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

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

Abstract

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

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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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(20)63710-X

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

Figures/Tables 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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