Controlled Synthesis And Gas-sensing Property Of MoO₃ Microstructure

Molybdenum trioxide(Mo O3), an n-type semiconductor with a wide-band gap of 3.3 e V, has been attracting wide attention due to its excellent sensitive performance for toxic or flammable gas. Mo O3 exist in three type phases: orthogonal phase(α-Mo O3), hexagonal phase(h-Mo O3) and monoclinic phase(β-Mo O3), orthogonal phase is an thermodynamic stable phase, hexagonal phase and monoclinic phase are thermodynamic metastable phases. Controlled synthesis h-Mo O3 and α-Mo O3 microstructure were prepared using solution method, such as water bath method or hydrothermal method, under mild conditions without adding any surfactant, and gas sensing properties of the obtained samples were also studied.h-Mo O3 rod was prepared using water bath method by heating hydrogen nitrate acidized ammonium molybdate saturated solution, through changing the reaction medium and reaction time at 65 oC. The trimethylamine(TMA) gas-sensing properties of h-Mo O3 rod were evaluated with different TMA gas concentrations at the working temperature of 340 oC. The response of h-Mo O3 rod sensor to 500 ppm TMA was 95.2, with a response and recovery time of 10 s and 7 s, respectively. The response to low concentration as 1 ppm TMA can reach the 2.61. The good gas sensing performance can be attributed to the addition of ethanol, which resulted in the h-Mo O3 with lamellar structure; furthermore, the obtained sample size can be optimized at the moderate reaction time. Hence, the specific area of the obtained sample can be large, which is good for the surface reaction controlled gas-sensing process.By extending the acidification time of the saturated ammonium molybdate in the water bath method at 95 oC, flower-like h-Mo O3 can be prepared. Sample prepared at the condition of acidification time 40 d and the reaction time 1 h showed a gas-sensing response of 272 to 500 ppm TMA, which is 2.85 times of h-Mo O3 rods, with response and recovery time of 8 s and 7 s, respectively. The response to 1 ppm low concentration TMA can reach 4.44. The enhanced TMA response can be attributed to the rods self-assembly resulting in the flower-like structure, which increased the specific area and the ratio of(001) surface.α-Mo O3 nanobelt was synthesized through a hydrothermal method at 170 oC, the obtained α-Mo O3 nanobelt was doped with different proportion of Cu O to improve its gas sensing performance. The sensor response of 0.5:1 Cu O/Mo O3 nanobelt sample to 500 ppm TMA at working temperature of 300 oC was 84.2, which is 3.89 times of α-Mo O3 nanobelt. The enhanced TMA response is mainly attributed to a wider depletion layer on the Cu O/Mo O3 surface resulted from the p-n heterojunctions formed between the p-Cu O nanoparticles and n-Mo O3 nanobelt.