REP。RTs recorded from the same batch(XRD) pattern 0 58. For the slightly truncated nanocubes (111) facets(28). When PVP was intro- Fig. ID. The x-ray diffracti of sample(sup- illustrated in Fig. ID, the ratio R should have duced, it is believed that the selective inter ported on a silicon substrate as in Fig. 1A)is a value close to 0.7. If PVP was not present, action between PVP and various crystalle also displayed in Fig. ID, and the peaks were the silver atoms generated by reducing silver graphic planes of fcc silver could greatly assigned to diffraction from the(111),(200), nitrate with ethylene glycol nucleated and reduce the growth rate along the <100> and(220) planes of fcc silver, respectively. grew into MTPs bounded by the most stable direction and/or enhance the growth rate The lattice constant calculated from this pat tern was 4.088A, a value in agreement with the literature report(a=4.086A, Joint Com- mittee on Powder Diffraction Standards file no. 04-0783. It is worth noting that the ratio between the intensities of the(200)and(lll) diffraction peaks was higher than the conven tional value(0.67 versus 0. 4), indicating that our nanocubes were abundant in 100) fac- ets, and thus their 100) planes tended to preferentially oriented (or textured) parallel to the surface of the supporting substrate (220) and (111) peaks was also slightly high- 5 8 22895s--1 um 100nm er than the conventional value (0.33 versus 0. 25) because of the relative abundance of (111) (110) facets on the surfaces of our silver oE anocubes product were found to strongly depend reaction conditions such as temperature, the concentration of AgNO3, and the molar ratio between the repeating unit of PVP and AgNO,. For example, when the temperature 5ooo was reduced to120° C or increased to I90°C, the product was dominated by nanoparticles with irregular shapes. The initial concentra- tion of AgNO, had to be higher than 0.1 M otherwise silver nanowires were the major Fig. 1. (A) Low-and (B)high-magnification SEM images of slightly truncated silver nar product. If the molar ratio between the repeat age: ng unit of PVP and AgNO, was increased A TEM image of the same batch of silver nanocubes. The inset shows the diffraction from 1.5 to 3, MTPs became the major prod- cube. (D)An XRn ing the electron beam perpendicular to one of the square faces of an i recorded by al uct. Silver nanocubes of various dimensions a u, arbitrary units. could be obtained by controlling the growth time(25). Figure 2, A and B, show TEM images for 17-and 14-min growth times, and A the nanocubes had a mean edge l15±9and95±7nm, respectively. Figure 2C shows a TEM image of the sample that was synthesized using a lower concentration time(30 min). The mean edge length of these silver nanocubes decreased to 80+ 7 nm 哪世血 nm, Fig. 2D)have also been obtained at a shorter growth time(25 min), although some c of these particles have not been able to evolve into complete cubes. These demonstrations suggest that it is possible to tune the size of silver nanocubes by controlling the experi- As illustrated by Wang(27), the shape of an fcc nanocrystal was mainly de the ratio(r) between the growt <100> and <111> direction and tetrahedra bounded by the most stable Fig. 2. TEM images of silver nanocubes synthesized under different conditions. (A and B)The same 111) planes will be formed when R=1.73, as in Fig. 1, except that the growth time was shortened from 45 min to 17 and 14 min and perfect cubes bounded by the less stable (C and D) The same as in Fig. 1, except that the AgNO, concentration was reduced 1100) planes will result if R is reduced to 0.125 M and the growth time was shortened to 30 and 25 min, respectively. Scale bar www.sciencemag.orgSciEnceVol29813DecemBer2002 2177Fig. 1D. The x-ray diffraction (XRD) pattern recorded from the same batch of sample (sup￾ported on a silicon substrate as in Fig. 1A) is also displayed in Fig. 1D, and the peaks were assigned to diffraction from the (111), (200), and (220) planes of fcc silver, respectively. The lattice constant calculated from this pat￾tern was 4.088 Å, a value in agreement with the literature report (a  4.086 Å, Joint Com￾mittee on Powder Diffraction Standards file no. 04-0783). It is worth noting that the ratio between the intensities of the (200) and (111) diffraction peaks was higher than the conven￾tional value (0.67 versus 0.4), indicating that our nanocubes were abundant in {100} fac￾ets, and thus their {100} planes tended to be preferentially oriented (or textured) parallel to the surface of the supporting substrate (26). The ratio between the intensities of the (220) and (111) peaks was also slightly high￾er than the conventional value (0.33 versus 0.25) because of the relative abundance of {110} facets on the surfaces of our silver nanocubes. The morphology and dimensions of the product were found to strongly depend on reaction conditions such as temperature, the concentration of AgNO3, and the molar ratio between the repeating unit of PVP and AgNO3. For example, when the temperature was reduced to 120°C or increased to 190°C, the product was dominated by nanoparticles with irregular shapes. The initial concentra￾tion of AgNO3 had to be higher than 0.1 M, otherwise silver nanowires were the major product. If the molar ratio between the repeat￾ing unit of PVP and AgNO3 was increased from 1.5 to 3, MTPs became the major prod￾uct. Silver nanocubes of various dimensions could be obtained by controlling the growth time (25). Figure 2, A and B, show TEM images for 17- and 14-min growth times, and the nanocubes had a mean edge length of 115  9 and 95  7 nm, respectively. Figure 2C shows a TEM image of the sample that was synthesized using a lower concentration (0.125 M) of AgNO3 and a shorter growth time (30 min). The mean edge length of these silver nanocubes decreased to 80  7 nm. Silver nanocubes with smaller sizes (50 nm, Fig. 2D) have also been obtained at a shorter growth time (25 min), although some of these particles have not been able to evolve into complete cubes. These demonstrations suggest that it is possible to tune the size of silver nanocubes by controlling the experi￾mental conditions. As illustrated by Wang (27), the shape of an fcc nanocrystal was mainly determined by the ratio (R) between the growth rates along 100 and 111 directions. Octahedra and tetrahedra bounded by the most stable {111} planes will be formed when R  1.73, and perfect cubes bounded by the less stable {100} planes will result if R is reduced to 0.58. For the slightly truncated nanocubes illustrated in Fig. 1D, the ratio R should have a value close to 0.7. If PVP was not present, the silver atoms generated by reducing silver nitrate with ethylene glycol nucleated and grew into MTPs bounded by the most stable {111} facets (28). When PVP was intro￾duced, it is believed that the selective inter￾action between PVP and various crystallo￾graphic planes of fcc silver could greatly reduce the growth rate along the 100 direction and/or enhance the growth rate Fig. 1. (A) Low- and (B) high-magnification SEM images of slightly truncated silver nanocubes synthesized with the polyol process. The image shown in (B) was taken at a tilting angle of 20°. (C) A TEM image of the same batch of silver nanocubes. The inset shows the diffraction pattern recorded by aligning the electron beam perpendicular to one of the square faces of an individual cube. (D) An XRD pattern of the same batch of sample, confirming the formation of pure fcc silver. a.u., arbitrary units. Fig. 2. TEM images of silver nanocubes synthesized under different conditions. (A and B) The same as in Fig. 1, except that the growth time was shortened from 45 min to 17 and 14 min, respectively. (C and D) The same as in Fig. 1, except that the AgNO3 concentration was reduced from 0.25 to 0.125 M and the growth time was shortened to 30 and 25 min, respectively. Scale bars, 100 nm. R EPORTS www.sciencemag.org SCIENCE VOL 298 13 DECEMBER 2002 2177 on September 23, 2011 www.sciencemag.org Downloaded from