1.Key Laboratory of Instrumentation Science and Dynamic Measurement, North University of China, Taiyuan 030051, China 2.Department of Applied Physics, Graduate School of Engineering, Osaka University, Osaka 5650871, Japan
Fund Project:Project supported by the National Key Research and Development Program of China (Grant No. 2017YFE0130200), the National Natural Science Foundation of China (Grant Nos. 51905500, 61874100), and the Natural Science Foundation of Shanxi Province, China (Grant No. 201901D211253)
Received Date:21 May 2020
Accepted Date:06 July 2020
Available Online:22 October 2020
Published Online:05 November 2020
Abstract:Transition-metal-oxide as a typical model surface for investigating the catalytic mechanism has been widely studied. Over the past years, the TiO2 properties have been reported. It is commonly accepted that the catalytic activity of reduced TiO2 is related to its defects, with the accompanying excess electrons leading to n-type conductivity. It is realized that subsurface charge is of key importance for the redox chemistry of TiO2 (110).Subsurface charge is explored by atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM). Subsurface charge exerts an additional attractive force on the scanning AFM tip, resulting in the relative retraction of tip motion in order to keep a constant frequency shift. As a result, the subsurface charged region is imaged as protrusion in an AFM topographic image. The height of bright hillock is mainly distributed in three different ranges, which means that the subsurface charges are at three different subsurface layers. The AFM results show such subsurface charges repel the electropositive oxygen vacancy, hydrogen atoms and step edges. It is obvious that there is not only an Ov depletion zone but also the subsurface charge free region in the proximity of the $\left\langle {001} \right\rangle $ and $\left\langle {1\bar 11} \right\rangle$ step edge.The KPFM image indicates that the subsurface charges are the positive charges. which is consistent with common sense. After oxygen exposure, it is found that the oxygen adatom is electronegative, but it is absent in the vicinity of positive subsurface charges. Irrespective of adsorbate being electropositive or electronegative, an adsorbate-free zone generally exists in the proximity of the charged region. Obviously, the present study is expected to provide some insights into clarifying the nature of subsurface charge and improving catalytic design. Keywords:TiO2/ subsurface charge/ atomic force microscopy/ Kelvin probe force microscopy
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--> --> --> 1.引 言半导体金属氧化物二氧化钛(TiO2)因其独特的物理化学特性被研究人员所关注[1-8]. 在过去几年中, 已经报道了有关TiO2性能的研究成果. 理论和实验工作表明, 还原性TiO2的物理化学性质与其表面和亚表面的缺陷密切相关, 缺陷是在样品制备过程中由离子轰击和高温退火形成的, 与缺陷伴随的剩余电子使得TiO2为n型半导体特性. 目前, 有关缺陷对其物理化学特性的影响机制仍在研究中. 表面缺陷主要包括二配位桥氧O2c列上的桥氧空位(Ov)和H原子(通常标记为羟基OH), 已使用不同的方法对其特性进行了广泛的研究. 具有低配位和剩余电子特性的Ov成为大多数吸附物的优先吸附和解离点. 在TiO2 (110)-(1 × 1)面上$\left\langle {1\bar11} \right\rangle$方向的台阶边缘上我们直接观察到了表面缺陷Ov, 这解释了$\left\langle {1\bar 11} \right\rangle$台阶边缘的活性起源[9]. 因此, 表面和台阶边缘上的Ov在最初被认为是TiO2(110)的表面活性的来源. 近几年的研究结果也表明源自体缺陷的亚表面电荷对于TiO2 (110)的电学和光学性能也至关重要[10-16]. 体缺陷主要包括本征缺陷(钛间隙缺陷Tiint)和掺杂缺陷. TiO2 (110)在O2氛围下退火会形成TiOx (x < 2)岛的实验表明, 近表面区域的Tiint是最丰富的本征缺陷. 在碱性介质中还原的TiO2 (110)面上, 亚表面电荷可以增强电导率并促进电子催化中的电子电荷转移和氢解吸[17]. 在扫描隧道显微镜(STM)和原子力显微镜(AFM)实验中经常观察到亚表面电荷, 但是很少有研究关注它们对表面性质的影响. 此外, 需要注意的是, Ov在O2反应期间极易于愈合, 这表明表面Ov在实际反应环境中不可持续, 其伴随的剩余电荷会被消耗掉. 因此, 探索TiO2 (110)亚表面电荷的特性对于进一步了解TiO2 (110)的基本物理特性具有重要意义. 国内从事STM的相关研究团队在国际上占有重要地位, 如清华大学的薛其坤院士团队和中国科技大学的侯建国院士团队. AFM已发展为一种在原子尺度上探索表面特性的有力表征工具, 并具有了研究表面单原子/分子反应的能力. 基于AFM发展的开尔文探针力显微镜(KPFM)通过测量探针与样品之间的接触电势差(CPD), 以及吸附物与基底之间的电子电荷转移行为来表征材料表面/界面的电子/电学特性[18-20]. 目前, AFM和STM两种表征工具是一种优势互补的关系. 由于AFM原子尺度的表征技术难度相对较大, 国内在该研究领域起步较晚, 但是发展较迅速, 如裘晓辉、江颖和程志海教授课题组已经取得了很多重要成果[21-23]. 本研究团队长期从事AFM性能的改进和二氧化钛物理特性的研究, 积累了丰富的扫描技术经验和重要的研究成果. 本文首先展示高分辨率AFM成像中TiO2 (110)亚表面电荷的特征, 并基于AFM图像分析亚表面电荷的物理特性. 通过KPFM测量亚表面缺陷的电学特性, 并结合亚表面电荷的深度和库仑电势, 系统地研究亚表面电荷与Ov、H原子、台阶边缘和吸附O原子的相互作用. 2.实 验表征实验是在超高真空(压力: 3 × 10–11 Torr (1 Torr = 1.33322 × 102 Pa))、低温(78 K)非接触式原子力显微镜中进行的. 该系统配备的光束偏转系统可检测悬臂梁探针的振动幅值和频率. 工作中的探针以其共振频率振荡, 并保持振动幅度恒定. 探针是商购Ir镀层的Si基悬臂梁探针(Nano sensors SD-T10 L100, f0为800 kHz). 早期的实验表明, 使用诸如W和Ir的金属镀层针尖可以获得稳定的成像模式[24-25]. 因此, 金属镀层探针被广泛地用于探索物质表面性质. 在进行AFM成像之前, 先将探针在600 K中退火5 min脱气, 然后通过Ar离子溅射进行清洁以去除表面污染物. 实验样品TiO2 (110)(由Furuuchi Chemical Corporation提供)在准备腔室中通过20次左右的Ar离子清洗和高温退火循环制备, 待新制备的样品冷却至室温, 将其转移到观察腔室中. 在样品温度降至78 K后开始AFM扫描成像实验. TiO2 (110)氧表面是通过在室温下将O2暴露于新制备的表面而得. 该研究中使用的低温环境和高共振频率探针可以提高对短程相互作用力的灵敏度, 这使得我们能够获得高空间分辨率AFM图像. 3.实验结果与讨论为了便于后续的分析和讨论, 首先简单介绍金红石TiO2 (110)-(1 × 1)的表面结构模型, 如图1所示. 它是由交替的五配位Ti5c列(橙色球)和六配位的Ti列(Ti6c)组成, 其中Ti6c列被共面的三配位氧气(O3c)列(蓝色球)和二配位桥氧O2c列(灰色球)所包围. 如引言中所述, 表面缺陷主要包含Ov (虚黑圈)和OH缺陷(绿色小球). 由于同一O2c列上成对Ov之间的排斥作用, Ov通常为单一状态[26]. OH对是由腔体残留水分子在Ov位点的解离而形成的, 在室温下H原子在表面上扩散而形成单个OH (如图2(c)所示). O吸附原子(Oad为浅绿色的大球)是O2在Ov或Ti5c位点上解离而产生的, OH原子的电子特性与Ov的相似. O2c列在AFM图像中的亮暗特征, 与探针针尖的极性相关, 详细的介绍可参考我们前期的研究[5,9]. 当探针针尖为正电特性时, 由于针尖和O2c列的作用力为吸引力, 因此O2c列被成像为亮列, 如图2(a)和图2(b)所示. 当针尖为负电特性时, Ti5c列和表面缺陷被分别扫描成亮列和亮点, 且OH比Ov缺陷明亮[27-29]. 图 1 金红石TiO2 (110)-(1 × 1)的表面模型结构的 (a)俯视图和(b)侧视图(由交替的五配位Ti5c列(橙色球)和六配位的Ti6c组成; Ti6c列被共面的三配位氧气O3c列(蓝色球)和二配位桥氧O2c列(灰色球)所包围; 表面缺陷主要包含Ov (虚黑圈)和OH缺陷(绿色小球)) Figure1. (a) Top view and (b) side view of ball model of rutile TiO2 (110)-(1 × 1) surface consist of alternating O2c (gray ball) and Ti5c (orange ball) atom rows. H atom (green small ball) is generated by water dissociation on Ov site, and O adatom (light green large ball) on Ti5c row is formed by O2 dissociation on Ov and/or Ti5c sites.
图 2 TiO2 (110)样品表面的代表性AF??M形貌图 (a)和(b)中亮列是O2c, 暗点是表面缺陷; (c)中亮列是Ti5c, 亮点是缺陷; (d)是亮丘的轮廓线, 表明亚表面缺陷分布于3个不同的亚表面原子层. Figure2. AFM topographic images of TiO2 (110) surface obtained at different experiments at 78 K. (a), (b) The bright rows are the O2c row, and dark spots are the surface defects. (c) The bright rows are the Ti5c row. Bright spot is the Ov, and brighter spots are the H atoms. Bright regions are observed in all three images. (d) Line profiles cross the bright region along the $ \left[ {1\bar10} \right] $ direction on TiO2 (110) surface, taken from different AFM images. The height shows three specific regions, corresponding to three different depths of subsurface charge.