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lsf_eclab木虫 (小有名气)
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厦门大学孙世刚课题组长篇研究论文登上美国《科学》杂志
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厦门大学化学化工学院孙世刚和美国佐治亚理工学院王中林等科学家采用一种新的电化学方法，首次制备出具有高表面能的二十四面体铂纳米晶粒催化剂，显著提高了铂纳米催化剂的活性和稳定性，在能源、催化、材料、化工等领域具有重大的意义和应用价值。5月4日出版的美国《科学》杂志以三页半篇幅的长篇报道刊登了这项最新成果。这是我校第一篇发表在《科学》杂志上的长篇研究报告。 该研究发展了一种新的电化学方法，能够控制纳米晶体的表面结构和生长，合成具有高表面能的金属纳米晶体，突破了传统化学法只能合成低表面能金属纳米晶体的局限。 提起铂，人们并不陌生，不过，铂的最大用途并不是做成漂亮的白金首饰。铂纳米材料是燃料电池、石油化工、汽车尾气净化等领域中广泛使用的催化剂，全世界用于催化剂的铂每年近100吨，占总产量的一半，价值高达40多亿美元。由于铂的资源匮乏，价格高昂，如何进一步提高铂纳米材料的催化活性、稳定性和利用效率是氢能源和相关领域发展的重大关键问题。 然而，科学家面临的挑战是：传统的化学法只能合成低表面能的金属纳米晶体，一般是立方体、八面体或是截角八面体，其表面为“低指数”晶面结构。科学家认为，“高指数”晶面结构围成的多面体的铂纳米晶粒催化剂能提高催化效率。二十四面体是一种十分罕见的晶体形状，其表面通常由高指数晶面围成。迄今从未见到人工合成二十四面体金属纳米晶体的报道。在自然界中，也仅发现金刚石、萤石和铜矿等极少数矿物能以不完美的二十四面体形式存在。 孙世刚说：“我们在研究中发现，铂族金属单晶的高指数晶面不仅催化活性很高、而且稳定性好，这是因为高指数晶面具有开放的表面结构和高密度的台阶原子,而且处于短程有序环境。因此，制备表面为高指数晶面结构的铂纳米晶粒，是显著提高催化剂活性和稳定性的重要途径。但是，铂晶体在生长过程中沿高指数晶面方向的生长速度远快于沿低指数晶面方向，导致高指数晶面趋于消失，最终生成的铂晶体表面主要由原子排列密集的低能低指数晶面组成。” 孙世刚教授与其博士生田娜和周志有博士，利用高指数晶面在氧化条件下稳定性高的特点，通过方波电位产生的周期性氧化/还原的驱动，调控铂纳米晶体生长过程的表面结构，首次制备出高指数晶面结构的二十四面体铂纳米晶体。同时控制条件，可使铂纳米晶体的尺度在二十到几百纳米内变化。 王中林教授和丁勇博士通过高分辨电子显微镜研究证实，所制备的二十四面体铂纳米晶体由高指数晶面围成，并具有很高的热稳定性，可耐高达800摄氏度的高温。 该中——美团队的电催化研究还证实，二十四面体铂纳米晶体具有很高的催化活性：以单位铂表面积来计算，它对甲酸、乙醇等有机小分子燃料电氧化的催化活性是目前商业铂纳米催化剂的2到4倍，显示其在燃料电池、电催化等领域中的重大应用价值。 孙世刚是国家杰出青年科学基金和国家级教学名师奖获得者，长期从事电化学和电催化研究。王中林是佐治亚理工学院杰出讲座教授和厦门大学客座教授，是著名的电子显微学和纳米材料学专家。上述研究是在孙世刚小组前期工作基础上，两个研究小组一年多紧密合作的结果。 孙世刚深有体会地介绍，在世界顶级的学术期刊《科学》上发表论文是非常不容易的。其实，早在2005年，他与博士生田娜和周志有博士，就已经制备出了这种二十四面体金属纳米晶体，但课题组并未急着发表论文，而是又花了两年功夫静下心来进一步追踪弄清楚这种晶体的生长过程、表征，并不断完善研究方法。论文前后修改了二十多次，博士生田娜还为此申请延期二年毕业。今年一月底，这一中美团队将研究成果投稿到《科学》，三月份《科学》就发来了用稿通知，这其间课题组还与杂志的编辑做了反复沟通，并就编辑所提问题进行了补充说明。 孙世刚和王中林认为该研究所发展的表面结构控制生长的电化学方法可以拓展到其它铂族金属，如钯、铑等，也可以运用到制备其它高指数晶面组成的不同形状的金属纳米晶体，深化了对金属晶体生长规律的认识；该研究不仅开辟了一条通过控制纳米粒子表面原子排列结构提高催化剂性能的崭新途径，也是将模型电催化剂的基础研究推进到实际催化剂设计和研制过程中的一个重大进展。 《科学》杂志的三位评审人也对该工作的原创性和重要性给予高度的评价。认为这一科研成果不仅指明了一种控制纳米粒子生长使高指数晶面暴露在外的新思路和新方法，而且将导致异相催化中的新发现。 孙世刚研究小组得到国家自然科学基金委、教育部、科技部、厦门大学以及固体表面物理化学国家重点实验室的长期支持和资助，还得到厦门大学“优秀博士学位论文”计划的支持。 [ Last edited by 小红豆 on 2007-6-12 at 22:53 ] |
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lsf_eclab
木虫 (小有名气)
- 应助: 0 (幼儿园)
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4楼2007-05-11 19:18:31
lsf_eclab
木虫 (小有名气)
- 应助: 0 (幼儿园)
- 金币: 2311
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近日，国际化学界再次将目光聚焦到厦门大学。5月4日出版的美国《科学》杂志以长篇报告的形式发表了厦门大学化学化工学院、固体表面物理化学国家重点实验室孙世刚教授等关于铂纳米催化剂的最新成果。同期杂志还配发了美国科罗拉多大学化学和生物化学系纳米材料专家丹 L. 费尔德门教授的题为“催化剂的新面孔”的评论，他指出，未来的能源安全很大程度上取决于化学家和材料科学家能够针对生产和应用替代燃料，发现更有效、更安全和更经济的催化剂。美国能源部将此确定为重大挑战，而深入认识催化剂结构和化学性能的关系，开发先进的纳米尺度材料合成技术是关键。他认为孙世刚等人的最新成果是纳米尺度催化剂合成的一个重大突破。 论文发表后，在国际学术界迅速引起重大反响，美国《化学工程新闻》周刊、《每日科学》、英国的《新科学家》和英国皇家化学会的《化学世界》等国际主流科技媒体都在第一时间，分别以“多面体催化剂－铂多面体的催化性能胜过其球形表兄”，“铂纳米晶体提升了其在燃料氧化、氢能源中的催化活性”，“纳米晶体‘珍宝’是高性能化学催化剂”和“铂的多面”等为题，报道和评述了这项重要研究成果。我国科学网和《科学时报》在第一时间和头版头条发表了题为“科学家首次制备二十四面体铂纳米晶体：显著提高了铂纳米催化剂的活性和稳定性”的报道，5月9日的《科技日报》也在第一版显要位置刊发了消息。人民网等门户网站也做了报道。此外，一些重要学术机构，如美国科学促进会、美国材料研究学会、麻省理工学院和著名的国际催化剂制备公司以及中国科学院和国家自然科学基金委等的网站也在第一时间转载和评述了这项成果。 孙世刚是1995年度国家杰出青年科学基金和2006年度国家级教学名师奖获得者。他在电化学和电催化研究所取得的原创性研究成果为国内外同行广泛认可。2007年，孙世刚当选国际电化学会特别会员。这项荣誉专门授予为电化学科学做出杰出贡献的科学家，至今全世界仅有24人获此称号，孙世刚是其中唯一的中国学者。 [ Last edited by lsf_eclab on 2007-5-11 at 13:59 ] |
2楼2007-05-11 13:54:00
3楼2007-05-11 16:32:30
lsf_eclab
木虫 (小有名气)
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小红豆(金币+5):感谢你的好资源!辛苦你了!
小红豆(金币+5):感谢你的好资源!辛苦你了!
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Science 4 May 2007: Vol. 316. no. 5825, pp. 732 - 735 DOI: 10.1126/science.1140484 Prev | Table of Contents | Next Reports Synthesis of Tetrahexahedral Platinum Nanocrystals with High-Index Facets and High Electro-Oxidation Activity Na Tian,1 Zhi-You Zhou,1 Shi-Gang Sun,1* Yong Ding,2 Zhong Lin Wang2* The shapes of noble metal nanocrystals (NCs) are usually defined by polyhedra that are enclosed by {111} and {100} facets, such as cubes, tetrahedra, and octahedra. Platinum NCs of unusual tetrahexahedral (THH) shape were prepared at high yield by an electrochemical treatment of Pt nanospheres supported on glassy carbon by a square-wave potential. The single-crystal THH NC is enclosed by 24 high-index facets such as {730}, {210}, and/or {520} surfaces that have a large density of atomic steps and dangling bonds. These high-energy surfaces are stable thermally (to 800°C) and chemically and exhibit much enhanced (up to 400%) catalytic activity for equivalent Pt surface areas for electro-oxidation of small organic fuels such as formic acid and ethanol. 1 State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China. 2 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332–0245, USA. * To whom correspondence should be addressed. E-mail: sgsun@xmu.edu.cn (S.G.S.); zhong.wang@mse.gatech.edu (Z.L.W.) Generally, catalytic performance of nanocrystals (NCs) can be finely tuned either by their composition, which mediates electronic structure (1, 2), or by their shape, which determines surface atomic arrangement and coordination (3, 4). Fundamental studies of single-crystal surfaces of bulk Pt have shown that high-index planes generally exhibit much higher catalytic activity than that of the most common stable planes, such as {111}, {100}, and even {110}, because the high-index planes have a high density of atomic steps, ledges, and kinks, which usually serve as active sites for breaking chemical bonds (5–7). For example, a bulk Pt(210) surface possesses extremely high catalytic reactivity for electroreduction of CO2 (8) and electro-oxidation of formic acid (9). The bulk Pt(410) surface exhibits unusual activity for catalytic decomposition of NO, a major pollutant of automobile exhaust (10). Thus, the shape-controlled synthesis of metal NCs bounded by high-index facets is a potential route for enhancing their catalytic activities. It is, however, rather challenging to synthesize shape-controlled NCs that are enclosed by high-index facets because of their high surface energy. Crystal growth rates in the direction perpendicular to a high-index plane are usually much faster than those along the normal direction of a low-index plane, so high-index planes are rapidly eliminated during particle formation (11). During the past decade, a variety of face-centered cubic (fcc) structured metal NCs with well-defined shapes have been synthesized, but nearly all of them are bounded by the low-index planes, such as tetrahedron, octahedron, decahedron, and icosahedron, enclosed by {111} facets (12–14), cube by {100} (12, 15), cuboctahedron by {111} and {100} (16), and rhombic dodecahedron by {111} (17). Here we describe an electrochemical method for the synthesis of tetrahexahedral (THH) Pt NCs at high purity. The THH shape is bounded by 24 facets of high-index planes {730} and vicinal planes such as {210} and {310}. The synthesized THH Pt NCs show enhanced catalytic activity in electro-oxidation of small organic fuels of formic acid and ethanol, demonstrating their potential for use in the traditional applications of Pt group metal nanoparticles, including catalysts (18), automotive catalytic converters (19), fuel cells (20), and sensors (21). Several electrochemical methods have been reported for the synthesis of Pt NCs. Sun and co-workers used a fast potential cycling to grow nanostructured film on a Pt microelectrode surface (22), and Arvia et al. employed an electrochemical square-wave potential route to shape bulk Pt electrode (23), which could yield several high-index planes of bulk crystals in some cases (24). However, these methods have limited practical applications because of a low specific surface area and the high cost of the bulk Pt electrode. We have developed a route for shape-controlled synthesis of Pt NCs through a square-wave potential. Starting from Pt nanospheres electrodeposited on glassy carbon (GC) substrate instead of bulk Pt, we obtained THH Pt NCs at high yield. Electrochemical preparation was carried out in a standard three-electrode cell at room temperature (25). All electrode potentials are reported on the scale of a saturated calomel electrode (SCE). In a typical experiment, polycrystalline Pt nanospheres of size about 750 nm (fig. S1) were electrodeposited on a GC electrode in a solution of 2 mM K2PtCl6 + 0.5 MH2SO4. The Pt nanospheres were then subjected to a square-wave treatment, with upper potential 1.20 V and lower potential between –0.10 and –0.20 V, at 10 Hz in a solution of 0.1 M H2SO4 + 30 mM ascorbic acid for 10 to 60 min. As illustrated schematically by Fig. 1A, THH Pt NCs were grown exclusively on GC surface at the expense of Pt nanospheres. Fig. 1. (A) Scheme of electrochemical preparation of THH Pt NCs from nanospheres. The Pt nanosphere is an agglomeration of tiny Pt nanoparticles of irregular shapes (fig. S1). Under the influence of the square-wave potential, new Pt NCs of THH shape grow at the expense of the large nanospheres (the large nanosphere is "dissolved" into smaller ones, which eventually transform into THH shape). (B) Low-magnification SEM image of THH Pt NCs with growth time of 60 min. (C and D) High-magnification SEM images of Pt THH viewed down along different orientations, showing the shape of the THH. (E) Geometrical model of an ideal THH. (F) High-magnification SEM image of a THH Pt NC, showing the imperfect vertices as a result of unequal size of the neighboring facets. Scale bars in (C), (D), and (F), 100 nm. [View Larger Version of this Image (45K GIF file)] The shape of the NCs was determined initially by scanning electron microscopy (SEM). An SEM image of THH Pt NCs produced with a growth time of 60 min is shown in Fig. 1B (an overview SEM image including THH Pt NCs and residual Pt nanospheres is shown in fig. S2). The yield of the THH Pt NCs in the final product is >90%, and most other shapes are an agglomeration of imperfect THH NCs (fig. S3). Their average size (Heywood diameter) was 217 nm, with a standard deviation of 23 nm. By controlling the experimental conditions, the smallest THH NCs received and identified by SEM are 20 nm (fig. S3). The THH Pt NCs on GC surface are randomly oriented. High-magnification SEM images of THH Pt NCs oriented nearly along the three- and four-fold axes are presented in Fig. 1, C and D, respectively. The THH shape is based on a cube with each face capped by a square-based pyramid (Fig. 1E). Three perfect square-based pyramids in Fig. 1C and nearly octagonal projection in Fig. 1D can be seen clearly, which confirms the THH shape of the Pt NC. In some cases, imperfection can be introduced at vertices (fig. S4), as presented by the SEM image in Fig. 1D, which is not caused by the {100} truncation of the THH but rather because crystal facets of different sizes are produced by substrate effects (Fig. 1F). Transmission electron microscopy (TEM) analysis of such structure clearly reveals the three-dimensional (3D) structure of the NC (fig. S5). In comparison with previous electrochemical faceting of bulk Pt electrodes (24), the use of Pt nanospheres deposited on GC substrate is vital to the production of nearly perfect THH Pt NCs, because GC is an inert substrate on which isolated Pt NCs grow in island (Volmer-Weber) mode but not columnar mode (26). THH-shaped crystals in nature exist occasionally in fluorite and diamond but very rarely in metals. To the best of our knowledge, only copper crystals of truncated THH shape were found in copper minerals (27). The THH (Oh symmetry) belongs to Catalan solids or Archimedean duals; the THH shape is bounded by 24 high-index planes of {hk0} (hk0) (28). We determined the facets of the THH Pt NCs by selected-area electron diffraction (SAED) and high-resolution TEM (HRTEM). The facets are best revealed by imaging the NC along [001], parallel to which 8 of the 24 facets are imaged edge-on (Fig. 2, A and B). The fourfold symmetry of the SAED pattern confirms that the THH Pt NC is a single crystal. The HRTEM image in Fig. 2C recorded from a boxed area in Fig. 2A shows continuous lattice fringes with lattice spacing of 0.20 nm, which corresponds to the {200} planes of Pt. The Miller indices of exposed facets of the THH can be identified by a conjunction of the angles between the facets, the TEM image, and the ED pattern of the Pt NC (Fig. 2, A and B), whose border can be looked at as the projection of eight {hk0} facets parallel to the [001] zone axis. As shown in Fig. 2A, two surface angles of 133.6 ± 0.3° and 137.6 ± 0.3° are measured, which are in good agreement with theoretical values of angle = 133.6° and angle ß = 136.4° between {730} facets (fig. S6). The results show that the dominant facets of the THH Pt NC are {730}. Although most of the THH Pt NCs are bounded by the {730} facets, some THH Pt NCs bounded by {210}, {310}, or {520} had also been observed [fig. S7 (25)]. Fig. 2. (A) TEM image of THH Pt NC recorded along [001] direction. A careful measurement of the angles between surfaces indicates that the profiles of the exposed surfaces are {730} planes ( = 133.6°, ß = 136.4°) (see fig. S6). The inset is a [001] projected model of the THH. (B) Corresponding SAED pattern with square symmetry, showing the single-crystal structure of the THH Pt NC. (C) High-resolution TEM image recorded from the boxed area marked in (A). An amorphous thin layer is shown at the surface, which may be introduced by contamination during specimen handling and/or TEM observation. (D) Atomic model of Pt(730) plane with a high density of stepped surface atoms. The (730) surface is made of (210) and (310) subfacets. The local surface of THH Pt NC can be (210) if the size of the crystal surface increases, although the overall profile of the facets is (730). (E) HRTEM image recorded from another THH Pt NC to reveal surface atomic steps in the areas made of (210) and (310) subfacets. The image reveals the surface atomic steps. [View Larger Version of this Image (102K GIF file)] The atomic arrangement of the Pt(730) surface (Fig. 2D) is periodically composed of two (210) subfacets followed by one (310) subfacet, that is, a multiple-height stepped structure (6). These steps have been directly captured in an HRTEM image recorded from another THH, as indicated by arrows in Fig. 2E. Thus, the single-crystal THH is enclosed by 24 high-index facets of {730}. In addition to the HRTEM results, the surface structure of THH Pt NCs can also be characterized by cyclic voltammograms (fig. S8), in which the features of oxygen adsorption and desorption are similar to those on a bulk Pt(210) surface but are different from those on a polycrystalline Pt surface. In a common point, these high-index facets exhibit an open structure. In the case of a Pt(730) surface, its density of stepped atoms is as high as 5.1 x 1014 cm–2, that is, 43% of the total number of atoms on the surface. It is thus reasonable to expect that the THH Pt NCs will display high catalytic activity. The size of the THH Pt NCs can be controlled by varying the growth time. The SEM images of THH Pt NCs produced with growth times of 10, 30, 40, and 50 min are shown in Fig. 3, A to D, respectively. The corresponding average sizes are measured, respectively, to be 53, 100, 126, and 144 nm. The histograms of size distributions of these particles are shown in Fig. 3E, with a relative standard deviation (RSD) of 12%, 10%, 14%, and 14%, respectively. The particle size is also correlated with the density of THH Pt NCs on the substrate surface. The (210) and (310) planes, the major subfacets for composing of the (730) as shown in Fig. 2D, exhibited high stability under electrochemical conditions and solid-gas environments in both oxidative and reducing atmospheres (9, 29–31). The THH Pt NCs are thermally stable; in situ TEM observation showed that the THH Pt NCs were thermally stable to temperatures of 800°C with the preservation of the shape and facets (Fig. 3F). (Note the large size of the particles.) |
5楼2007-05-11 19:21:20
