Enhanced optical absorption of Fe-, Co- and Ni- decorated Ti₃C₂ MXene: A first-principles investigation

Abstract

Surface functionalization on novel transition metal carbides Ti₃C₂ MXene has been demonstrated to be effective strategy to broaden potential application. Herein, the effect of surface decoration of transition magnetic metals (Fe-, Co-, Ni-) on the electronic structure and optical properties of Ti₃C₂ MXene is studied using first principle calculations. Energy band structures and corresponding density of electronic states investigations have shown that interlamellar spacing between Ti-layer and C-layer, as well as specific surface area, is inclined to expand after surface decoration. Besides, obvious peaks of d orbit have been observed, and demonstrated to play a dominant role in electronic states in Fe-, Co-, Ni–Ti₃C₂ composite. More importantly, surface decoration of transition magnetic metal Fe-, Co-, and Ni-could greatly promote optical absorption coefficient of Ti₃C₂ nanomaterials, covering ultraviolet region, visible region and infrared region. The increment rate reaches 50% within visible and ultraviolet region for Ni–Ti₃C₂ composite, while the increment rate of Fe-substituted Ti₃C₂ exhibits even greater than 100%. The remarkable enhanced optical absorbing properties over wide spectral region may be associated with interlamellar space expansion accompanied with more active sites and increased electronic mobility. All these findings are considered to broaden practical application of low-dimensional Ti₃C₂ nanomaterial towards photoelectric devices.

Introduction

As ignited by the fabrication of graphene [1,2], two-dimensional (2D) materials have experienced an increasing attention in recent years and have found wide applications toward electronics, photonics, energy, and environmental devices [[3], [4], [5], [6], [7], [8], [9]]. However, it remains great challenge to apply 2D materials for efficient optical absorption [10,11]. Many efforts through increasing numbers of layers or surface decoration [12,13] have been devoted to further improve optical absorption, but the effect is very limited. New optical absorption materials that can overcome this problem are needed.

MXenes, a large class of 2D carbides, nitrides, and carbonitrides, are recognized as a new family to the 2D world in 2011 [14,15]. The general formula of MXene is M_n+1X_nT_x (n = 1,2,3), where M represents a transition magnetic metals element, such as Ti, V, Mo, and X represents C, N, or CN, T generally refers to terminal groups distributed on the surface, such as –O, –OH, –F. MXene can be produced by selectively etching the A layer (HF aqueous solution as an etchant) from its corresponding MAX phase at room temperature, where A is a group IIIA to VIA element [16,17]. MXene has high electrical conductivity and extensive chemical properties, in energy storage, electromagnetic interference (EMI) shielding [[18], [19], [20]], transparent conductors [21,22], gas and pressure sensors [[23], [24], [25], [26]], water purification [[27], [28], [29], [30]], photocatalysis [31], Electrocatalysis [32,33], thermoelectrics [34,35] and plasmons have important research value and huge application potential [36].

Lots of theoretical calculations [[37], [38], [39], [40], [41], [42]] have shown that MXenes are mostly conductors, and the conductivity is mainly contributed by the d-orbital electrons of transition magnetic metals. The types of functional groups [41], temperature [43], stress [44], and layer thickness of MXene [45] can all affect the electronic structure and physical properties of MXene to varying degrees. Berdiyorov G R⁴⁶ studied the effect of surface termination on the dielectric and optical properties of Ti₃C₂T₂ (T = F, O, OH) MXene. It was found that the surface functionalization have a significant impact on the optical properties of MXene [47], where the oxidized samples showed greater absorption compared to the original MXene. Hart J L et al. [48] correlate MXene surface de-functionalization with increased electronic conductivity through in situ vacuum annealing, electrical biasing, and spectroscopic analysis within the transmission electron microscope. These indicate that MXene has the potential to improve electronic conductivity and enable MXene's transition between metals and semiconductors. Anasori B et al. [49] have demonstrated that two-dimensional transition magnetic metals carbides can transform MXene from metal to semiconductor behavior by replacing titanium on the surface of MXene with molybdenum. Calculations of density functional theory (DFT) indicate that Ti₃C₂T_x is a metal, while OH-terminated Mo–Ti MXene is a semiconductor with a narrow band gap. Bandyopadhyay A et al. [50] use the first-principles simulations to study the structure and magnetoelectronic behavior of point defects that are most likely to occur in the MXene system. The results show that the layered material exhibits outstanding metal-to-semiconductor or semiconductor-to-metal transitions. In addition, unpaired electrons in the spin-splitting d orbitals are predicted, which may lead to novel visible-light response behavior.

The variety of transition magnetic metals and surface functional groups provide MXene with a great deal of flexibility to extend applications, by controlling the combination of transition magnetic metals and X elements to control its surface chemical properties Peyghan A A et al. [51] used first-principles calculations to study the application of graphene decorated with Ni (Pd) as a methylphenol storage material. The results show that, unlike the original graphene, metal-modified flakes can effectively interact with CH₃OH molecules and decoration of the Ni and Pd atoms on the surface of graphene induces some changes in the electronic properties of the sheet and its E_g remained unchanged after the adsorption of CH₃OH molecules. Sun M et al. [52] also studied the structure, electrons, and magnetic properties of Sn atoms adsorbed on the original graphene through first-principles calculations. The results show that the adsorption of Sn atoms on the original graphene increases the Fermi level, but the unique Dirac point structure remains intact, and the semiconductor graphene with zero band gap also becomes metallic and obtains magnetic moment. It is well demonstrated that surface decoration on low-dimensional nanomaterials is one of effective way to open energy gap, giving rise to promotion optical absorption. Besides, from experience of our previous work [53], magnetic Ni-decoration on graphene which accomplished by enhancing the plasma energy could effectively improve the optical absorption. Comparatively, it is reasonable to assume that transition magnetic metals (Fe-, Co-, Ni-) decoration is favorable to promote optical absorption performance.

Driven by the demand of broadening applications in terms of optical absorption and photoelectric devices, the effect of surface adsorption of transition magnetic metals (Fe-, Co-, Ni-) on the electronic structure and optical properties of Ti₃C₂ MXene is studied comparatively. Based on first-principles density functional theory, depending on the location and type of adsorption, the electronic structure and optical performance of MXene are regulated and enhanced. Improving the electrical conductivity and light absorption properties of MXene through doping modification (adsorption/embedding) will provide theoretical support and technical means for the regulation of the microscopic physical properties of graphene-like two-dimensional materials.