In this thesis, the electronic properties of two-dimensional magnetic materials, transition metal trichlorides and transition metal phosphorus trichalcogenides, are studied by means of various spectroscopic techniques including photoelectron spectroscopy (PES), electron energy-loss spectroscopy (EELS) and optical spectroscopy. The experiments on transition metal trichlorides mainly focus on manipulating the electronic structure of α−RuCl3 — a Kitaev spin liquid candidate material that, however, hosts an antiferromagnetic ground state at temperatures below 7 K. Such manipulation attempts include transition metal substitution by Cr, Ar+ sputtering of exfoliated flakes and the creation of an interface of α−RuCl3 with the organic semiconductor manganese (II) phtalocyanine (MnPc). To study the influence of transition metal substitution by Cr, the parent compounds α−RuCl3 and CrCl3, and the mixed compound Cr0.5Ru0.5Cl3 were studied by PES and EELS. The mixed compound preserves the +III oxidation state of Cr and Ru. The valence band resembles a superposition of the parent compounds and EELS reveals the appearance of a new optical absorption channel assigned to a Cr-Ru charge transfer. Ar+ sputtering decreases the chlorine content of exfoliated α−RuCl3 flakes. However, the properties of the sputtered film, namely the rate of chlorine loss and the work function, depend heavily on the initial flake thickness. The work function spans a remarkable range from Φ = 4.6 eV to 6.1 eV. The interface of α−RuCl3 with MnPc demonstrates the potential of α−RuCl3 as a strong electron acceptor. The work function and electron affinity of α−RuCl3 are characterized and the charge transfer from MnPc to α−RuCl3 is experimentally verified. In the second part of the thesis, two transition metal phosphorus trichalcogenide compounds are studied: FePS3 and NiPS3. Both are antiferromagnetic materials with FePS3 being of Ising-type and NiPS3 of anisotropic Heisenberg-type. Their electronic structure is spectroscopically investigated and the results are used as input for advanced density functional theory calculations (DFT+U) characterizing FePS3 as a Mott insulator and NiPS3 as a charge-transfer insulator. In the magnetically ordered state, magnetism and electronic properties are intertwined with the giant linear dichroism (LD) of FePS3 measured in optical transmission being the most impressive example. A microscopic understanding of the LD is provided with the DFT+U results giving confidence to the described model. For NiPS3, the origin of an extremely sharp magnetic exciton is studied bearing some analogy to the famous Zhang-Rice singlet state initially proposed for cuprates.:Contents iii List of Figures v Acronyms ix 1. Introduction 1 2. Experimental Techniques 3 2.1. Photoelectron Spectroscopy (PES) . . . . . . . . . . . . . . . . . . . 3 2.2. Three-step-model of Photoemission . . . . . . . . . . . . . . . . . . . 4 2.2.1. Photoabsorption . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2.2. Propagation to the Surface . . . . . . . . . . . . . . . . . . . 6 2.2.3. Escape into the Vacuum . . . . . . . . . . . . . . . . . . . . . 7 2.3. Spectral Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.4. Energy-filtered Photoemission Electron Microscopy (PEEM) . . . . . 9 2.5. Background Signal of XPS and UPS Measurements . . . . . . . . . . 9 2.6. Electron Energy-loss Spectroscopy (EELS) . . . . . . . . . . . . . . . 10 2.6.1. EELS Cross Section . . . . . . . . . . . . . . . . . . . . . . . 12 2.7. The Dielectric Function . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.7.1. The Drude-Lorentz-model . . . . . . . . . . . . . . . . . . . . 16 2.7.2. Related functions . . . . . . . . . . . . . . . . . . . . . . . . . 17 2.7.3. Kramers-Kronig relations . . . . . . . . . . . . . . . . . . . . 19 2.8. Optical Microscopy and Spectroscopy . . . . . . . . . . . . . . . . . . 20 2.8.1. Optical Microscopy . . . . . . . . . . . . . . . . . . . . . . . . 20 2.8.2. Optical Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . 21 2.8.3. Optical Contrast of Thin Films . . . . . . . . . . . . . . . . . 22 2.9. Core Level Spectroscopy of Solids . . . . . . . . . . . . . . . . . . . . 25 2.9.1. Spin-orbit Splitting and Notation . . . . . . . . . . . . . . . . 25 2.9.2. Core Level Spectroscopies: XPS and EELS/XAS . . . . . . . 26 2.9.3. Multiplet and Charge Transfer Effects . . . . . . . . . . . . . 26 2.10. Atomic Force Microscopy (AFM) . . . . . . . . . . . . . . . . . . . . 29 2.11. Details on Spectrometers . . . . . . . . . . . . . . . . . . . . . . . . . 31 2.11.1. nanoARPES . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 2.11.2. nanoESCA . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 2.11.3. Transmission EELS . . . . . . . . . . . . . . . . . . . . . . . . 37 3. Manipulating the Electronic Structure of α−RuCl3 41 3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 3.2. Tuning the Electronic Structure of the Trichlorine Honeycomb Lattice by Transition Metal Substitution: α−RuCl3, Cr0.5Ru0.5Cl3, CrCl3 . 47 3.2.1. Electron diffraction . . . . . . . . . . . . . . . . . . . . . . . . 48 3.2.2. Core Level Spectroscopy . . . . . . . . . . . . . . . . . . . . . 49 3.2.3. UPS Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 3.2.4. EELS Results in the Low Energy Region . . . . . . . . . . . . 52 3.2.5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 3.3. Work Function Engineering of Atomically Thin α−RuCl3 by Arsputtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 3.3.1. Characterization . . . . . . . . . . . . . . . . . . . . . . . . . 56 3.3.2. Work Function . . . . . . . . . . . . . . . . . . . . . . . . . . 57 3.3.3. XPS Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 3.3.4. UPS Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 3.3.5. Discussion and Summary . . . . . . . . . . . . . . . . . . . . 64 3.4. Charge Transfer at the α−RuCl3/MnPc Interface . . . . . . . . . . . 66 3.4.1. Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 3.4.2. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 3.4.3. Discussion and Summary . . . . . . . . . . . . . . . . . . . . 73 4. Spectroscopic Investigation of NiPS3 and FePS3 75 4.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 4.1.1. Crystal Structure and Magnetic Properties . . . . . . . . . . 76 4.1.2. Electronic Structure . . . . . . . . . . . . . . . . . . . . . . . 79 4.2. FePS3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 4.2.1. UPS Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 4.2.2. XPS Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 4.2.3. Electron Diffraction . . . . . . . . . . . . . . . . . . . . . . . 86 4.2.4. EELS Results in the Energy Region between 4 eV and 80 eV . 87 4.2.5. EELS Results in the Low Energy Region . . . . . . . . . . . . 88 4.2.6. Optical Spectroscopy and Linear Dichroism (LD) . . . . . . . 89 4.2.7. Discussion and Conclusion . . . . . . . . . . . . . . . . . . . . 92 4.3. NiPS3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 4.3.1. UPS Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 4.3.2. Core Level Spectroscopy . . . . . . . . . . . . . . . . . . . . . 98 4.3.3. Electron Diffraction . . . . . . . . . . . . . . . . . . . . . . . 101 4.3.4. EELS Results in the Energy Region between 4 eV and 70 eV . 102 4.3.5. EELS in the Low Energy Region . . . . . . . . . . . . . . . . 103 4.3.6. Multiplet theory and RIXS . . . . . . . . . . . . . . . . . . . 105 4.3.7. Discussion and Conclusion . . . . . . . . . . . . . . . . . . . . 107 5. Summary and Outlook 109 A. Appendix 111 A.1. The Pseudo-Voigt Profile . . . . . . . . . . . . . . . . . . . . . . . . . 111 A.2. Calculation of Reciprocal Lattice Vectors . . . . . . . . . . . . . . . . 111 Bibliography 113 / In dieser Arbeit wurden die elektronischen Eigenschaften von zweidimensionalen magnetischen Materialien, Übergangsmetall-Trichloriden und Übergangsmetall-Phosphor-Trichalkogeniden, untersucht. Dabei kamen verschiedene Spektroskopie-Techniken zum Einsatz: Photoelektronenspektroskopie (PES), Elektronen-Energieverlust-Spektroskopie (EELS) und optische Spektroskopie. Die Experimente an Übergangsmetall-Trichloriden zielen hauptsächlich auf die Manipulation der elektronischen Eigenschaften von α−RuCl3 ab. α−RuCl3 ist ein Kandidat für eine Kitaev Quantenspinflüssigkeit, das jedoch bei tiefen Temperaturen unter 7K einen antiferromagnetischen Grundzustand besitzt. Die Manipulationsversuche beinhalten die Substitution des Übergangsmetalls durch Cr, Ar+ sputtern von exfolierten Kristallflocken und die Erzeugung einer Grenzfläche zwischen α−RuCl3 und dem organischen Halbleiter Mangan (II) Phthalocyanin (MnPc). Um den Einfluss der Substitution des Übergangsmetalls durch Cr zu untersuchen, wurden die Ausgangsverbindungen α−RuCl3 und CrCl3, und die gemischte Verbindung Cr0.5Ru0.5Cl3 mittels PES und EELS untersucht. In der gemischten Verbindung liegen Cr und Ru weiterhin mit Oxidationszahl +III vor. Das Valenzband lässt sich als Überlagung der Ausgangsverbindungen darstellen und EELS Daten zeigen einen neuen optischen Absorptionskanal durch Ladungstransfer von Cr zu Ru. Ar+ sputtern reduziert den Chloranteil von exfolierten α−RuCl3-Flocken. Die Eigenschaften der gesputterten Filme, insbesondere Austrittsarbeit und Chlorverlust, hängen jedoch stark von der ursprünglichen Dicke der exfolierten Flocke ab. Die Austrittsarbeit zeigt eine beachtliche Spanne von Φ = 4.6 eV bis 6.1 eV. Die Grenzfläche von α−RuCl3 mit MnPc demonstriert das Potential von α−RuCl3 als starken Elektronenakzeptor. Die Austrittsarbeit und die Elektronenaffinität von α−RuCl3 wurden charakterisiert und der Ladungstransfer von MnPc zu α−RuCl3 wurde experimentell bestätigt. Im zweiten Teil der Arbeit werden zwei Vertreter der Übergangsmetall-Phosphor-Trichalkogeniden untersucht: FePS3 und NiPS3. Beide Materialien sind antiferromagnetisch, wobei FePS3 dem Ising-Typ entspricht und NiPS3 einem anisotropen Heisenberg-Modell. Die elektronische Struktur der beiden Materialien wurde durch spektroskopische Methoden untersucht und als Grundlage für DFT+U Rechnungen verwendet, wodurch FePS3 als Mott-Isolator und NiPS3 als Ladungstransfer-Isolator charakterisiert wurden. Im magnetisch geordneten Zustand sind elektronische und magnetische Eigenschaften verflochten, das sich am eindrucksvollsten im großen linearen dichroismus (LD) Effekt von FePS3 gemessen in optischer Transmission zeigt. Ein mikroskopisches Modell zur Erklärung des LD wird beschrieben und durch Ergebnisse aus DFT+U Rechnungen unterlegt. Bei NiPS3 wurde die Ursache für ein energetisch extrem scharfes, magnetisches Exziton untersucht, das Analogien zum bekannten Zhang-Rice-Singulett aufweist, welches ursprünglich für Kuprate vorgeschlagen wurde.:Contents iii List of Figures v Acronyms ix 1. Introduction 1 2. Experimental Techniques 3 2.1. Photoelectron Spectroscopy (PES) . . . . . . . . . . . . . . . . . . . 3 2.2. Three-step-model of Photoemission . . . . . . . . . . . . . . . . . . . 4 2.2.1. Photoabsorption . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2.2. Propagation to the Surface . . . . . . . . . . . . . . . . . . . 6 2.2.3. Escape into the Vacuum . . . . . . . . . . . . . . . . . . . . . 7 2.3. Spectral Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.4. Energy-filtered Photoemission Electron Microscopy (PEEM) . . . . . 9 2.5. Background Signal of XPS and UPS Measurements . . . . . . . . . . 9 2.6. Electron Energy-loss Spectroscopy (EELS) . . . . . . . . . . . . . . . 10 2.6.1. EELS Cross Section . . . . . . . . . . . . . . . . . . . . . . . 12 2.7. The Dielectric Function . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.7.1. The Drude-Lorentz-model . . . . . . . . . . . . . . . . . . . . 16 2.7.2. Related functions . . . . . . . . . . . . . . . . . . . . . . . . . 17 2.7.3. Kramers-Kronig relations . . . . . . . . . . . . . . . . . . . . 19 2.8. Optical Microscopy and Spectroscopy . . . . . . . . . . . . . . . . . . 20 2.8.1. Optical Microscopy . . . . . . . . . . . . . . . . . . . . . . . . 20 2.8.2. Optical Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . 21 2.8.3. Optical Contrast of Thin Films . . . . . . . . . . . . . . . . . 22 2.9. Core Level Spectroscopy of Solids . . . . . . . . . . . . . . . . . . . . 25 2.9.1. Spin-orbit Splitting and Notation . . . . . . . . . . . . . . . . 25 2.9.2. Core Level Spectroscopies: XPS and EELS/XAS . . . . . . . 26 2.9.3. Multiplet and Charge Transfer Effects . . . . . . . . . . . . . 26 2.10. Atomic Force Microscopy (AFM) . . . . . . . . . . . . . . . . . . . . 29 2.11. Details on Spectrometers . . . . . . . . . . . . . . . . . . . . . . . . . 31 2.11.1. nanoARPES . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 2.11.2. nanoESCA . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 2.11.3. Transmission EELS . . . . . . . . . . . . . . . . . . . . . . . . 37 3. Manipulating the Electronic Structure of α−RuCl3 41 3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 3.2. Tuning the Electronic Structure of the Trichlorine Honeycomb Lattice by Transition Metal Substitution: α−RuCl3, Cr0.5Ru0.5Cl3, CrCl3 . 47 3.2.1. Electron diffraction . . . . . . . . . . . . . . . . . . . . . . . . 48 3.2.2. Core Level Spectroscopy . . . . . . . . . . . . . . . . . . . . . 49 3.2.3. UPS Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 3.2.4. EELS Results in the Low Energy Region . . . . . . . . . . . . 52 3.2.5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 3.3. Work Function Engineering of Atomically Thin α−RuCl3 by Arsputtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 3.3.1. Characterization . . . . . . . . . . . . . . . . . . . . . . . . . 56 3.3.2. Work Function . . . . . . . . . . . . . . . . . . . . . . . . . . 57 3.3.3. XPS Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 3.3.4. UPS Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 3.3.5. Discussion and Summary . . . . . . . . . . . . . . . . . . . . 64 3.4. Charge Transfer at the α−RuCl3/MnPc Interface . . . . . . . . . . . 66 3.4.1. Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 3.4.2. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 3.4.3. Discussion and Summary . . . . . . . . . . . . . . . . . . . . 73 4. Spectroscopic Investigation of NiPS3 and FePS3 75 4.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 4.1.1. Crystal Structure and Magnetic Properties . . . . . . . . . . 76 4.1.2. Electronic Structure . . . . . . . . . . . . . . . . . . . . . . . 79 4.2. FePS3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 4.2.1. UPS Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 4.2.2. XPS Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 4.2.3. Electron Diffraction . . . . . . . . . . . . . . . . . . . . . . . 86 4.2.4. EELS Results in the Energy Region between 4 eV and 80 eV . 87 4.2.5. EELS Results in the Low Energy Region . . . . . . . . . . . . 88 4.2.6. Optical Spectroscopy and Linear Dichroism (LD) . . . . . . . 89 4.2.7. Discussion and Conclusion . . . . . . . . . . . . . . . . . . . . 92 4.3. NiPS3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 4.3.1. UPS Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 4.3.2. Core Level Spectroscopy . . . . . . . . . . . . . . . . . . . . . 98 4.3.3. Electron Diffraction . . . . . . . . . . . . . . . . . . . . . . . 101 4.3.4. EELS Results in the Energy Region between 4 eV and 70 eV . 102 4.3.5. EELS in the Low Energy Region . . . . . . . . . . . . . . . . 103 4.3.6. Multiplet theory and RIXS . . . . . . . . . . . . . . . . . . . 105 4.3.7. Discussion and Conclusion . . . . . . . . . . . . . . . . . . . . 107 5. Summary and Outlook 109 A. Appendix 111 A.1. The Pseudo-Voigt Profile . . . . . . . . . . . . . . . . . . . . . . . . . 111 A.2. Calculation of Reciprocal Lattice Vectors . . . . . . . . . . . . . . . . 111 Bibliography 113

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Anthropogenic release of the greenhouse gas carbon dioxide is believed to be a leading cause in the global rise in temperature. The main source of the carbon dioxide released is from combustion of fossil fuels. Thus, its necessary to mitigate the release of CO2, look for alternatives for fossil fuels and capture and sequester or capture and convert CO2 to other useful fuels and chemicals hence creating carbon neutral or carbon negative energy cycles. This thesis work was primarily focused on design, adapt and understand the chemistry of two-dimensional (2D) layered materials, particularly transition metal dichalcogenide (TMD) molybdenum disulfide and transition metal carbides (MXenes) as catalytic materials for the conversion of renewable energy into fuels and chemicals as an alternative for fossil fuels. This investigation was accomplished by combining electrochemistry, state of the art characterization and density functional theory (DFT) calculations. We hypothesized that it would be possible to improve the electrocatalytic hydrogen evolution reaction (HER) on MoS2 by engineering catalytically active sites on the plane, their edges and their interlayer regions. We also hypothesized 2D MXene sheets would serve as good carbon dioxide reduction reaction (CO2RR) catalysts under aprotic conditions. Conceivably the broad impact of this thesis work utilizing experimental and theoretical studies is the realization of transition metal doped metallic MoS2 as a potential candidate towards HER in alkaline conditions. Initially the interlayer region of MoS2 were investigated for the HER by introducing Na+, Ca2+, Ni2+ and Co2+ cations in the interlayers of metallic phase MoS2. Experimental results show that intercalation of cations (Na+, Ca2+, Ni2+, and Co2+) into the interlayer region of 1T-MoS2 to lower the overpotential for the HER. In acidic media the overpotential to reach 10 mAcm-2 for 1T-MoS2 with intercalated ions is lowered by ~60 mV relative to pristine 1T-MoS2 (~230 mV). DFT calculations suggest that the introduction of states from the intercalated metals whether sp or d, to lower the Gibbs free energy for H-adsorption (ΔGH) relative to intercalant-free 1T-MoS2. The DFT calculations suggest that Na+ intercalation results in ΔGH closest to zero, which is consistent with our experiments where the lowest overpotential for the HER is observed with Na+ intercalation. In order to explore the activity of the edge sites of MoS2 and the effect of a conductive support we used a microwave-assisted growth technique to synthesize interlayer expanded MoS2 with a vertically orientation on conductive two-dimensional Ti3C2 MXene nanosheets (MoS2⊥Ti3C2). Judicious choice of reaction temperature allows a control over the density of the edges obtained. Compared to pure MoS2 this unique inorganic hybrid structure allows an increased exposure of catalytically active edge sites of MoS2. The produced materials were investigated as electrocatalysts for the hydrogen evolution reaction (HER) in acidic conditions. The MoS2⊥Ti3C2 catalyst synthesized at 240 0C exhibited a low onset potential (-95 mV vs RHE) for the HER and a low Tafel slope (~40 mV dec-1). The decrease in the overpotential is linked to decrease in the charge transfer resistance of the materials with the electrode and the increased edge site density. In a third study the basal plane of metallic MoS2 was engineered by doping with transition metals Co and Ni to be evaluated as a catalyst for the alkaline HER. Due to a lack of oxygen evolution catalysts that can oxidize water at the anode under acidic conditions, there is an urgency to realize HER catalysts that can efficiently reduce water to hydrogen gas under alkaline conditions. Though metallic MoS2 has an optimum H binding free energy for the HER, the sluggish water dissociation step under alkaline conditions has made the implementation of MoS2 as a catalyst at higher pHs harder. We hypothesized that doping transition metals in the basal plane of metallic MoS2 that can efficiently catalyze the water dissociation step in alkaline conditions would help to reduce the overpotential required for the HER under alkaline conditions. Ni and Co were doped in orthorhombic MoO3 which was then converted metallic MoS2 under hydrothermal conditions. The polarization plots obtained in 1.0 M KOH solution shows a low onset overpotential of -75 mV vs RHE for the 10% Ni doped metallic MoS2 with an overpotential of -145 mV to reach a current density of 10 mA/cm2. Pure metallic MoS2 reaches the same current density at an overpotential of -238 mV vs RHE while samples doped with 10% Co atoms reached 10 mA/cm2 at -165 mV. This improvement in the doped samples is attributed to the improved kinetics of the water dissociation step under the alkaline reaction conditions. DFT calculations suggests that an optimal binding of water for the water dissociation step, H binding free and low free energy of binding for OH intermediates. Rigorous cycling of the catalysts shows extremely high stability with the doped samples while the pure metallic MoS2 loses its activity with continuous cycling. DFT calculations show that the doped samples provide extra stability to the metastable metallic MoS2 thus improving their long-term stability. Photo/electrochemical conversion of CO2 is an important step in the path to renewable production of carbon-based fuels and chemicals. Activity and selectivity have been major concerns on the CO2RR catalysts. The activity of known materials are hindered by the scaling relationship in the binding energies of the many intermediates involved in the CO2RR. Thus, the simplest of CO2RR products CO and HCOOH are of great value. Nano structured precious metals like silver and gold have shown promise as cathode materials for the conversion of CO2 to CO. In this thesis work we evaluate the electrocatalytic properties of Mo2C and Ti3C2 MXenes towards the electrochemical CO2 reduction reaction (CO2RR) as cheaper alternatives for precious metals. Though there have been theoretical predictions of the ability of MXenes with certain composition to have the ability to reduce CO2 to hydrocarbons, there are no experimental findings to support these calculations. In this study we observe very high faradaic efficiencies, ~90% for the CO2 reduction to CO at low overpotentials ~250 mV in acetonitrile/ionic liquid electrolytes on Mo2C MXene while Ti3C2 shows ~65% FE at an overpotential of ~600 mV for the cathodic half reaction. Density functional theory calculations suggests that the enhanced activity of Mo2C relative to Ti3C2 is due to relative lowering of the energy barrier for the initial proton couple electron transfer step of CO2 and the spontaneous dissociation of the absorbed *COOH species to *CO and H2O on the Mo2C surface. The calculations also predict the most probable active sites for the CO2 conversion to be vacant oxygen sites. High selectivity and high FE of CO2 reduction to CO makes these earth abundant materials an attractive electrocatalyst for the CO2RR. / Chemistry