Theoretical Atomic-Scale Modification of Two-Dimensional Materials for Environmental and Energy Applications

Abstract

With the increasing pressure from the energy crisis and environmental pollution, developing green functional materials for clean energy and environmental protection has become increasingly urgent. Two-dimensional materials (2DMs) are widely proposed for various energy and environmental issues due to their unprecedented properties and multiple surface engineering. However, they are limited in precise control and mechanism understanding of designed active sites. Therefore, the central issue is to design 2DMs not only to improve reaction performance, but also to possess attributes of low cost, effective active site, and understandable mechanism. In this thesis, a series of 2DMs modified by vacancy-defect or transition metal (TM) dopants were designed for various environmental and energy applications via density functional theory (DFT). The microstructural stability, reaction performance, and intrinsic mechanisms were systematically evaluated.

In the first part of this thesis, MXenes of Ti₃C₂O₂, Ti₃C₂F₂, and Ti₃C₂(OH)₂ are evaluated as the sensor platform for SF₆ decomposed gases (SOF₂, SO₂, and H₂S). The results indicate that as compared with the weak adsorption on pristine Ti₃C₂O₂ and Ti₃C₂F₂, SOF₂, SO₂, and H₂S gas molecules tend to be chemisorbed on Ti₃C₂O₂ and Ti₃C₂F₂ containing point vacancy with high adsorption energies. Furthermore, the study of electronic properties suggests that all adsorption systems show high electronic conductivity and the exposed Ti atoms by point defect mainly contribute to the formation of ionic bond with high adsorption energy. Thus, the present results show that Ti₃C₂O₂ and Ti₃C₂F₂ with point vacancy are feasible novel sensing materials to detect SF₆ decomposed species with high sensitivity and low electronic noise. The sensitive detection capability of SO₂ is particularly noticeable.

Second, all single TMs embedded in Ti₂N were screened for hydrogen evolution reaction (HER) via DFT. Our results show that embedded single TMs can tune the hydrogen adsorption with a volcano-like relationship between the exchange current density and the Gibbs free energy (ΔG_H), and Mo-Ti₂N stands out with an optimal ΔG_H of -0.097 eV. This can be further improved with an optimal ΔG_H close to thermal neutrality by applying 1.2% tensile strain to Mo-Ti₂N. The excellent exchange current density of Mo-Ti₂N under 1.2% tensile strain is superior to the majority of traditional metal catalysts. The chemical nature of the Mo-H bond under strain was analyzed by revealing the changes in internal electronic structure and establishing the relationship with adsorption energy. The results reveal the bonding mechanisms to clearly show that single TM atom embedded in Ti₂N such as Mo-Ti₂N is a feasible electrocatalyst for HER and provides guidelines for improving the electrocatalysts.

Third, ten candidates of TMs embedded in phosphorus carbide (PC) monolayer were studied for direct NO-to-NH₃ electroreduction by DFT, realizing the dual benefit of NH₃ synthesis and NO removal. The employment of machine learning-aided theoretical calculations helps to identify the critical role of TM-d orbitals in regulating NO activation. A V-shape tuning rule of TM-d orbitals for the ΔG_H change of NO or U_L is further revealed as the design principle of TM embedded PC (TM-PC) for NO-to-NH₃ electroreduction. Moreover, after employing effective screening strategies including surface stability, selectivity, kinetic barrier of potential-determining step, and thermal stability comprehensively studied for the ten TM-PC candidates, only Pt embedded PC monolayer has been identified as the most promising direct NO-to-NH₃ electroreduction with high feasibility and catalytic performance.

Fourth, considering the influence of low-nuclearity clusters, which exist in SACs as well, on catalytic performance is always overlooked, 9 single-TMs, namely, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu, were designed to anchor on three Ti₃C₂O₂-based models (Ti₃C₂O₂-V₀, Ti₃C₂O₂-O₁, and Ti₃C₂O₂-Ti₁) as catalysts for NO electroreduction to NH₃ (NORR), and further evaluate the impact of low-nuclearity clusters on optimal SAC via DFT. Our results identified Cu/Ti₃C₂O₂-V₀ and Ni/Ti₃C₂O₂-Ti₁ as promising SACs for ultrahigh performance of NORR. Furthermore, NORR activity on both SACs diminished with the formation of the low-nuclearity cluster, especially for Ni clusters on Ti₃C₂O₂-Ti₁. From Ni single atoms to Ni clusters, the projected density of states increases significantly near the Fermi level, leading to stronger interactions between Ni clusters and reaction intermediates, thereby hindering hydrogenation steps and greatly slowing down the NORR on Ni-cluster. Meanwhile, a pH-dependent catalytic activity analysis revealed that the rate-determining step changed as pH increased on Ni/Ti₃C₂O₂-Ti₁, and an acidic environment facilitates the effective NO-to-NH3 conversion on Cu/Ti₃C₂O₂-V₀.

The present discoveries are vital to the atomic-scale modifications on 2DM for solving energy and environmental issues. Furthermore, in-depth mechanisms and effective active sites have been revealed to guide the future experimental design of catalysts and sensor applications.

Date of Award	30 Apr 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Lawrence WU (Supervisor)