Quantum Dots Compete at the Acme of MXene Family for the Optimal Catalysis

Abstract

It is well known that two-dimensional (2D) MXene-derived quantum dots (MQDs) inherit the excellent physicochemical properties of the parental MXenes, as a Chinese proverb says, "Indigo blue is extracted from the indigo plant, but is bluer than the plant it comes from." Therefore, 0D QDs harvest larger surface-to-volume ratio, outstanding optical properties, and vigorous quantum confinement effect. Currently, MQDs trigger enormous research enthusiasm as an emerging star of functional materials applied to physics, chemistry, biology, energy conversion, and storage. Since the surface properties of small-sized MQDs include the type of surface functional groups, the functionalized surface directly determines their performance. As the Nobel Laureate Wolfgang Pauli says, "God made the bulk, but the surface was invented by the devil," and it is just on the basis of the abundant surface functional groups, there is lots of space to be thereof excavated from MQDs. We are witnessing such excellence and even more promising to be expected. Nowadays, MQDs have been widely applied to catalysis, whereas the related reviews are rarely reported. Herein, we provide a state-of-the-art overview of MQDs in catalysis over the past five years, ranging from the origin and development of MQDs, synthetic routes of MQDs, and functionalized MQDs to advanced characterization techniques. To explore the diversity of catalytic application and perspectives of MQDs, our review will stimulate more efforts toward the synthesis of optimal MQDs and thereof designing high-performance MQDs-based catalysts.

Keywords: Catalysis; MXene; Quantum dots; Structure; Surface groups.

Fig. 1

a Jöns Jacob Berzelius (1779–1848) [1]. Copyright @1948, American Chemical Society. b Schematic illustration of the energy band structure of materials with different sizes, and typical images of MAX and MXene from 3D multilayer to 2D nanosheets to 1D nanowires to 0D nanodots [6]. Copyright @2021, The American Association for the Advancement of Science. The morphology was obtained by field emission scanning electron microscopy [–10], Copyright @2011, WILEY–VCH, @2019, WILEY–VCH, @2018, Elsevier and @2018, American Chemical Society

Fig. 2

The classification of quantum dots

Fig. 3

a State of the art and prospect of 0D MQDs. b Number of journal publications related to publication time and applications science 2017 (Source: Web of Science)

Fig. 4

The timeline showing the development of MQDs in catalysis in the past few years. Reproduced with permission from Refs. [–79]. Copyright @2019, Elsevier, @2019, WILEY–VCH, @2020, Elsevier, @2020, American Chemcial Society, @2020, WILEY–VCH, @2020, American Chemical Society, @2020, WILEY–VCH, @2021, American Chemical Society, @2021, Elsevier, @2021, Elsevier, @2022, The Royal Society of Chemistry, @2022, Elsevier, @2022. MDPI and the author, @2022, Zhengzhou University and Wiley

Fig. 5

Development of synthesis methods and synthesis of MQDs. a Number of synthesis methods publications on MQDs. (Source: Web of science, 2017 to 2022s). b Scheme of hydrothermal synthesis method [107]. Copyright @2021, American Chemical Society. c-h Morphology of MQDs at different reaction temperature of 100, 120, and 150 °C. The data was obtained by transmission electron microscopy; i XRD characterization of MQDs [58]. Copyright @2017, WILEY–VCH. j Scheme of synthesis MQDs at different solvents of DMSO, DMF, and ethanol; k XRD characterization of MQDs [96]. Copyright @2018, WILEY–VCH

Fig. 6

Schematic illustration for the synthesis of MQDs by using different methods. a Probe ultrasound [108]. Copyright @2020, Wiley–VCH. b Bath sonication [109]. Copyright @2020, American Chemical Society. c A combination of probe sonication and bath sonication [110]. Copyright @2017, The Royal Society of Chemistry. d Mechanical stirring method [111]. Copyright @2021, Wiley–VCH

Fig. 7

Schematics, structural and optical behavior characterizations of MQDs modified by single/dual heteroatoms. a Schematic illustration of the synthesis of N, P-Ti₃C₂ MQDs; b Charge density difference of N, P functionalized Ti₃C₂ MQDs; c Fluorescence emission spectra of N, P-Ti₃C₂ MQDs; d Photoluminescence decay spectra of the N-Ti₃C₂ MQDs, P-Ti₃C₂ MQDs, N, P-Ti₃C₂ MQDs [144]. Copyright @2019, The Royal Society of Chemistry. e Schematic illustration of the synthesis of S-Ti₃C₂ MQDs, N-Ti₃C₂ MQDs, S, N-Ti₃C₂ MQDs; f–h UV–Vis adsorption spectra of S-Ti₃C₂ MQDs, N-Ti₃C₂ MQDs, S, N-Ti₃C₂ MQDs [133]. Copyright @2019, Elsevier. i Schematic illustration of the synthesis of N-Ti₂C MQDs; j Antioxidants performance test at KMnO₄ solutions; k Mechanism of antioxidants [145]. Copyright @2021, American Chemical Society

Fig. 8

Schematic and morphological and structural characterizations of 0D MQDs/0D nanocomposite. a Schematic illustration of the synthesis of CsPbBr₃–Ti₃C₂T_x MQD/QD; b XRD patterns of CsPbBr₃ QDs, Ti₃C₂T_x MQD, CsPbBr₃–Ti₃C₂T_x MQD/QD; c TEM image of CsPbBr₃–Ti₃C₂T_x MQD/QD; d HRTEM image of CsPbBr₃–Ti₃C₂T_x MQD/QD [156]. Copyright @2020, American Chemical Society. e Schematic of the formation of Ni@Ti₃C₂ MQDs; f-g TEM image of Ni@Ti₃C₂ MQDs. insets of A and B represent HRTEM image of Ni and MQDs, respectively; h EDS of Ni@ Ti₃C₂ MQDs [77]. Copyright @2022, Elsevier

Fig. 9

Schematic and morphological and structure characterizations of 0D MQDs/1D nanomaterials heterostructure. a Synthesis process of Ti₃C₂ QDs/Cu₂O NWs/Cu heterostructure; b TEM image of the Ti₃C₂ QDs/Cu₂O NWs heterojunction; c HRTEM image of the interface in Ti₃C₂ QDs and Cu₂O; d-g EDX elemental mapping of Ti₃C₂ QDs/Cu₂O NWs [67]. Copyright @2019, WILEY–VCH. h Schematic illustration of WO₃/TQDs/In₂S₃ heterostructure; i TEM image of WO₃/TQDs; j HRTEM image of WO₃/TQDs; k TEM image of WO₃/TQDs/In₂S₃; l HRTEM image of WO₃/TQDs/In₂S₃ [74]. Copyright @2021, Elsevier. m Schematic for the preparation of Au NRs/Ti₃C₂ MQDs/Ti₃C₂ nanosheets; n TEM image of Au NRs/TDTS. Inset illustration is HRTEM image of Ti₃C₂ MQDs [157]. Copyright @2021, Elsevier

Fig. 10

Schematic and morphological and structure characterizations of 0D MQDs/2D nanosheets heterostructure. a Schematic diagram of Ti₃C₂ MQDs/Ni-MOF; b SEM images of Ti₃C₂ MQDs/Ni-MOF; c TEM images of Ti₃C₂ MQDs/Ni-MOF. The inset illustration is the size distribution of Ti₃C₂ MQDs. Reproduced with permission [69]. Copyright @2020, American Chemical Society. d Synthesis process schematic of Ti₃C₂ QDC/N-C nanocomposites and e TEM image of Ti₃C₂ QDC/N-C, inset illustration is SAED pattern; f HRTEM image of Ti₃C₂ QDC/N-C [137]. Copyright @2021, Wiley–VCH. g Schematic preparation of Ti₃C₂ MQDs/TiO₂/C₃N₄ hierarchical structure; h HRTEM image of T-CN-TC heterostructure [159]. Copyright @2020, Elsevier

Fig. 11

Schematic and morphological and structure characterizations of 0D/3D heterostructure. a The prepared process diagram of C₃N₄/r-Ti₃C₂ QDs; b TEM image of C₃N₄/r-Ti₃C₂ QDs; c HRTEM image of C₃N₄/r-Ti₃C₂ QDs; d XRD pattern of C₃N₄, C₃N₄/r-Ti₃C₂ QDs [76]. Copyright @2022, The Royal Society of Chemistry. e Schematic preparation of Ti₃C₂ MQDs/3D Inverse Opal g‑C₃N₄ heterojunction; f SEM image of TC/CN-20 after adding 20 mL of MQDs solution and 20 mL water; g HRTEM image of TC/CN-20 [73]. Copyright @2020, American Chemical Society. h Schematic illustration of Ti₃C₂-QDs/ZnIn₂S₄/Ti(IV) heterostructure; i-k TEM image of Ti₃C₂-QDs/ZnIn₂S₄/Ti(IV) at different magnifications; l-p Elemental mappings of Ti₃C₂-QDs/ZnIn₂S₄/Ti(IV) [78]. Copyright @2022, MDPI and the authors

Fig. 12

Schematic and morphological and structure characterizations of MXene-derived inorganic QDs. a Schematic of synthesis for TiO₂/C-QDs; b TEM image of TiO₂/C-QDs, the inset illustration is size distribution of TiO₂/C-QDs; c HRTEM image of TiO₂/C-QDs; d UV–Vis adsorption spectra of TiO₂/C-QDs [88]. Copyright @2020, The Royal Society of Chemistry. e Illustration of preparation of oxygen-vacancy-rich Ti_nO_2n−1 QDs @PCN; f-g TEM image of OV–T_n QDs @PCN at different magnification; h-i HRTEM image of Ti_nO_2n−1 QDs @PCN with different number of oxygen vacancies [90]. Copyright @2021, Wiley–VCH. j Schematic illustration of the preparation of graphene quantum dots (GQDs); k TEM image of GQDs, and the inset illustration is size distribution of GQDs [195]. Copyright @2020, Elsevier

Fig. 13

Characterization techniques of MQDs

Fig. 14

Morphology and structure characterization of MQDs. a TEM image of Ti₃C₂ MQDs; b Size distribution of Ti₃C₂ MQDs; c HRTEM image of Ti₃C₂ MQDs, the inset illustration is corresponded Fourier transform; d AFM image of Ti₃C₂ MQDs; e The height distribution based on AFM; f Height profiles of Ti₃C₂ MQDs along d image [110]. Copyright @2017, The Royal Society of Chemistry. g XRD patterns of Ti₃AlC₂, Ti₃C₂ MXene, and Ti₃C₂ MQDs [204]. Copyright @2019, The Royal Society of Chemistry. h XRD patterns of bulk Ti₃C₂ MXene, Ti₃C₂ nanosheet, and Ti₃C₂ MQDs [59]. Copyright @2019, WILEY–VCH. i XRD patterns of Nb₂AlC and Nb₂C MQDs [205]. Copyright @2020, Elsevier

Fig. 15

Composition and optical spectral characterization of MQDs. a XPS survey spectra of Ti₃C₂ MQDs; b High-resolution spectra of Ti 2p [204]. Copyright @2019, The Royal Society of Chemistry. c FTIR spectra of Ti₃C₂ MQDs [215]. Copyright @2018, The Royal Society of Chemistry. d Raman spectra of Ti₃C₂(OH)₂ MQDs [70]. Copyright @2020, WILEY–VCH. e UV–Vis adsorption, PL, and PLE spectra of Ti₃C₂ MQDs; f PL spectra at different excitation wavenumbers [58]. Copyright @2017, WILEY–VCH

Fig. 16

a-b Simulation diagram of energy levels and charge transfer processes in the MQDs and N-MQDs; c-d DOS calculation of MQDs and N-MQDs; e Work function of MQDs and N-MQDs [147]. Copyright @2018, The Royal Society of Chemistry. f-g DFT calculation of total and projected DOS of Nb₂CO₂ QDs and S, N-doped Nb₂CO₂ QDs [86]. Copyright @2020, The Royal Society of Chemistry

Fig. 17

Electrocatalytic performance of MQDs. a Number of journal publications on different catalytic aspects (Source: Web of Science). b Reaction mechanism of electrocatalytic N₂ reduction and free energy calculation on the Ti edge of Ti₃C₂, Ti₃C₂F, Ti₃C₂OH MXene from the adsorption of N₂ to the reduction of NH₃; c The average NH₃ yield and faradaic efficiency of Ti₃C₂OH MQDs at different applied voltages [70]. Copyright @2020, WILEY–VCH. d Charge density difference of Ti₃C₂ MQDs/Cu; e The NH₃ yield of Cu, MQDs, MQDs/Cu at applied voltage of -0.5 V, inset diagram is chronoamperometry test [79]. Copyright @2022, Zhengzhou University and Wiley

Fig. 18

a Synthesis of Ti₂CT_x MQDs/Cu₂O/Cu foam nanocomposite (top), the evolution process of functional groups and Soft X-ray emission spectrum (SXES) image; b The LSV image [164]. Copyright @2022, Zhengzhou University and Wiley. c TEM image of MoS₂QDs @Ti₃C₂T_xQDs@MWCNTs and d ORR performance of sample [66]. Copyright @2019, Elsevier. e AC-STEM image of Ti₃C₂ nanosheet; f AC-STEM of Ti₃C₂ MQDs; g Initial deep discharge–charge curves of the three samples at 200 mA g⁻¹; h Cycling stability and terminal discharge–charge voltages of Ti₃C₂ QDC/N–C electrode at 200 mA g⁻¹ [137]. Copyright @2021, Wiley–VCH

Fig. 19

Photocatalytic water splitting performances of MQDs-based heterostructure. a Schematic react mechanism of g-C₃N₄@Ti₃C₂ QD; b Steady photoluminescence spectra of g-C₃N_4, MQDs@g-C₃N₄; c Time-resolved fluorescence decay spectra under the 325 nm excitation wavelength; d The transient photocurrent response; e Photocatalytic HER rate plot of catalyst [160]. Copyright @2019, American Chemical Society. f Schematic photocatalytic mechanism of BV@ZIS/TC QDs; g UV–visible diffuse reflectance spectra of BV@ZIS/TC QDs and control sample; h Photocatalytic gas production of BV@ZIS/TC QDs and control sample [68]. Copyright @2020, Elsevier

Fig. 20

Photocatalytic CO₂ reduction performances of MQDs-based photocatalysts. a TEM image of Ti₃C₂ MQDs/Cu₂O/Cu; b Photocatalytic reaction mechanism of Ti₃C₂ MQDs/Cu₂O/Cu; c UV–Vis diffuse reflectance spectra (DRS) of samples; d Nyquist plots of samples; e Methanol yield of samples [67]. Copyright @2019, WILEY–VCH. f TEM of TiO₂/C₃N₄ with core–shell; g S-scheme heterojunction before and after contact, and light irradiation; h Schematic diagram of structure of TiO₂/C₃N₄/Ti₃C₂ MQDs [159]. Copyright @2020, Elsevier

Fig. 21

Photocatalytic performances of NH₃, H₂O₂ production. a SEM image of Ti₃C₂ MQDs/Ni-MOF; b Energy band structure of Ti₃C₂ MQDs/Ni-MOF; c UV–Vis diffuse reflectance spectra of Ti₃C₂ MQDs, Ni-MOF, Ti₃C₂ MQDs/Ni-MOF nanocomposites with different loads of MQDs; d The NH₃ yield of Ti₃C₂ MQDs/Ni-MOF [69]. Copyright @2020, American Chemical Society. e TEM image of C₃N₄/r-Ti₃C₂ MQDs Schematic diagram of photocatalytic N₂ fixation; f Photocurrent test in Ar and N₂ of samples; g UV–Vis DRS spectra of C₃N₄, C₃N₄/r-Ti₃C₂ MQDs; h Photocatalyst NH₃ production rate and side reaction of H₂ production [76]. Copyright @2022, The Royal Society of Chemistry. i Transfer of photogenerated electrons and holes near the carbon vacancies of C₃N₄ and C₃N₄/ r-Ti₃C₂ MQDs; j The energy band structure of C₃N₄/ r-Ti₃C₂ MQDs; k The photocatalytic reaction mechanism diagram of C₃N₄/r-Ti₃C₂ MQDs [73]. Copyright @2021, American Chemical Society

Fig. 22

Photocatalytic pollutant degradation performances of MQDs-based heterojunction. a UV–Vis diffuse reflectance spectra (DRS) of samples; b Calculation of band gap of Bi₂O₃, BiOTIC-75; c Photocatalytic reaction mechanism illustration of BiOTIC-75; d Photocatalytic degradation of TC by Bi₂O₃, Ti₃C₂ MQDs/Bi₂O₃ with different content of MQDs [75]. Copyright @2021, Elsevier. e Energy band structure of MQDs/SiC, SiC, MQDs [71]. Copyright @2020, American Chemical Society

Fig. 23

The photoelectrocatalytic performances of MQDs as co-catalyst. a UV–Vis adsorption spectra of Ti₃C₂ MQDs, Co-MQDs with different rates of Co/Ti; b Photoelectric conversion efficiency of Ti₃C₂ MQDs, Co-MQDs with different rates of Co/Ti; c Time-resolved photoluminescence (TRPL) spectra; d Photoelectrocatalytic water splitting mechanism under light irradiation; e Photocurrent test of Ti₃C₂ MQDs, Co-MQDs with different rates of Co/Ti [72]. Copyright @2020, WILEY–VCH. f Schematic illustration of the charge transfer process for NiFeOOH/MoOx/MQD/BiVO₄ photoanodes; g Photoelectrochemical water splitting device [161]. Copyright @2022, Wiley–VCH