2D Titanium Carbide (MXene): Band Gap Modulation
Authors: Swapnalin Jhilmil , Banerjee Prasun 
Published: December 2024

In the past few years, due to the growing demand for energy worldwide, research interest has been triggered to boost the photovoltaic performance of solar cells. Solar cells, the green energy processing media source, require an excellent absorber material to collect the incident photons and generate electrical energy. In this aspect, perovskite materials have been widely used as the absorber layer forming perovskite solar cells (PSC). Due to its tunable band gap, high charge transportation, reduced grain boundary and surface recombination properties, perovskite materials are extensively used in solar cells. Various research has been conducted on the upgradation of PSCs, mainly focusing on enhancing power conversion efficiency (PCE), stability, toxicity of Pb-based PSCs, less production cost, easy fabrication, and improved operation rate by reducing recombination of charge carriers. Several studies have been done to come up with appropriate electron/hole transport layers (ETL/HTL) arranged in a PSC.
An advanced 2D hierarchical structure, MXene refer to transition metals carbides (C)/nitrides (N) or carbonitrides (CN). It has a general formula of Mn+1XnTx, where n = 1–3, M denotes the transition metals (Ti, Mo, Zr, Hf, Nb), X indicates C or N, and Tx indicates functional groups (–O, –OH, –F, –Cl). Owing to their unique optoelectronics properties, MXenes has proved itself to be a promising candidate for photovoltaic technology applications. MXenes are transparent, electrically conducting, have good charge mobility, and have an optimizable work function. The rich surface termination groups present on the MXene surface are uniform and offer great possibilities for tuning electrical properties. Also, the synthesis process of MXene makes it naturally functionalized, which alters the electrostatic potential, leading to a work function shift in thin films. For the first time, MXene was used in a perovskite solar cell in 2018, and since then, it has been explored in different ways.
But MXene, being metallic in nature, offers nearly zero bandgap. So, various hybrid preparations or changes in the synthesis route should be opted to tune the bandgap of MXene. Thus, MXene-based composite, which offers a wide range of bandgap tunability, is extensively used as the light absorber material in various solar cells. In many research works, it is evident that intentionally introducing the required bandgap in metallic MXene makes it more useful for photovoltaic applications. Bandgap engineering is a very significant method to come up with different selective semiconductors for fabricating advanced photovoltaic devices. Solar intensity mainly falls in the range of infrared and visible part of the spectrum. Accordingly, the ideal bandgap of the active material should be roughly 1.2-1.6 eV. Therefore, the recent research aims to prepare 2D MXene semiconductors that have good absorption in this range with suitable bandgap.
In this work, we synthesized a composite of MXene/multiferroic, taking bismuth ferrite (Ti3C2Tx@BiFeO3) nanocomposites via facile hydrothermal synthesis. To prepare the nanocomposites, we used two reducing agents: hydrazine hydrate (N2H4.H2O) and Ammonia (NH3.H2O). Through the same synthesis method but with different reducing agents, the resultant nanocomposite was noted to be different with two different multiferroics. When the MXene was prepared using N2H4.H2O, it resulted in (Ti3C2Tx@BiFeO3) and with NH3.H2O outcome was (Ti3C2Tx@Bi25FeO39) as shown in Figure 1. X-ray diffraction peak analysis showed proper composite formation. The characteristic peaks of layered MXene and bismuth ferrite were evident indicating no impurity formation and suitability of the synthesis route. Thorough bandgap studies of both MXene@multiferroic nanocomposite were done using reflectance spectra (Figure 2). In the range of 3000-8000 Å, reflectance spectra were seen to be continuous for (Ti3C2Tx@BiFeO3), but a reflectance edge was observed for (Ti3C2Tx@Bi25FeO39), indicating it as a better light absorber. Using Tauc’s plot and the kubelka-Munk equation, i.e., Eg = hν − [F (R (∞)) hν]2, the direct bandgap of (Ti3C2Tx@Bi25FeO39) was obtained as 2.2 eV. The analyzed reports point towards a new possibility of researching more on preparing 2D semiconducting MXene composite with a wide bandgap using a suitable reducing agent and weight percentage, which is crucial for solar cell application. Creating a modified MXene composite, which is non-toxic, cheap, provides favourable bandgap and can be commercially produced, is highly challenging. This work addresses such challenges and gives a different direction to the current work on the band gap modulation of 2D titanium carbide (MXene).

REFERENCES:

[1] Koneru, Bhargavi, Jhilmil Swapnalin, Srinivasan Natarajan, Adolfo Franco Jr, and Prasun Banerjee. "Intercalation of nanoscale multiferroic spacers between the two-dimensional interlayers of MXene." ACS omega 7, no. 23 (2022): 20369-20375.

Figure 1