REP。RTs 8. L Rustad, Nature 413, 578(2001). calculated as the product of 12.7% of the cumulative °sedh10 and again in 1999. Samples were analyze increase in net nitrogen 10.w CHN analyzer. Concentrations of inorganic nitrogen 11 wea ter tbe ihing siow the reting alone were meas: 18. This conclusion is based on a paired student'st test with aggregated data from the six experimental pling the headspace at s-min intervals Samples were ears of the experiment, then once monthly. Ly Indmark, B. Elfving, For. Ecol. iy n were ree d na sas ate het fuang ters were placed at a depth of 50 cm and evacuated to 15 inches of mercury for 24 hours before sampling. 20. P. Jarvis, S. Linder, Nature 405, 904(2000) 12.cHRm过40481o tatic chan measured 22. Y. Luo, S Wan, SHui, L Wallace, Nature 413,622 (2001 13. E.A. Davidson, S E. Trumbore, R. Amundson, Nature analyzed 23. J. Merriam, W. H. McDowell, w.S. Currie, Soil trace gas concentrations by gas chromatography. soc.Am.J60.1050(1996) 14. Net nitroge d the changes in concentration were used to cal- 24. Supported by the Office of Science, Biological and buried bag incubations Incubations were for 6 15. During 1996, we analyzed the lysimeter water sar of Energ collected and analyzed for extractable NH4+ an stimated as the difference between total nd analysis with standard autoanalyzer methods trogen(TDN)and dissolved inorganic nitrogen, Program(contract no. EPA-CR 823713-01-0): and th 8 -catalyzed combustion(23) between initial and incubated soils miner- 16. A H Magill et al, Ecosystems 3, 238(2000) 8 through 17. This additional carbon storage in woody tissue wi May 2002; accepted 7 November 2002 Shape-Controlled Synthesis of The primary reaction involved the reduc- tion of silver nitrate with ethylene glycol at o Gold and silver Nanoparticles the ethylene glycol served as both reductant o and solvent. We recently demonstrated that Yugang Sun and Younan xia this reaction could yield bicrystalline silver anowires in the presence of a capping re- monodisperse samples of silver nanocubes were synthesized in large quantities ent such as poly( vinyl pyrrolidone)(PVP) by reducing silver nitrate with ethylene glycol in the presence of poly(vinyl (24). Subsequent experiments suggested that pyrrolidone)(PVP). These cubes were single crystals and were characterized by the morphology of the product had a st a slightly truncated shape bounded by (100), (110), and (111]facets. The dependence on the reaction conditions. When presence of PVP and its molar ratio(in terms of repeating unit relative to silver the concentration of AgNO, was increased by nitrate both played important roles in determining the geometric shape and size a factor of 3 and the molar ratio between the of the product. The silver cubes could serve as sacrificial templates to generate repeating unit of PVP and AgNO, was kept at E single-crystalline nanoboxes of gold: hollow polyhedra bounded by six (100) and eight [111 facets. Controlling the size, shape and structure of metal nanoparticles is technologically important because of the strong correlation canning electron microscope(SEM) images t between these parameters and optical, electrical, and catalytic properties of a typical sample of silver nanocubes and o ndicate the large quantity and good unifor Metal nanoparticles play important roles in Many metals can now be processed into ity that were achieved using this approach. 3 many different areas. For example, they can monodisperse nanoparticles with controllable These silver nanocubes had a mean edge serve as a model system to experimentally composition and structure(13)and some- length of 175 nm, with a standard deviation probe the effects of quantum confinement on times can be produced in large quantities of 13 nm. Their surfaces were smooth, and electronic, magnetic, and other related proper- through solution-phase methods(14, 15). De- some of them self-assembled into ordered ties(1-3). They have also been widely exploit- spite this, the challenge of synthetically con- two-dimensional(2D) arrays on the silicon ed for use in photography (4), catalysis(5), trolling the shape of metal nanoparticles has substrate when the SEM sample was biological labeling(6), photonics (7), optoel been met with limited success. On the nano- pared. It is also clear from Fig. 1B that all tronics(&), information storage (9), surface- meter scale, metals(most of them are face- comers and edges of these nanocubes were enhanced Raman scattering (SERS)(10, ID), centered cubic, or fcc) tend to nucleate and slightly truncated Figure IC shows the trans- and formulation of magnetic ferrofluids(12). grow into twinned and multiply twinned par- mission electron microscope(TEM)image of The intrinsic properties of a metal nanoparticle ticles(MTPs) with their surfaces bounded by an array of silver nanocubes self-assembled are mainly determined by its size, shape, com- the lowest-energy(111) facets(16). Other on the surface of a TEM grid. The inset position, crystallinity, and structure(solid ver- morphologies with less stable facets have shows the electron diffraction pattern ob- sus hollow ). In principle, one could control any only been kinetically achieved by adding tained by directing the electron beam perpen one of these parameters to fine-tune the prop- chemical capping reagents to the synthetic dicular to one of the square faces of a cube erties of this nanoparticle. systems(17-22). Here we describe a solu- The square symmetry of this pattern indicates tion-phase route to the large-scale synthesis that each silver nanocube was a single ci ment of Chemistry, University of Washington, of silver nanocubes. Uniform gold nanoboxes bounded mainly by 1100) facets. On the Seattle wa 98195-1700 USA. with a truncated cubic shape were also gen- basis of these SEM and TEM studies, it * To whom correspondence should be addressed. E- erated by reacting the silver cubes with an clear that the slightly truncated nanocube mail xia@chem aqueous HAuCla solution could be described by the drawing shown in 2176 13DecemBer2002Vol298SciEncewww.sciencemagorg8. L. Rustad, Nature 413, 578 (2001). 9. W. T. Peterjohn, J. M. Melillo, F. P. Bowles, P. A. Steudler, Oecologia 93, 18 (1993). 10. W. T. Peterjohn, J. M. Melillo, P. A. Steudler, K. M. Newkirk, Ecol. Appl. 4, 617 (1994). 11. CO2 flux measurements were made by placing cham￾ber lids over anchored collars for 15 min and sam￾pling the headspace at 5-min intervals. Samples were analyzed for trace gas concentrations by gas chro￾matography or infrared analysis, and the changes in concentration were used to calculate net flux rates. On each sampling date, fluxes were measured at early morning and afternoon intervals. 12. J. Grace, M. Rayment, Nature 404, 819 (2000). 13. E. A. Davidson, S. E. Trumbore, R. Amundson, Nature 408, 789 (2000). 14. Net nitrogen mineralization was measured for the organic horizon and upper 10 cm of mineral soil using in situ buried bag incubations. Incubations were for 6 weeks at a time, from April through November, and for 4 months during the winter. Initial samples were collected and analyzed for extractable NH4
and NO3 – content (extraction with 2N KCl for 48 hours and analysis with standard autoanalyzer methods). The same analysis was carried out on the incubated samples. The difference in total mineral N content between initial and incubated soils is the net miner￾alization rate. Soil nitrogen was assessed through sampling of organic and mineral soils in all plots in 1992 and again in 1999. Samples were analyzed for carbon and nitrogen content with a Perkin-Elmer CHN analyzer. Concentrations of inorganic nitrogen in water leaching below the rooting zone were mea￾sured with high-tension lysimetry. Soil water samples were collected from one porous cup lysimeter per plot on two occasions every month for the first 2 years of the experiment, then once monthly. Lysim￾eters were placed at a depth of 50 cm and evacuated to 15 inches of mercury for 24 hours before sampling. Samples were frozen until they were analyzed for NH4
and NO3 –. Nitrous oxide fluxes were measured along with CO2, with the same static chamber meth￾od, from 1991 through 1995. Samples were analyzed for trace gas concentrations by gas chromatography, and the changes in concentration were used to cal￾culate net flux rates. 15. During 1996, we analyzed the lysimeter water sam￾ples for dissolved organic nitrogen (DON). We found very low levels of DON in lysimeters from all treat￾ments with no clear treatment differences. DON was estimated as the difference between total dissolved nitrogen (TDN) and dissolved inorganic nitrogen, where TDN was measured by high-temperature plati￾num–catalyzed combustion (23). 16. A. H. Magill et al., Ecosystems 3, 238 (2000). 17. This additional carbon storage in woody tissue was calculated as the product of 12.7% of the cumulative increase in net nitrogen mineralization over the de￾cade (41 g of nitrogen m2) and the measured carbon:nitrogen mass ratio of the wood (300 :1) as follows: 0.127 4 g of nitrogen m2 300. 18. This conclusion is based on a paired Student’s t test with aggregated data from the six experimental blocks. 19. J. Bergh, S. Linder, T. Lundmark, B. Elfving, For. Ecol. Manage. 119, 51 (1999). 20. P. Jarvis, S. Linder, Nature 405, 904 (2000). 21. J. Harte et al. Ecol. Appl. 5, 132 (1995). 22. Y. Luo, S. Wan, S. Hui, L. Wallace, Nature 413, 622 (2001). 23. J. Merriam, W. H. McDowell, W. S. Currie, Soil Sci. Soc. Am. J. 60, 1050 (1996). 24. Supported by the Office of Science, Biological and Environmental Research Program, U.S. Department of Energy, through the Northeast Regional Center of the National Institute for Global Environmental Change under cooperative agreement no. DE-FC03- 90ER61010; NSF’s Long-Term Ecological Research Program (contract no. NSF-DEB 0080592); the U.S. Environmental Protection Agency’s Global Change Program (contract no. EPA-CR 823713-01-0); and the ExxonMobil Corporation. 20 May 2002; accepted 7 November 2002 Shape-Controlled Synthesis of Gold and Silver Nanoparticles Yugang Sun and Younan Xia* Monodisperse samples of silver nanocubes were synthesized in large quantities by reducing silver nitrate with ethylene glycol in the presence of poly(vinyl pyrrolidone) (PVP). These cubes were single crystals and were characterized by a slightly truncated shape bounded by {100}, {110}, and {111} facets. The presence of PVP and its molar ratio (in terms of repeating unit) relative to silver nitrate both played important roles in determining the geometric shape and size of the product. The silver cubes could serve as sacrificial templates to generate single-crystalline nanoboxes of gold: hollow polyhedra bounded by six {100} and eight {111} facets. Controlling the size, shape, and structure of metal nanoparticles is technologically important because of the strong correlation between these parameters and optical, electrical, and catalytic properties. Metal nanoparticles play important roles in many different areas. For example, they can serve as a model system to experimentally probe the effects of quantum confinement on electronic, magnetic, and other related proper￾ties (1–3). They have also been widely exploit￾ed for use in photography (4), catalysis (5), biological labeling (6), photonics (7), optoelec￾tronics (8), information storage (9), surface￾enhanced Raman scattering (SERS) (10, 11), and formulation of magnetic ferrofluids (12). The intrinsic properties of a metal nanoparticle are mainly determined by its size, shape, com￾position, crystallinity, and structure (solid ver￾sus hollow). In principle, one could control any one of these parameters to fine-tune the prop￾erties of this nanoparticle. Many metals can now be processed into monodisperse nanoparticles with controllable composition and structure (13) and some￾times can be produced in large quantities through solution-phase methods (14, 15). De￾spite this, the challenge of synthetically con￾trolling the shape of metal nanoparticles has been met with limited success. On the nano￾meter scale, metals (most of them are face￾centered cubic, or fcc) tend to nucleate and grow into twinned and multiply twinned par￾ticles (MTPs) with their surfaces bounded by the lowest-energy {111} facets (16). Other morphologies with less stable facets have only been kinetically achieved by adding chemical capping reagents to the synthetic systems (17–22). Here we describe a solu￾tion-phase route to the large-scale synthesis of silver nanocubes. Uniform gold nanoboxes with a truncated cubic shape were also gen￾erated by reacting the silver cubes with an aqueous HAuCl4 solution. The primary reaction involved the reduc￾tion of silver nitrate with ethylene glycol at 160°C. In this so-called polyol process (23), the ethylene glycol served as both reductant and solvent. We recently demonstrated that this reaction could yield bicrystalline silver nanowires in the presence of a capping re￾agent such as poly(vinyl pyrrolidone) (PVP) (24). Subsequent experiments suggested that the morphology of the product had a strong dependence on the reaction conditions. When the concentration of AgNO3 was increased by a factor of 3 and the molar ratio between the repeating unit of PVP and AgNO3 was kept at 1.5, single-crystalline nanocubes of silver were obtained (25). Figure 1, A and B, show scanning electron microscope (SEM) images of a typical sample of silver nanocubes and indicate the large quantity and good unifor￾mity that were achieved using this approach. These silver nanocubes had a mean edge length of 175 nm, with a standard deviation of 13 nm. Their surfaces were smooth, and some of them self-assembled into ordered two-dimensional (2D) arrays on the silicon substrate when the SEM sample was pre￾pared. It is also clear from Fig. 1B that all corners and edges of these nanocubes were slightly truncated. Figure 1C shows the trans￾mission electron microscope (TEM) image of an array of silver nanocubes self-assembled on the surface of a TEM grid. The inset shows the electron diffraction pattern ob￾tained by directing the electron beam perpen￾dicular to one of the square faces of a cube. The square symmetry of this pattern indicates that each silver nanocube was a single crystal bounded mainly by {100} facets. On the basis of these SEM and TEM studies, it is clear that the slightly truncated nanocube could be described by the drawing shown in Department of Chemistry, University of Washington, Seattle, WA 98195–1700, USA. *To whom correspondence should be addressed. E￾mail: xia@chem.washington.edu R EPORTS 2176 13 DECEMBER 2002 VOL 298 SCIENCE www.sciencemag.org on September 23, 2011 www.sciencemag.org Downloaded from