高灵敏度二氧化钛氢气传感器选择性响应的理论设计和优化

Hydrogen sensors are of increasing importance in connection with the development and expanded use of hydrogen gas as an energy source and as a chemical reactant. Oxides-based H2 gas sensors such as SnO2, ZnO, and TiO2 are cheap, long service life and easy to be fabricated. TiO2 are more stable and corrosion resistant, and has a higher position of the edge of conduction band caused by the wider band gap. Dense thin film of rutile TiO2 grew along the [002] preferred orientation showed excellent H2 sensing properties in our previous works owing to following three factors. (1)Rutile (002) surface has the highest surface energy compared with other crystal face in all TiO2’s phases. It is helpful to the adsorption of small molecules. (2) Dense surface can obviously reduce the resistant between Pt electrodes and TiO2, which contributes to the improvement of the sensitivity of device.(3) H2 molecular can be dissociated in atomic H at the Ti(4) site on the (002) surface at room temperature. And the adsorption of atomic H on surface plays a doping role in the influence of band structure, which leads to an obvious shift down of conduction band of TiO2 and reduce the system’s resistance. In our precious experiment, we have achieved that the hydrogen sensor device is working at room temperature and atmospheric environment with excellent properties, eg. The sensitivity of 4% in 1ppm H2, response time is short of 9s, recover time is of 160s. However the humid air brings a lot of noise in hydrogen gas detection process. In order to restrain the effects from water and further improve the sensitivity at low H2 concentration, we start our work through modeling work to illuminate mechanism of hydrogen sensor as shown in following three aspects. (a) To understand the behavior of the adsorption of H2O on TiO2(002), we will set up possible H2O adsorbing or reaction product configurations on (002) surface. By searching transition states between reactant and product, we will find the possible reaction paths of H2O at room temperature. According to the explored paths, the band structures of important configurations, including reactant, product and transition states, will be calculated by GGA+U or HSE06 method. Then, employing (002) surface reconstruction by surface steps, Ti or O vacancy on the surface, it is helpful to find a way to prevent adsorption of water. Finally, comparing the results of H2O adsorbing with the adsorption of H2 in our previous work, the results will help us to understand the competitive adsorption between H2O and H2 molecular on rutile TiO2 (002). (b) To understand the height of schottky barrier between Pt and TiO2 under different H2 concentrations, it is helpful for us to describe the behavior of electronic transport in device, especially to explain the jump point in the resistance test at H2 of 100ppm. Then through N, Zn or Ag doping, it will change TiO2 into P-type semiconductor. The further up shift of the edge of conduction band will lead to a higher schottky barrier between Pt and TiO2. Before and after H2 adsorbing, the change of the resistance of system will be more obvious，which will enhance the sensitivity of hydrogen sensor. (c) Reconstructing the band structure by doping, such as narrowing the band gap, shifting the position of valence band, or importing intermediate band, we will attempt to adjust and control the position of energy band associated with H2O always below the edge of valence band or middle gap band. Keeping energy band associated with H2O away from the edge of conduction band, it will weaken the influence of H2O on the electronic transport in device and achieve the device only response to H2.

氧化物半导体表面与氢气和水相互作用机理的揭示和对表面吸附小分子所导致体系电子结构改变的深入理解，是开发新型高效、选择性响应氢敏器件的根本出发点。针对目前氧化钛薄膜氢敏器件在低氢气浓度下响应不高且湿度对氢气探测干扰明显的问题，本项目在前期取向氧化钛纳米薄膜氢敏特性的研究基础上，以理论设计为出发点并结合实验，研究p型掺杂氧化钛对器件氢敏和抗水干扰特性改善的相关机理：（1）研究H2O和H2在氧化钛活性表面的竞争吸附关系，明确水吸附对表面吸氢和导电性的影响；（2）研究p型掺杂对铂电极/氧化钛间肖特基势垒的调控作用以及对低浓度氢气探测的影响；（3）研究p型掺杂调控氧化钛电子结构特征，明确抑制器件对水产生信号响应的电子结构调控机理。通过对氢气与水在活性表面竞争吸附过程的认识，深入揭示氧化钛表面在高湿度环境下低浓度氢气探测机制，为开发高灵敏度、复杂工作环境下、极低浓度的氢气探测器提供科学依据。

氢气作为一种清洁、高效的可再生能源以及重要的化工原料广泛应用于石油化工、电子工业、冶金工业、食品加工和航空航天等领域。氢气无色无味，与空气混合易形成爆炸物等物理特性，使得在氢气的存储、运输和使用过程中存在很多安全隐患。实现氢气的高灵敏度检测、防范危险于未然成为保障生产生活安全的重要手段。根据对近年来氢敏传感器的调研，我们发现对于导电性能较好的体系灵敏度往往不高，一般需要在较高的氢气浓度下才有明显的电学信号响应;而对于金属氧化物半导体系统如SnO2、ZnO、WO3、MnO2等，工作温度往往不在室温区域，响应时间较长。这些不同体系下的氢敏器件各有长处，然而一个性能优异的氢敏器件需要同时具备灵敏度高、响应时间短、工作温度低、工作寿命长以及好的选择性响应等特点。.针对该背景，我们已经分步骤地完成了：取向生长（002）方向的TiO2纳米薄膜的制备，工作温度室温、快速响应以及高灵敏度的原件封装，相关氢敏理论机理探讨和实验的优化等研究工作。(1) 通过计算发现金红石相TiO2(002)面具有最高的表面能，有利于吸附小分子。过第一性原理过渡态探寻计算，我们利用过渡态探寻明确了小分子与TiO2(002)相互作用过程。阐明了体系的能带结构的变化过程，以及与实验测得的阻抗曲线变化规律的对应关系；(2)通过实验计算证明了，水与金红石-锐钛矿异质结结构的相互作用机理；(3)通过实验与计算相结合，明确了水和氢气在金红石TiO2 {002}, {101}, {110} 不同暴露表面上响应机理；(4)通过水热制备 (004)取向的锐钛矿TiO2高性能氢气传感器，室温响应时间仅为5秒，恢复时间64秒，探测氢气极限浓度仅为1ppm。.总之，实验与计算相结合，较为圆满的完成了本项目的研究内容，制备两大类金红石TiO2 (002)和锐钛矿TiO2(004)氢气传感器，其关键指标均达到了行业要求。