, the application of the electric field improves the performance of hydrogen purification further. This finding opens up a new avenue to optimally tune the performance of 2D materials for gas separation by applying an electric field.

Furthermore, phosphorene exhibits great potential for catalyst due to superior electronic, chemical and optical properties. Thus, we used DFT calculation to screen and design the single transition metal atoms (Sc to Zn, Mo, W, Ru,Pd, Pt, Ag and Au) supported on experimentally available phosphorene monolayer for N2 fixation. In addition, the catalytic mechanism and all possible nitrogen reduction reaction steps were also studied. The results showed that single Ti atom supported on phosphorene exhibits high catalytic activity for N2 fixation at room temperature with fairly low overpotential. The N-N triple bond will be weakened by back-donation from the transition metal to N2 molecule.

Finally,metal-free phosphorus carbide monolayer was theoretically designed as electrocatalyst for hydrogen evolution reaction (HER). Our results showed that inert pristine phosphorus carbide monolayer can be activated by introducing C-vacancy and the adsorption free energy, ΔGH, can be effectively tuned by external strain. With multiple optimized combinations of external strain and defect concentration of C-vacancy, an optimal ΔGH near to zero can be achieved,indicating highly catalytic activity for HER. It is worth noting that a strain-free condition can be obtained with an experimentally available low defect concentration of 4.2%. Our study provides a new promising metal-free catalyst for HER and reveals an effective approach for activating and optimizing the catalytic activity.