Direct hydrothermal synthesis of monoclinic VO2(M) single-domain nanorods on large scale displaying magnetocaloric effect

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Monoclinic VO2(M) is a prototype material for interpreting correlation effects in solids with fully reversible phase transition between VO2(M) and VO2(R). Although VO2(M) is the thermodynamically most stable phase, few available solution strategies for VO2(M) have been realized since Morin's discovery in the 1950s. Here, we report a controlled oxidation reaction to realize the direct solution-based synthesis of monoclinic VO2(M). The established strategy also achieves highly uniform nanorods with single-domain structure, which is particularly fascinating due to the long-standing limited and sole method for one-dimensional (1D) nanostructures of monoclinic VO2(M). The orientation change of the electron clouds leads to the magnetization change on passing through the phase transition from VO2(M) to VO2(R), making VO2(M) the first case of a vanadium oxide compound to show magnetocaloric effect (MCE). And the MCE performance advantage of solution synthesis sample greatly benefits from the highly uniform nanorods having a single-domain structure, which realizes the rapid magnetization change on passing the phase transition and then leads to higher magnetic entropy change ΔS value for magnetocaloric effect (MCE). The clear physical picture of the first-order phase transition process comes from a series of time-resolved in situ techniques, including the first established in situXAFS technique for vanadium K-edge.