Cr掺杂ZnO(10$\overline{1}$0)表面的原子结构和电子性质研究

doi:10.1016/S1872-2067(20)63710-X

催化学报 ›› 2021, Vol. 42 ›› Issue (6): 971-979.DOI: 10.1016/S1872-2067(20)63710-X

Cr掺杂ZnO(10$\overline{1}$0)表面的原子结构和电子性质研究

黄武根^a^,^b, 蔡军^c, 胡俊^d, 朱俊发^d, 杨帆^a^,^c^,^*(), 包信和^a^,^#()

^a中国科学院大连化学物理研究所催化基础国家重点实验室, 辽宁大连116023
^b中国科学院大学, 北京100049
^c上海科技大学物质科学与技术学院, 上海201210
^d中国科学技术大学同步辐射国家实验室, 安徽合肥230029

收稿日期:2020-08-16 接受日期:2020-09-14 出版日期:2021-06-18 发布日期:2021-01-30
通讯作者: 杨帆,包信和
基金资助:
国家重点研发计划(2017YFB0602205);国家自然科学基金(21972144);国家自然科学基金.(91545204)

Atomic structures and electronic properties of Cr-doped ZnO(10$\overline{1}$0) surfaces

Wugen Huang^a^,^b, Jun Cai^c, Jun Hu^d, Junfa Zhu^d, Fan Yang^a^,^c^,^*(), Xinhe Bao^a^,^#()

^aState Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China
^bUniversity of Chinese Academy of Sciences, Beijing 100049, China
^cSchool of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China
^dNational Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, Anhui, China

Received:2020-08-16 Accepted:2020-09-14 Online:2021-06-18 Published:2021-01-30
Contact: Fan Yang,Xinhe Bao
About author:^#Tel: +86-411-84686637; Fax: +86-411-84694447; E-mail: xhbao@dicp.ac.cn
^*Tel: +86-411-84379765; Fax: +86-411-84379128; E-mail: fyang@dicp.ac.cn;
Supported by:
Ministry of Science and Technology of China(2017YFB0602205);National Natural Science Foundation of China(21972144);National Natural Science Foundation of China(91545204)

摘要/Abstract

摘要：

多相催化中ZnO基催化剂广泛应用于甲醇合成、水汽变换和合成气转化等诸多领域. 近期发展的ZnCrO_x-分子筛双功能催化剂(OX-ZEO)打破了传统合成气转化的ASF分布, 能够高选择性地实现CO加氢转化为低碳烯烃. 其中CO在ZnCrO_x表面活化被认为是OX-ZEO催化的关键基元过程, 但是ZnCrO_x表面的活性位组成和结构目前仍然缺乏原子尺度上的理解, 阻碍了人们对反应机理的研究. 因此, 本文构建了Cr掺杂ZnO(10$\overline{1}$0)单晶的模型催化剂, 结合低温扫描隧道显微镜(LT-STM)和X射线光电子能谱(XPS), 研究不同条件下Cr在ZnO(10$\overline{1}$0)表面生长的形貌结构, 并重点考察了Cr掺杂对表面电子结构和CO吸附的影响.
ZnO(101-0)是Zn-O混合终止的非极性面, 也是ZnO最稳定的晶面. 实验发现室温下沉积Cr在ZnO(10$\overline{1}$0)表面时, 出现多种生长结构. STM显示低覆盖度(< 0.1 ML)的沉积Cr在ZnO(10$\overline{1}$0)台阶面上出现单分散的亮点和暗点, 分别为位于表面O原子链上的Cr原子和嵌入ZnO晶格替代Zn²⁺的Cr. XPS显示这些Cr原子与ZnO之间存在电荷转移, 呈现出+3价. 由于Cr在[0001]方向扩散能垒高于沿着[1$\overline{2}$10]方向扩散的能垒, 能够观察到少量沿着[0001]台阶方向生长的长方形岛, 归属为Cr岛. 表面单分散Cr原子和Cr岛的密度都会随着Cr沉积量的增加而增多. 当在200 K沉积Cr来抑制Cr的表面扩散行为, 我们发现Cr主要在ZnO面上形成Cr团簇, 均匀分散在表面; 而在400 K沉积时, Cr则直接扩散进入ZnO晶格. STM和XPS结果都表明, 600 K以上的高温处理能够促进Cr进入ZnO晶格, 同时伴随Cr岛的分解, 这说明表面负载的Cr原子在高温不能稳定存在; 而Cr岛的再分散进入到近表层甚至体相, 也表明了Cr和ZnO之间的强相互作用. 这种强相互作用有利于Cr在ZnO表层进行原子级分散, 形成具有催化活性的结构; 与此同时, 实验发现Cr扩散到近表层会导致表面能带向上弯曲, 从而导致XPS芯能级向低结合能位移. 在超高真空条件下, CO与Cr掺杂的ZnO表面作用较弱, 室温下只观察到碳酸盐的形成.
因此, 通过对比不同条件下Cr在ZnO(10$\overline{1}$0)表面的生长结构, 热稳定性以及相应的电子结构变化, 发现Cr和ZnO存在强相互作用, 在600 K以上Cr以Cr³⁺形式分散到ZnO晶格中. 本文通过构建Cr/ZnO(10$\overline{1}$0)模型催化剂, 并研究其原子结构与电子结构的演变, 为进一步的分子尺度合成气转化机理研究开辟了道路.

关键词: ZnO, Cr/ZnO(10$\overline{1}$0)催化剂, CO吸附, STM, XPS

Abstract:

An integrated approach combining scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) is used to investigate the atomic structures and electronic properties of Cr-doped ZnO(10$\overline{1}$0) surfaces. When deposited at 300 K, Cr at low surface coverage (< 0.1 ML) appeared either as isolated atoms on the surface terrace of ZnO(10$\overline{1}$0) or substituting Zn atoms in the ZnO lattice. Their structural models could be identified from atomic-resolution STM images and their oxidation states were found as Cr³⁺ based on XPS measurements. Rectangular islands nucleated at step edges along the [0001] direction could also be observed during the initial growth of Cr at 300 K and were assigned as Cr islands. The density of Cr islands as well as their average size increased with the increasing of Cr surface loading. Thermal treatments at above 600 K could facilitate the decomposition of Cr islands and the re-dispersion of Cr atoms into the ZnO lattice, indicating a strong interaction between Cr and ZnO. The adsorption of CO at 78 K showed no preferential adsorption at Cr³⁺ sites embedded in the surface lattice of ZnO. However, the re-dispersion of Cr atoms into the ZnO bulk at above 600 K could induce a significant upward band bending, causing a negative shift of core level XPS peaks of Zn 2p and O 1s by ~0.5-0.7 eV. Our study has thus constructed a model catalyst for Cr-doped ZnO and provided atomic insight for understanding ZnO-based catalysts.

Key words: ZnO, Cr/ZnO(101-0) catalyst, CO adsorption, Scanning tunneling microscopy, X-ray photoelectron spectroscopy

黄武根, 蔡军, 胡俊, 朱俊发, 杨帆, 包信和. Cr掺杂ZnO(10$\overline{1}$0)表面的原子结构和电子性质研究[J]. 催化学报, 2021, 42(6): 971-979.

Wugen Huang, Jun Cai, Jun Hu, Junfa Zhu, Fan Yang, Xinhe Bao. Atomic structures and electronic properties of Cr-doped ZnO(10$\overline{1}$0) surfaces[J]. Chinese Journal of Catalysis, 2021, 42(6): 971-979.

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链接本文: https://www.cjcatal.com/CN/10.1016/S1872-2067(20)63710-X

https://www.cjcatal.com/CN/Y2021/V42/I6/971

图/表 8

Fig. 1. STM images of clean and CO adsorbed surface of ZnO(10$\overline{1}$0). (a) Large-scale image of clean ZnO(10$\overline{1}$0). Inset is the atomically resolved STM image. The squared area in (a) is magnified in (b). Current-mode STM image of (b) is shown in (c) to enhance the contrast of depressions. (d) Surface adsorbed with CO molecules. Line profiles in (b) and (d) are plotted in (e) to differentiate the apparent heights of surface defects and CO-adsorption-induced depressions. (f) Ball-stick model for CO adsorption and Zn-O vacancy (VZn-O). Scanning conditions: (a) Vs=1.5 V, It=0.1 nA; inset, Vs=0.66 V, It=0.5 nA; (b) Vs=0.8 V, It=0.5 nA; (d) Vs=2.0 V, It=0.05 nA.

Fig. 2. Surface structures of Cr deposited on ZnO(10$\overline{1}$0) at 300 K. (a-c) STM images of 0.075 ML Cr deposited on ZnO(10$\overline{1}$0). The white squared area in (a) is magnified in the inset. Cr atoms adsorbed on or embedded in the surface of ZnO(10$\overline{1}$0) are demonstrated in (b,c) and termed as CrA and CrB, respectively. CrA atoms were imaged as bright protrusions at below +1.5 V sample bias. Line profiles in (a) and (b) are plotted in (d). (e,f) High-resolution STM images of Cr atoms on ZnO(10$\overline{1}$0). CO adsorption on Zn sites was used to confirm the location of Zn rows. The red and blue arrows indicate the positions of CrA adatom on surface and CrB atoms in Zn rows, respectively. CrB atoms were resolved as bright dots with dark halos due to the change of tip state. (f) Side and top views of the structural model of Cr atoms on ZnO(10$\overline{1}$0). Gray, red and pink balls represent Zn, O and Cr atoms, respectively. Red and blue cycles indicate the locations of CrA on the O row, and CrB substituting Zn atom in the Zn row, respectively. Scanning conditions: (b) Vs=1.0 V, It=0.5 nA; (c) Vs=1.5V, It=0.2 nA; (e) Vs=1.2 V, It=0.3 nA.

Fig. 3. STM images of 0.2 ML Cr/ZnO(101-0) (a-c) and 0.4 ML Cr/ZnO(101-0) (d-f) prepared by depositing Cr atoms onto ZnO(101-0) at 300 K. Squared region in (a) is magnified in (c). With increasing Cr coverage, both the density of surface Cr atoms and the size of Cr islands increased. To enhance the contrast, current-mode STM images were displayed in (b) and (e).

Fig. 4. STM images of 0.06 ML Cu/ZnO(10$\overline{1}$0) prepared by depositing Cu onto ZnO(10$\overline{1}$0) at 300 K. (a,b) STM images showing the 3D growth of Cu clusters, which nucleated both at step edges and on the surface terrace. Line profile in (b) is plotted in (c) to exemplify the 3D growth of Cu clusters.

Fig. 5. STM images of the temperature effect on the surface structures of Cr/ZnO(101-0). (a,b) Cr atoms (0.075 ML) deposited at ~200 K. Cr clusters and surface Cr atoms were both observed. (c,d) Cr atoms (0.075 ML) deposited at 400 K. The incorporation of Cr atoms into the ZnO lattice is clearly observed in (d). (e,f) A typical surface of 0.15 ML Cr/ZnO(101-0), where Cr was deposited at 300 K. (g,h) The surface in (e) was annealed at 600 K under UHV. Cr islands would decompose and incorporate into the ZnO lattice after annealing. Scanning conditions: (b) Vs=1.0 V, It=0.2 nA; (d) Vs=1.2 V, It=0.1 nA; (f) Vs=1.0 V, It=0.5 nA; (h) Vs=0.8 V, It=0.2 nA.

Fig. 6. XPS spectra on the oxidation states of Cr on ZnO(101-0). (a-c) Cr 2p spectra collected with Al Kα source. (a) Cr 2p spectra on the clean ZnO(101-0) and 0.8 ML Cr/ZnO(101-0). Cr 2p spectra of the latter surface annealed in 1 × 10-6 mbar CO at different temperatures were displayed sequentially. (b) The differential spectra of 0.8 ML Cr/ZnO(101-0) obtained by subtracting Cr 2p spectra in (a) by the Cr 2p spectrum of clean ZnO(101-0) (black curve). (c) The differential Cr 2p spectra of 0.2 ML Cr/ZnO(101-0). (d,e) Cr 3p spectra of (d) 0.06 ML Cr/ZnO(101-0) and (e) 0.2 ML Cr/ZnO(101-0) before and after the annealing in 1 × 10-6 mbar CO at different temperatures.

Fig. 7. STM images on the effects of Cr doping on CO adsorption on ZnO(101-0). (a,b) CO adsorption at 78 K on 0.075 ML Cr/ZnO(10$\overline{1}$0) surface prepared by depositing Cr onto ZnO(10$\overline{1}$0) at 300 K. The squared region in (a) is magnified in (b). (c,d) In situ STM images before (c) and after (d) the adsorption of CO at 78 K on the 0.075 ML Cr/ZnO(10$\overline{1}$0) surface, which was prepared by Cr deposition at 400 K. (e,f) STM images of 0.075 ML Cr/ZnO after the annealing in 1×10-6 mbar CO at 600 K.

Fig. 8. O 1s and Zn 2p3/2 spectra of 0.2 ML Cr/ZnO(101-0) before and after the annealing at different temperatures in 1×10-6 mbar CO. (a) O 1s spectra taken with incident X-ray photon energy at 600 eV. After annealing in 1×10-6 mbar CO at 670 K, a negative shift by ~0.6 eV was observed. (b) Zn 2p3/2 spectra taken with Al Kα source. (c) Shift of core-level binding energies of O 1s and Zn 2p3/2 as a function of Cr coverage.

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Cr掺杂ZnO(10$\overline{1}$0)表面的原子结构和电子性质研究

Atomic structures and electronic properties of Cr-doped ZnO(10$\overline{1}$0) surfaces

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