Size-dependent electronic, optical and photocatalytic properties of Ti3C2O2 quantum dots studied by first-principles calculations

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Abstract

Due to quantum confinement effect, quantum dots (QDs) would show totally different electronic and optical properties from two-dimensional (2D) nanosheets. For the first time, we have systemically investigated the optical, electronic and photocatalytic properties of Ti3C2O2 QDs based on the first-principles calculations. Our results show that the most stable structures of Ti3C2O2 QDs have features of hexagonal unit centered by O atom. The energy gaps of QDs with H passivation vary from 2.76 to 1.14 eV with the increase of lateral size. The lowest unoccupied molecular orbital (LUMO) is mainly contributed by Ti:3d, while the highest occupied molecular orbital (HOMO) is mainly contributed by C:2p and Ti:3d. The corresponding partial charge density at HOMO or LUMO is distributed around Ti and C atoms in the outside surface of QDs. The QDs show large absorption coefficient with the order of 105 cm−1 at visible light range, while the first absorption peaks vary from 2.8 to 1.4 eV (443–886 nm). Most importantly, the Ti3C2O2 QDs present size-dependent photocatalytic selectivity for CO2 reduction and photocatalytic activity for water splitting, caused by the shift of energy level positions of HOMO and LUMO. Our study reveal the potential of Ti3C2O2 QD in electronic and photocatalytic applications and provide new ideals into the design of photocatalysis based on QD materials.

Introduction

In the past ten years, great efforts have been put on the exploring of new two-dimensional (2D) materials after the emergence of graphene [1]. The 2D materials usually show very distinctive properties, such as the ultrahigh carrier mobility of graphene (105 cm2 ⋅V−1 ⋅s−1) [1], indirect to direct band gap transition of transition-metal dichalcogenides (TMDCs) [2] and moderate direct band gap from bulk to monolayer as well as a relatively high hole mobility of 103–104 cm2 ⋅V−1 ⋅s−1 in black phosphorus (BP) [[3], [4], [5], [6]]. Recently, a new kind of 2D materials, layered transition metal carbide/nitride materials (MXenes), has been attracting a lot of research interest, due to its advantages including excellent structural stability, high carrier mobility, good electrical conductivity and tunable band gap [[7], [8], [9], [10]].

Generally, 2D MXenes can be denoted as Mn+1XnTx (n = 1, 2 and 3), in which M is a transition metal (TM), Tx represents surface functional groups (i.e. –F, –O, or –OH) and X is nitrogen and/or carbon. Until now, more than 20 kinds of MXenes have been successfully synthesized and dozens of hundreds of new MXenes have been theoretically simulated [11]. The first member of MXene, Ti3C2Tx nanosheets, was produced by the room temperature exfoliation of Ti3AlC2 in hydrofluoric acid [12]. As a representative MXene material, Ti3C2Tx shows excellent structural stability and good electrical conductivity, which makes it suitable for numerous applications, such as electrodes of lithium/sodium-ion batteries, electrochemical catalysts, supercapacitors, hydrogen storage, molecular sensors, antibacterial and bioimaging probes materials [8,10,13]. Unfortunately, most of MXenes including Ti3C2Tx have exhibited metallic conductivity without an intrinsic band gap, which limits their applications in laser diode (LD), field-effect transistor (FET), and light emitting diode (LED) devices.

In order to find out semiconducting 2D MXenes, two different strategies have been proposed. One is the adjustment of element composition. Previous studies show that Sc2CF2, Sc2CO2, Sc2COH2, Zr2CO2 and Hf2CO2 have an intrinsic band gap [14], and two transition metals MXene Mo2TiC2O2 was found to be a 2D topological semiconductor with a band gap of 0.17 eV [15]. Another is the reduction of dimensionality from 2D nanosheet to one-dimensional (1D) nanoribbon or zero-dimensional (0D) quantum dot (QD). In the past 5 years, a number of cross-sectional studies suggest that MXene QDs can be synthesized and show totally different photoluminescence (PL) and absorption properties from 2D MXenes. Wang et al. developed a generic and mild approach for producing ultrasmall MXene sheets (4 nm in lateral dimensions and 1 nm in thickness on average) and found that the emission quantum yield of the ultrasmall Ti3C2 sheets yields a high value of 8.9% at 405 nm [16]. Later on, a high photoluminescence quantum yield of 18.7% was realized by using nitrogen-doped Ti3C2 quantum dots with the emission wave-length of 450 nm [17]. Furthermore, the functionalized Ti3C2 QDs exhibit highly pH sensitive photoluminescence and can be used as intracellular pH sensors [18]. Zeng et al. developed a new heterostructure based on Ti3C2 QD and Cu2O nanowires (NL), which shows improved photocatalytic reduction of CO2 into methanol than that of Cu2O NWs or Ti3C2 sheets/Cu2O NWs [19]. More importantly, Ti3C2 QDs were found to present strong two-photon white fluorescence, which covers all of the visible region (400–800 nm) and is very stable under high pressure [20]. Compared with 2D MXene, one can see that 0D MXene quantum dos show more unique electronic and optical properties. There is an urgent need to investigate the fundamental electronic and optical properties of 0D MXene QDs so as to reveal the microscopic mechanism of PL spectra as observed in the experiment. However, this kind of study is scarce.

Herein, we study the electronic and optical properties of Ti3C2O2 quantum dot using first-principles calculations. We have constructed seven Ti3C2O2 QD models with different lateral size. The energy gap of Ti3C2O2 QD decreases progressively with the increase of lateral size. The highest occupied molecular orbital (HOMO) is mainly contributed by C:2p and Ti:3d, while the lowest unoccupied molecular orbital (LUMO) is mainly contributed by Ti:3d. The calculated absorption coefficient of Ti3C2O2 QDs increases with the increase of lateral size, and the first absorption peaks vary from 2.8 to 1.4 eV (443–886 nm), which is consistent with the experiment observation. Importantly, the QDs show size-dependent photocatalytic activity for CO2 reduction and water splitting.

Section snippets

Computational methods

Our first-principles calculations were performed using density functional theory (DFT) framework, where the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional for electron exchange and correlation potentials were adopted, as implemented in the Vienna ab initio simulation package (VASP) [[21], [22], [23]]. The electron-ion interaction was described using the projector augmented wave (PAW) approach, and the energy cutoff of plane waves was set to 500 eV.

Results and discussion

Pristine 2D MXene Ti3C2O2 consists of seven atomic layers with the hexagonal lattice constant of a = b = 3.04 Å, where the atomic arrangement is O–Ti–C–Ti–C–Ti–O from the top to down surface. On the basis of structure relaxation, we find that only QDs with O atom as the center of hexagonal unit are energetically stable. We construct seven Ti3C2O2 QDs denoted as QD-n (n = 1–7) with the maximum diameter of 1.2 nm, based on the number of O atoms in top surface, as shown in Fig. 1. Considering the

Conclusions

In conclusion, we have constructed the most stable structures of Ti3C2O2 QDs with various lateral sizes and have systemically studied the electronic, optical and photocatalytic properties of these QDs based on the first-principles calculations. With the increase of lateral size, the energy gaps of QDs with H passivation vary from 2.76 to 1.14 eV. The HOMO of QDs is mainly contributed by C:2p and Ti:3d orbitals, while the LUMO is mainly determined by Ti:3d orbital. The partial charge density at

CRediT authorship contribution statement

Yi-min Ding: Conceptualization, Methodology. Xiaomin Nie: Software. Huilong Dong: Software. Nopporn Rujisamphan: Formal analysis. Youyong Li: Supervision, Project administration.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grants No. 51761145013, 21703145, 21673149), National Key R&D Program of China (Grants No. 2017YFB0701600 and 2017YFA0204800), the 111 Project, Joint International Research Laboratory of Carbon-Based Functional Materials and Devices, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and Collaborative Innovation Center of Suzhou Nano Science and Technology.

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