ARTICLE pubs.acs.org/JPCC
Stability of Hydrogen-Terminated Surfaces of Silicon Nanowires in Aqueous Solutions Chunzeng Peng,†,‡,§ Jing Gao,† Suidong Wang,† Xiaohong Zhang,*,|| Xinping Zhang,‡ and Xuhui Sun*,† †
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Institute of Functional Nano & Soft Materials (FUNSOM) and Jiangsu Key Laboratory for Carbon Based Materials and Devices, Soochow University, Suzhou 215123, China ‡ College of Applied Sciences, Beijing University of Technology, Beijing 100124, China Nano-organic Photoelectronic Laboratory and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China ABSTRACT: The surface stability of silicon nanowires (SiNWs) is crucial to their applications in nanodevices, such as transistors, sensors, photovoltaic cells, Li-ion batteries, etc. In this study, X-ray photoelectron spectroscopy (XPS) has been used to systematically investigate the stability of H-terminated surfaces of SiNWs at room temperature in aqueous solutions of different pH values. The hydrogen-terminated surfaces of SiNWs show relatively poor stability in aqueous solution, which depends on the solution pH. The oxygen in the solution had little effect on the oxidation of SiNW surfaces. With pH value increases in the solution, H-terminated SiNWs exhibit a greater extent of oxidation with higher oxidation rate. In low pH solution ( 7), Si1þ always maintains at a low level. Si hydride on the SiNW surfaces could be oxidized by the attack of OH- which comes from the dissociation of water molecules to form Si-OH species existing stably in low pH solutions. Simultaneously, oxygen atoms released from the surface SiOH bonds also attack Si-Si bonds with the assistance of OHions, to generate the Si-O-Si bridge bond which leads to the formation of SiO2 (Si4þ) finally.14,18,19 The Si4þ signal increases at the expense of the consumption of Si-OH bonds. In low pH solution, there are also a large number of Si-OH bonds on the SiNW surfaces during oxidation processes. However, in the high pH solution, a high concentration of OH- directly oxidized the H-terminated SiNW surfaces to form Si-OH species and then made them decompose readily to produce oxygen atoms which insert into the Si-Si back bonds. Thus, the Si1þ signal was weak due to the fact that a great number of Si-OH species were consumed at high OH- concentration. There is another possibility that the oxygen atoms from OH- directly oxidized the H-terminated SiNW surfaces to form SiO2 without intermediate processes. In the actual oxidation process, the two reaction paths may exist at the same time. In addition, the intensities of intermediate species (Si2þ and Si3þ) were very weak in all pH solutions which is due to the unstableness of these intermediate species in the solution. According to the above fact, two different oxidation processes of H-terminated SiNWs in acid and basic solution may exist. A schematic of the above reaction paths are shown in Figure 6. In the low pH solutions (pH e 7), OH- ions which come from the dissociation of H2O attack and replace the sites of hydrogen (a f b) to form relatively stable Si-OH species. Then, with the assistance of another OH-, Si-OH species decompose to produce oxygen atoms which attack Si-Si back bonds to generate the Si-O-Si bridge bonds (b f c). After several similar processes (c f d f e), fully oxidized SiO2 is eventually formed. Although the reaction path (a f b f c f d f e) may also occur in the high pH solution (pH > 7), intermediate species b is unstable. In high pH solution, it is also reasonable to assume an additional direct path (a f e) on the basis of the present observation that the amount of b, c, and d can be neglected. When the pH is high enough, surface SiO2 (e) continues to react with OH- to form silicate dissolving in the solution and SiNWs are etched away. The behavior of the Si1þ, Si2þ, Si3þ, and Si4þ intensities with increased dipping time in high pH solution is similar to that of SiNWs exposed to molecular oxygen,12 for which the intensities of suboxide states (Si1þ to Si3þ) almost stayed a small amount, while the Si4þ oxidation state continues to grow and becomes dominant. However, it is obviously different for that of Si wafer oxidized in oxygen.20 The results of ref 20 showed that Si2þ and especially Si3þ could relatively stably exist and their intensities were comparable with that of Si4þ during the oxidation process. This fact illustrates that there are distinctions between the oxidation processes of SiNWs and that of Si wafer and the nanoscale Si is more sensitive to OH- in solution.
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’ CONCLUSION The oxidation processes of H-terminated SiNW surfaces in aqueous solutions with different pHs were studied by XPS. The results show that the solution pH had a great influence on the surface oxidation of SiNWs. With pH value increases in the solution, H-terminated SiNWs exhibit a greater extent of oxidation. Time evolutions of the intensities of the oxidation states were used to reveal the oxidation processes of H-terminated SiNWs in low and high pH solution, and the differences between them were obtained. The Si1þ species rapidly increases in the initial oxidation and then stably exists in low pH solution, while it keeps a low concentration in high pH solution during the oxidation process. The fact is rationalized by the possible two different oxidation processes in acid and basic solution. Comparisons of the oxidation behaviors between SiNWs and Si wafers are also made. It is obvious that the stability of H-terminated surfaces of SiNWs in solution was much poorer than that of Si wafer due to the nanoscale defects in SiNWs. Such defects in SiNWs can be reduced by a thermal annealing process which improves the stability of H-terminated of SiNWs in aqueous solutions. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (X.S.);
[email protected] (X.Z.). Notes
§ Visiting student from College of Applied Sciences, Beijing University of Technology, Beijing 100124, China.
’ ACKNOWLEDGMENT The authors thank Junjun Zhu (TEM), Xiaoye Chen (XPS) and Huaixin Wei (XPS) for the experimental support. The authors also thank the referee for valuable comments on the thermal annealing process to improve the stability of H-terminated SiNWs in aqueous solutions. The work was supported by the National Basic Research Program of China (973 Program) (Grant No. 2010CB934500) and the National Nature Science Foundation of China (Grant Nos. 51072127, 50825304, 20971128). ’ REFERENCES (1) Cui, Y.; Zhong, Z. H.; Wang, D. L.; Wang, W. U.; Lieber, C. M. Nano Lett. 2003, 3, 149–152. (2) Cui, Y.; Wei, Q. Q.; Park, H. K.; Lieber, C. M. Science 2001, 293, 1289–1292. (3) Garnett, E. C.; Yang, P. D. J. Am. Chem. Soc. 2008, 130, 9224–9225. (4) Shao, M. W.; Cheng, L.; Zhang, X. H.; Ma, D. D. D.; Lee, S. T. J. Am. Chem. Soc. 2009, 131, 17738–17739. (5) Hochbaum, A. I.; Yang, P. D. Chem. Rev. 2010, 110, 527–546. (6) Teo, B. K.; Sun, X. H. Chem. Rev. 2005, 107, 1454–1532. (7) Sun, X. H.; Wang, S. D.; Wong, N. B.; Ma, D. D. D.; Lee, S. T.; Teo, B. K. Inorg. Chem. 2003, 42, 2398–2404. (8) Ma, D. D. D.; Lee, C. S.; Au, F. C. K.; Tong, S. Y.; Lee, S. T. Science 2003, 299, 1874–1877. (9) Sun, X. H.; Peng, H. Y.; Tang, Y. H.; Shi, W. S.; Wong, N. B.; Lee, C. S.; Lee, S. T.; Sham, T. K. J. Appl. Phys. 2001, 89, 6396–6398. (10) Sun, X. H.; Wong, N. B.; Li, C. P.; Lee, S. T.; Kim, P. S. G.; Sham, T. K. Chem. Mater. 2004, 16, 1143–1152. 3870
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