J. Phys. Chem. C 2009, 113, 2699–2703
2699
From Borax to Ultralong One-Dimensional Boric Acid Wei Wang, Kezheng Chen, and Zhikun Zhang* Key Laboratory of Nanostructured Materials, Qingdao UniVersity of Science & Technology, Qingdao 266042, People’s Republic of China ReceiVed: September 3, 2008; ReVised Manuscript ReceiVed: December 3, 2008
An extremely facile synthesis route to ultralong boric acid one-dimensional (1D) micron structures was developed, and the anisotropic growth of boric acid was interpreted by the rolling mechanism of layered structures. Studies on the microscale tribological properties of 1D boric acid were done with the help of atomic force and lateral force microscopy techniques, showing the results of excellent lubricating properties of the 1D boric acid micron structures. Introduction One-dimensional (1D) nanomaterials (rods, belts, wires, and cables) have kindled enormous research activity in recent years because of their potential applications in the fields of materials science, biotechnology, and medicine as nanoscale probes, sensors, and switches.1,2 Boric acid is a layered material with a special structure in which the atoms are tightly bonded to each other, whereas the layers themselves are weakly bonded. When the layers are stressed, they shear and slide over one another easily, so friction is low. Scientists at Argonne National Laboratory have discovered that boric acid offers excellent lubricant properties, providing friction coefficients as low as 0.02, without the drawbacks associated with many conventional lubricants.3 New lubricants should be nonflammable, environmentally safe, and easy to apply and remove. Boric acid appears to meet these requirements. Its potential importance for tribological applications demands serious consideration.4 Inspired by the properties mentioned above, the preparation of nanoscaled and microscaled boric acid tubes, tips, and rods has been reported recently.5 By far, a general method employed for the fabrication of boric acid involves crystal precipitation from a boric acid (H3BO3) solution6,7 and films that form spontaneously on boron oxide (B2O3) substrates when they are exposed to humid air.5 Moreover, from the very beginning of the fabrication of carbon nanotubes,8 much evidence has demonstrated that lamellar structures, regardless of whether they are natural or artificial, have a strong tendency to form wires or tubes.9 Onedimensional nanomaterials of compounds with layered structures analogous to that of graphite such as R-Bi,10 WS2,11 MoS2,12 and NiCl213 nanotubes and a series of metal nanowires (W, Ni, Co, Cu, and Cd)9,14 have been made during the past few years. Recently, the self-curling and self-assembling phenomena of boric acid clusters were reported employing the computer-aided molecular design approach, and the resulting predicted structures take the form of a petal, boat, bowl, cage, or tube, which makes boric acid clusters behave similar to the well-studied carbon clusters.15-18 However, this mechanism has not been proven by any experimental witness. In this context, a simple synthesis route to ultralong micron-scaled 1D boric acid structures was * To whom correspondence should be addressed. E-mail: zhangzk@ qust.edu.cn.
Figure 1. XRD pattern of 1D boric acid micron structures.
developed, and the growth mechanism and microscale tribological properties of 1D boric acid were studied in detail. Experimental Section Sample Preparation. In a typical process, borax solution (0.1-2.0 wt %) was added into a certain amount of ascorbic acid under vigorous stirring to regulate the pH value of the solution into the range of 1-5. The resulting homogeneous solution was transferred into a beaker, sealed, and maintained at ambient conditions for about 10 days to allow the complete hydrolysis of borax to boric acid under acidic conditions. The resulting white floccule product was collected by centrifugation and washed with distilled water several times. Sample Characterization. The product was characterized by powder X-ray diffraction (XRD) with a D/max-2500 X-ray diffractometer using Cu KR radiation (λ ) 1.5406 Å). Morphological investigations were carried out with field-emission scanning electron microscopy (FESEM, JEOL JSM-6700F) and transmission electron microscopy (TEM, JEOL JEM-2100F). Microscale Tribological Study. Measurement techniques of friction properties of lubricants by atomic force microscopy and lateral force microscopy (AFM-LFM) have been described elsewhere.19,20 Briefly, a commercially made AFM instrument (Veeco Multimode) equipped with microfabricated cantilevers was used. The torsional deflection of the cantilever is directly proportional to the lateral force acting on the tip, and the torsional force exerted on the cantilever is a measure of the friction between the tip and the sample. Because the torsional
10.1021/jp8078327 CCC: $40.75 2009 American Chemical Society Published on Web 01/22/2009
2700 J. Phys. Chem. C, Vol. 113, No. 7, 2009
Wang et al.
Figure 2. (A) SEM image and (B) TEM image of 1D boric acid micron structures.
Figure 3. SEM images of samples obtained on (A) the second day, (B-D) the fifth day, and (E and F) the 10th day of the growth process of the boric acid micron fibers. The white arrows in panel E indicate the strumaes on the branched structures. The white panes in panel F denote the Y-junctions observed in the final products.
force constant is not known, only voltage signals of torsional deflection will be used in these measurements. The applied load is calculated as the product of the piezo z displacement of the lever and the nominal cantilever force constant (0.12 N/m). By application of various loads on the sample surface, a typical friction-versus-load plot showing the relationship between applied load and frictional force can be obtained, and the slope of the plot indicates the friction coefficient of the sample, assuming that a linear relationship exists between frictional force and applied load. As a reference, the friction characteristics of freshly cleaved mica, which is a well-known solid lubricant,
were also studied using the same tip and lever. All of the experiments reported here were performed with Si3N4 tips acquired from Digital Instruments, and all data were taken with the same cantilever and tip, which is very important for meaningful quantitative comparisons. Results and Discussion Phase and Morphology Analysis. The phase of the products was examined by the XRD pattern of the product (Figure 1). All of the diffraction peaks can be readily indexed as boric acid [space group, P1 (No. 2)], which is in good agreement with the
From Borax to Ultralong 1D Boric Acid
J. Phys. Chem. C, Vol. 113, No. 7, 2009 2701
Figure 4. TEM images of 1D boric acid micron structures showing (A) a curled edge, (B) the transition from an uncurled edge to a curled edge, (C) the coexistence of curled edges and uncurled edges in the same structure, and (D) a tube structure.
Figure 5. AFM height images of 1D boric acid micron structures in AFM tapping mode: (A) 10 µm × 10 µm, (B) 10 µm × 10 µm threedimensional, and (C) 1 µm × 1 µm.
reported data (JCPDS No. 33-0596). The broad diffraction peak located at ∼23° indicates the poor crystallinity of the products. The morphologies and microstructures of the as-prepared products were investigated by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM), presented in panels A and B of Figure 2, respectively. As is shown, microscaled 1D boric acid can be synthesized in large scale. The 1D structures were ultralong, having lengths of more than 600 µm, which virtually correspondes to an aspect ratio greater than 300. The as-prepared 1D boric acid micron
structures were flexible and uniform with diameters ranging from 2 to 4 µm. The high-resolution TEM (HRTEM) image (Figure S1 of the Supporting Information) further confirmed the poor cystallinity of the products. Growth Mechanism Study. Figure 3 outlines the growth process of the 1D boric acid micron structures studied by FESEM of the samples obtained at various stages, which clearly exhibits the evolution of the 1D morphology. As shown in Figure 3A, gemmiform sample rooting from a looming film was obtained on the second day. These preliminary products exhibited the rudiment of 1D morphology with an average length of 1-4 µm and a diameter of about 1 µm. As the process proceeded to the fifth day, longer branched products were obtained with an average length of several tens of micrometers (Figure 3B). From a high-magnification SEM image (Figure 3C), we can see there are many strumaes along the branched structures (indicated by white arrows in Figure 3C). The bottom of an individual branched structure is magnified in Figure 3D, which confirms that the 1D structures originate from platelike ground film. With continuous anisotropic growth of boric acid, ultralong uniform 1D structures were obtained (Figure 3E). As is shown, the final products are much longer than the earlier products (Figure 3A,B) and display many Y-junctions (indicated by white panes in Figure 3F), which may be derived from the strumaes shown in Figure 3C. On the basis of the present results, we concisely explain the growth mechanism of the ultralong 1D boric acid micron structures as follows. First, the hydrolysis of borax to boric acid under acidic conditions led to the nucleation of boric acid
B4O72-+ 2H++ 5H2O ) 4H3BO3 When the nuclei reached the critical size, the growth of boric acid nanostructures and microstructures occurred.5 The 1D
2702 J. Phys. Chem. C, Vol. 113, No. 7, 2009
Wang et al.
Figure 6. Plots of friction versus applied load obtained with a bare Si3N4 tip (k ) 0.12 N/m) on (A) 1D boric acid micron structures and (B) mica.
growth of the boric acid may be derived from the lamellar structure of boric acid. It has been found that boric acid is a layered material with a triclinic crystal structure.21 In each layer, one boron atom is connected to three oxygen atoms to form triangular BO3 groups, and OH · · · O hydrogen bonds link the planar BO3 groups together. The layers are 0.318 nm apart and are held together by weak van der Waals forces. The analogy between the layered structures of boric acid and other layered compounds such as graphite, black phosphorus, WS2, and MoS2, which have been fabricated into 1D nanostructures successfully, suggests to us that the 1D growth of boric acid may result from the rolling of layered boric acid as has been predicted theoretically, employing the computer-aided molecular design approach.19,20 In fact, we indeed observed curled edges of platelike boric acid during TEM observation (Figure 4A). In addition, there is more direct evidence for the rolling mechanism found in a halftube and half-plate structure (Figure 4B,C). As shown in Figure 4B, curling that occurred at the edge was clearly observed in zone A; while in zone B, only platelike structures could be observed, and in zone C, transition from an uncurled edge to a curled edge is found. Figure 4C clearly shows the coexistence of curled edges (zone A) and uncurled edges (zone B) in the same structure. As has been mentioned before, lamellar structures have a strong tendency to form tubes as well as wires. During TEM observation, boric acid tubes were also occasionally observed (Figure 4D), which further confirms the assumption of the rolling mechanism for the 1D growth of boric acid structures. Further experiments demonstrated that ambient conditions such as the pH value and temperature of the system influence the morphology of the products. A proper temperature ranging from 10 to 30 °C was found to be appropriate for the formation of uniform 1D boric acid structures with high yield. When the temperature was below 0 °C, almost no product could be obtained, keeping other conditions constant. In addition, less uniform micron fibers were obtained when the pH was greater than 5 (Figure S2A of the Supporting Information). Uniform micron structures were obtained when the acids used were selected from ascorbic acid, nitric acid (HNO3), and hydrochloric acid (HCl), whereas sausagelike micron fibers were prepared when citric acid was used (Figure S2B of the Supporting Information), keeping other conditions constant. Microscale Tribology Analysis. Surface roughness and microscale tribological properties of the as-prepared 1D boric acid were investigated by AFM-LFM. AFM-LFM can be used
to measure normal and lateral forces on the nanometer scale and thus represents a powerful tool for probing the microscopic mechanisms of friction.22-27 In AFM-LFM, it is possible to simultaneously measure topography and frictional force images. The topography image is derived from monitoring the vertical forces on the cantilever. The frictional image is acquired simultaneously by monitoring the lateral motions of the cantilever. Panels A-C of Figure 5 are typical tapping mode AFM images obtained from boric acid micron structures. Surface roughness values were calculated from 10 surface images using Nanoscope (R) III software (version 5.12r2, Digital Instruments, Veeco Instruments, Inc.), and the results showed that the values of surface roughness average (Ra) and roughness root-meansquare (Rq) are 3.051 and 3.764 nm, respectively. Panels A and B of Figure 6 give ∆V versus load plots of 1D boric acid and mica, respectively, in which ∆V is half of the difference between the friction voltage signals in trace and retrace images. From the linear fitting results shown in panels A and B of Figure 6, we can derive that the coefficient value of mica (i.e., the slope of the line in panel B of Figure 6) in our measurement is close to the reported values (with an average value of 0.045),28-30 confirming the reliability of our experiment. In addition, the coefficient of 1D boric acid (i.e., the slope of the line in panel A of Figure 6) is 0.010, much lower than that of mica, demonstrating the expected excellent lubricating properties of boric acid. Conclusion In summary, we present an extremely facile synthesis method for the fabrication of ultralong uniform 1D boric acid micron structures in large scale. The growth mechanism of the 1D morphology was interpreted as a rolling mechanism ascribed to the layered structure of boric acid. Studies on the surface roughness and microscale tribological properties of this system were done with the help of AFM-LFM techniques, showing the results of the excellent lubricating properties of the boric acid micron structures. This method may provide a simple, economical, and completely green route to the fabrication of the nanoscaled and microscaled boric acid materials, which may find potential applications in tribology and in the manufacture of lubricants, brake fluids, metalworking fluids, water treatment chemicals, and fuel additives. Supporting Information Available: Characterization methods, HRTEM, and additional SEM and TEM images of 1D boric acid micron structures. This material is available free of charge via the Internet at http://pubs.acs.org.
From Borax to Ultralong 1D Boric Acid References and Notes (1) Alivisatos, A. P. Science 1996, 271, 933–937. (2) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, H. Q. AdV. Mater. 2003, 15, 353–398. (3) Erdemir, A.; Fenske, G. R. Article title. SAE International Congress & Exposition, Detroit, MI, 1998. (4) Wei, J. J.; Erdemir, A.; Fenske, G. R. Article title. STLE/ASME Tribology Conference, Kissimmee, FL, 1999. (5) Li, Y.; Ruoff, R. S.; Chang, R. P. H. Chem. Mater. 2003, 15, 3276– 3285. (6) Ma, X. D.; Unertl, W. N. J. Mater. Res. 1999, 14, 3455–3466. (7) Prioli, R.; Ponciano, C. R.; Freire, F. L. Appl. Phys. A: Mater. Sci. Process. 2000, 71, 233–236. (8) Iijima, S. Nature 1991, 354, 56–58. (9) Wang, J. M.; Li, Y. D. AdV. Mater. 2003, 15, 445–454. (10) Li, Y. D.; Wang, J. M.; Deng, Z. X.; Wu, Y. Y.; Sun, X. M.; Yu, D. P.; Yang, P. D. J. Am. Chem. Soc. 2001, 123, 9904–9905. (11) Li, Y. D.; Li, X. L.; He, R. R.; Zhu, J.; Deng, Z. X. J. Am. Chem. Soc. 2002, 124, 1411–1416. (12) Tenne, R.; Margulis, L.; Genut, M.; Hodes, G. Nature 1992, 360, 444–446. (13) Hachohen, Y. R.; Grunbaum, E.; Sloan, J.; Hutchison, J. L.; Tenne, R. Nature 1998, 395, 336–337. (14) Li, Y. D.; Lin, X. L.; Deng, Z. X.; Zhou, B. C.; Fan, S. S.; Wang, J. W.; Sun, X. M. Angew. Chem. 2002, 114, 343–345. (15) Wang, W. Z.; Zhang, Y.; Huang, K. X. Chem. Phys. Lett. 2005, 405, 425–428.
J. Phys. Chem. C, Vol. 113, No. 7, 2009 2703 (16) Wang, W. Z.; Zhang, Y.; Huang, K. X. J. Phys. Chem. B 2005, 109, 8562–8564. (17) Enyashin, A. N.; Ivanovskii, A. L. Chem. Phys. Lett. 2005, 411, 186–191. (18) Elango, M.; Parthasarathi, R.; Subramanian, V.; Sathyamurthy, N. J. Phys. Chem. A 2005, 109, 8587–8593. (19) Xiao, X. D.; Hu, J.; Charych, D. H.; Salmeron, M. Langmuir 1996, 12, 235–237. (20) Li, J. W.; Wang, C.; Shang, G. Y.; Xu, Q. M.; Zhang, L.; Guan, J. J.; Bai, C. L. Langmuir 1999, 15, 7662–7669. (21) Erdemir, A. Lubr. Eng. 1991, 47, 168–173. (22) McClelland, G. M.; Erlandsson, R.; Chiang, S. Phys. ReV. Lett. 1987, 59, 1942–1945. (23) Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071–2074. (24) Noy, A.; Frisbie, C. D.; Rozsnyai, L. F.; Wrighton, M. S.; Lieber, C. M. J. Am. Chem. Soc. 1995, 117, 7943–7951. (25) Bhushan, B.; Israelachvili, J. N.; Landman, U. Nature 1995, 374, 607–616. (26) Rozman, M. G.; Urbakh, M.; Klafter, J.; Elmer, F.-J. J. Phys. Chem. B 1998, 102, 7924–7930. (27) Carpick, R. W.; Salmeron, M. Chem. ReV. 1997, 97, 1163–1169. (28) Liu, E.; Blanpain, B.; Celis, J. P. Wear 1996, 192, 141–150. (29) Labardi, M.; Allegrini, M.; Salerno, M.; Frediani, C.; Ascoli, C. Appl. Phys. A: Mater. Sci. Process. 1994, 59, 3–10. (30) Liu, E.; Blanpain, B.; Celis, J. P.; Roos, J. R. J. Appl. Phys. 1998, 84, 4859–4865.
JP8078327
