Direct Synthesis of Molybdenum Carbide Nanosheets via A Pseudo-Topotactic Solid- State Reaction of Exfoliated Molybdate Nanosheets Katsutoshi Fukuda* and Satoshi Toyoda Kyoto University, Yoshida-honmachi, Kyoto, 611-0011, Japan l Introduction In the last decade, primitive structural conversion of exfoliated nanosheets has demonstrated significant potential in the design of 2D materials; for instance, the structural conversion of chalcogenide nanosheets such as MoS, produces several polymorphs with different properties. In contrast to these structural conversions, reduction of graphite oxide nanosheets into graphene without disruption of the sheet-structure can be depicted as a simplification of the composition from a C-O binary system to a C mono-component system. Thus, structural conversions using the exfoliated nanosheets are known to be mainly attained in the same composition system or a simpler composition system. Should the exfoliated nanosheets topotactically react with foreign species in a solid-state reaction, various derivative nanosheets would be created depending on the ingredient used. In this study, we propose a direct synthetic route for Mo,C nanosheets based on a pseudo-topotactic reaction of electrostatic self-assembled monolayer pairs consisting of MoO , nanosheets and organic polymers [1].
2 Experiment
Layered molybdates, Na 9, Mo, O 4 , were soft-chemically delaminated into a mass of dispersed elemental host layers, i.e. MoO, nanosheet suspension [2]. Immersing Si substrate pre-coated with cationic polymers used as a binder into the obtained suspension yields sub-monolayer films in which a pair of the anionic nanosheets and polymer monolayers lay flat to the substrate. Multilayer films of the nanosheets/polymers pairs were then fabricated by repeating the aforementioned process up to ten times. And heat-treatments of these films were performed by increasing the temperature from that of the surrounding environment at a rate of 10°C min 1 to set temperatures of 200, 300, 400, 500, 600 and 700˚C under H, gas-flow-controlled conditions. After heating at the selected temperature for 1 hour, the samples were left to cool in a quartz tube furnace.
3  Results and Discussion
Topographical and structural changes occurring within MoO, nanosheets deposited on the substrate before and after heat-treatment under the H, gas atmosphere were examined by AFM and in-plane XRD analyses. As shown in Fig. 1A, the as-deposited nanosheets are visualized as lamellar objects below 2 nm in thickness and sub- micrometer in lateral size in this study. Most of the MoO, nanosheets were adsorbed onto the substrate as monolayers despite the inevitability of some overlap and gaps between the nanosheets under the present fabrication process. The in-plane XRD pattern of MoO, nanosheets (Fig. 1a) is indexable to a 2D rectangular unit cell of b = 0.290(1) and c = 0.502(5) nm [2]. The XRD patterns and morphology remained nearly unchanged until the MoO 2 nanosheets were heated to 300°C, indicating the structural robustness of the MoO 2 nanosheets. However, heating at 400°C triggered peak-boarding and a change in the intensity ratios of reflections 11 and 20 , and at 500°C the 2D rectangular unit cell seemed to transform into a 2D hexagonal one ( d = 0.2887(2) nm) due to the loss of reflections 12 and 03 . This symmetry improvement may originate from the reduction of the MoO, nanosheets induced by heating under the reduction atmosphere. After heating above 600°C, these XRD peaks distinctively shift to smaller d 1 values, yielding a larger 2D hexagonal unit cell of d = 0.3000(3) nm. This shift cannot simply be attributed to deoxygenation as is often the case with several exfoliated nanosheets. Considering the d -value of ~ 0.3 nm in the heated film, the most likely phase is β- Mo,C (JCPDS No. 075-6678). Interestingly, lamellar objects are still observed in the sample heated to 600°C (see Fig. 1B). The average thickness of the generated Mo,C sheets (~ 2 nm), acquired from the AFM image, seems to be similar to those of conventional oxide-type nanosheets including its precursor nanosheet, which
Fig.1 Left panel shows AFM images of (A) as-grown film of the MoO, nanosheet, and  (B) its annealed samples at 600°C for 1h under H, flow. Right panel shows in-plane XRD patterns for (a) as-grown film, and its heated films at (b) 200, (c) 300, (d) 400, (e) 500, (f) 600, and (g) 700˚C for 1h under H, flow. The asterisk refers to 110 of bcc-Mo.