NOTES Chinese Science Bulletin Vol. 46 No. 15 August 2001 1317 Evaluation of diameter dis- tribution of inside cavities of open CNTs by analyses of nitrogen cryo-adsorption isotherm YANG Quanhong, LI Feng, HOU Pengxiang, LIU Chang, LIU Min & CHENG Huiming Institute of Metal Research, Chinese Academy of Sciences Shenyang 110015, China Correspondence should be addressed to Cheng Huiming (e-mail: cheng @imr.ac.cn) Abstract Precise evaluation of innerdiameter distribu- tion of open carbon nanotubes (CNTs) is the basis of syste- matic investigation of physico-chemical processes occurring in nano-sized quasi-1-dimensional carbons. Due to the po- rosity characteristics and adsorption properties, this study evaluated the innerdiameter and its distribution of carbon nanotubes by analyses of nitrogen cryo-adsorption iso- therms, and proved that the gas adsorption method is an effective method to characterize the inner cavity structure in comparison with that of electron microscopy observations and Raman measurements. The advantages of this method are as follows: Firstly, statistical information for inner- diameter distribution of open nanotubes can be obtained; Secondly, the method based on the adsorption process in inner cavities is of importance for investigation of other physico-chemical pro- cesses inside the cavities of carbon nanotubes. And finally, if combining with other characteri- zation methods, complete structural information for cavity can be acquired and these basic parameters are important for theoretical investigations and practical applications of carbon nanotubes. Keywords: carbon nanotubes (CNTs), innerdiameter distribution, gas adsorption method, adsorption isotherm. Since their discovery in 1991, carbon nanotubes have been found many interesting properties and poten- tial applications. Many physical or chemical properties of nanotubes are connected with the smooth, straight, 1- dimensional channel in their cores. Experiments and calculations showed that the inside channels can hold atoms or molecules of other species by capillarity or adsorption, which thus brings about some unique possi- bilities of using nanotubes as super-adsorbent (e.g. high hydrogen adsorption capacity), nano-sized container, reactor and templates for fabrication of novel 1- dimensional nanomaterials such as nanowires [1 4] . In particular, recent tremendous interests have been aroused by the discovery and reproduction [5 9] of high hydrogen adsorption capacity in carbon nanotubes and other low- dimensional carbon materials. Most recently, it was dis- covered that trace gas adsorption was able to modify the electric properties of nanotubes, which may lead to cre- ating new types of chemical sensors [10,11] . And it is be- lieved that inner cavity diameter determines the physico- chemical process, and the precise evaluations of diameter distributions are of great importance for understanding the process initiated in the quasi- 1-dimensional inside channel of these carbon nano-mate- rials. By now, the main methods to evaluate the diameter distributions of carbon nanotubes can be divided into two categories. One is the direct microscopy observation [12,13] (e.g. high resolution transmission electron microscopy (HRTEM)), and the other is the measurement of the ra- dial breathing mode (RBM) frequencies at a variety of laser excitation energies, which provides the other tool for the measurement of the diameter distribution of sin- gle-walled carbon nanotubes (SWNTs) due to the depen- dence of the RBM frequency on the inverse nanotube diameter [14,15] . The former method is a sampling survey method, and it is not suitable for obtaining complete in- formation especially for a large amount of samples; the latter can only be adapted to SWNTs, but not for multi- walled carbon nanotubes (MWNTs). With the develop- ment of large-scale production methods and the investi- gation on the physico- chemical process in the inside channel of these novel materials, an effective inner- diameter analysis method applicable to a large amount of