Microwave synthesis of gram scale millimeter size layered transition metal oxide crystals

Abstract Layered transition metal oxides (TMOs) have been utilized for centuries for their abundance and diverse applications in various fields. Developing efficient synthesis methods of layered TMOs is crucial for exploring novel properties and potential applications. This work introduces a direct microwave synthesis method for producing transition metal oxides and crystals. Using alpha molybdenum trioxide as a representative example, a microwave synthesis protocol is outlined for the rapid and scalable synthesis of layered transition metal oxides. Transition metal oxides have been synthesized using various state-of-the-art (SOTA) techniques, such as hydrothermal and sol-gel, among others. In contrast to these established methods, our direct microwave process is simple, requiring only a microwave and minimal handling, and it is highly scalable (1 g/h of α-MoO ₃ crystals). Additionally, it consumes just 0.5 kWh of energy per gram of α-MoO ₃ , which is 8-140 times better than typical SOTA methods. It is environmentally friendly as it produces ~10-150 times less CO ₂ emissions (0.3 kg CO ₂ eq) compared to SOTA methods. It produces long crystals with a length of up to 8 mm, which is comparable to hazardous chemical and physical vapor deposition methods. We further demonstrate the advantages of these high-quality crystals by fabricating a MoO ₃ -based Metal-Interlayer-Oxide-Semiconductor (MIOS) memristor incorporating an ultrathin Al ₂ O ₃ interlayer. The device exhibits low SET (~ −2 V) and RESET (~2 V) voltages with stable endurance over multiple cycles, attributed to field-driven oxygen vacancy migration. These findings establish microwave synthesis as a transformative approach for producing high-performance metal oxides and underscore the significant potential of MoO ₃ for next-generation energy-efficient and reliable memory technologies.