Investigation On The Controllable Growth Of Molybdenum-trioxide Nanomaterials And Their Room-temperature Hydrogen Sensing Performance

Hydrogen is one of the clean energies and has the potential to become an ideal alternative source to fossil fuels.It has been widely used in industrial production and peoples’ lives.However,hydrogen molecules are so small that it is easily to be leaked out,which will cause the explosion accident.It is of great importance to develop the hydrogen-sensitive semiconductor materials with good stability,quick response and recovery at room temperature for promoting the popularization and utilization of hydrogen energy.In addition,the systemic simulation of hydrogen-sensitive semiconductor materials will be helpful to develop the high-quality hydrogen sensor.In this paper,we made a research on hydrogen sensing mechanism of MoO3 through the computer simulations.Then we composite MoO3 nanoribbons with graphene and assemble the composite to be room-temperature sensor with high sensitivity and fast response/recovery.The main works and results are shown as follows:(1)Calculation about hydrogen sensing property of MoO3 material.Based on density functional theory,the hydrogen sensing properties of MoO3(010)surface are simulated.The ideal stoichiometric MoO3(010)surface do not have the available adsorbed sites for gas molecules,therefore O2 and H2 molecules can’t be adsorbed on the surface.While the terminal oxygen vacancies can be formed on the non-stoichiometric MoO3(010)surface with the formation energy of 4.41 eV,Free O2 molecules will be chemisorbed at Mo5+ in horizontal way and capture the electrons from the MoO3 material,resulting in the decrease in the density of electron carrier.Then the H2 molecules can react with pre-adsorbed O2-on the surface of non-stoichiometric MoO3(010)to form the H2O molecules,releasing the electrons into the MoO3 material.Accordingly,the number of carriers in the system will increase.This work can provide theoretical guidance for the development of MoO3-based hydrogen gas sensor.(2)Hydrogen sensing properties of single MoO3 nanoribbons at room temperature.[001]-oriented orthorhombic MoO3 nanoribbons with average length of more than 500 μm were synthesized via a facile hydrothermal method.The hydrogen sensing behavior stems from the redox reaction between the chemosorbed oxygen ions(mainly O2-)and hydrogen molecules on the nanoribbon surface.The annealed atmosphere has an important impact on the content of Mo5+ in the samples.The nanoribbon annealed in H2 atmosphere has the highest content of Mo5+ than the ones in O2 or vacuum,showing the optimal hydrogen sensing properties with the response and recovery times of 54.6 s and 12.6 s to 1000 ppm H2,respectively.(3)Hydrogen sensing behavior of MoO3 nanoribbons at room temperature.The MoO3 nanoribbons with different sizes can be assembled into the membrane to form the hydrogen sensor with the method of brush coating.It is found that the hydrogen sensing prosperities of MoO3 nanoribbons has a positive relationship with the size of the nanoribbons.The average length of nanoribbons synthesized at 200 0C is the longest.The response and recovery times of this sample to 1000 ppm H2 is 14 s and 110 s,respectively.There are numerous MoO3/MoO3 homojunctions in the sensing layer,which has an important contribution to the hydrogen sensing behavior.However,the slow diffusion rate of the gas molecules at this nanostructure is the main reason for long recovery time of pure MoO3 nanoribbon based gas sensor.(4)Hydrogen sensing behavior of MoO3 nanoribbon/graphene composites at room temperature.The MoO3 nanoribbon/graphene composites with various compositions were successful synthesized by a one-step hydrothermal route.The response and recovery time of the composite with optimal graphene loading level of 1.5%towards 1000 ppm H2 largely decreased from～15 s to～10 s and～110 s to～30 s,respectively.Then the MoO3 NRBs were covered by the graphene nanosheets with further increasing the content of graphene.The accelerated response speed of the M/G-1.5 sensor could also be attributed to the less contribution of MoO3/MoO3 homojunctions to the sensing behavior.The surface reaction between absorbed oxygen and hydrogen became the predominant factor with the decreased MoO3/MoO3 nanojunctions.In addition,the suppressed contribution of MoO3/MoO3 nanojunctions,enlarged specific surface area and improved electronic conductivity should also be the positive factors leading to the enhanced hydrogen sensing performance.