1 贵金属高表面能及其特殊性质综述，孙志刚，2011

This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 4167–4185 4167 Cite this: Chem. Soc. Rev., 2011, 40, 4167–4185 Nanomaterials of high surface energy with exceptional properties in catalysis and energy storage Zhi-You Zhou, Na Tian, Jun-Tao Li, Ian Broadwell and Shi-Gang Sun* Received 10th November 2010 DOI: 10.1039/c0cs00176g The properties of nanomaterials for use in catalytic and energy storage applications strongly depends on the nature of their surfaces. Nanocrystals with high surface energy have an open surface structure and possess a high density of low-coordinated step and kink atoms. Possession of such features can lead to exceptional catalytic properties. The current barrier for widespread industrial use is found in the difficulty to synthesise nanocrystals with high-energy surfaces. In this critical review we present a review of the progress made for producing shape-controlled synthesis of nanomaterials of high surface energy using electrochemical and wet chemistry techniques. Important nanomaterials such as nanocrystal catalysts based on Pt, Pd, Au and Fe, metal oxides TiO2 and SnO2, as well as lithium Mn-rich metal oxides are covered. Emphasis of current applications in electrocatalysis, photocatalysis, gas sensor and lithium ion batteries are extensively discussed. Finally, a future synopsis about emerging applications is given (139 references). 1. Introduction Nanomaterials are of immense importance in today’s modern society. The development of chemical industries, environ- mental protection and new-energy resources (e.g., fuel cells, lithium ion batteries) have long relied on nanomaterials with exceptional properties. The fields of catalysis, electrocatalysis, photocatalysis and photoelectricity are all examples of where nanotechnology is impacting on current science.1–4 As particle dimensions reduce towards the nanoscale, the surface-to-volume ratio proportionally increases and small- size effects associated with nanoparticles become more pronounced. Understanding the nanoscale topography of surface sites, such as terraces, steps, kinks, adatoms and vacancies, and their effects on catalytic and other physico- chemical properties is the key to designing nanoscale functional materials by nanotechnology.5–7 The performance of nanocrystals used as catalysts depends strongly on the surface structure of facets enclosing the crystals. Thermodynamics usually ensures that crystal facets evolve to have the lowest surface energy during the crystal growth process. For a pure metal, the surface energy relies on coordination numbers (CNs) of surface atoms as well as their density. For example, it increases in the order of g{111}o g{100} o g{110} o g{hkl} on a face-centered cubic (fcc) metal, where State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, School of Energy Research, Xiamen University, Xiamen 361005, China. E-mail: sgsun@xmu.edu.cn; Fax: +86 0592 2183047 Zhi-You Zhou Zhi-You Zhou received his PhD degree in 2004 from Xiamen University. He is an associate professor at the Department of Chemistry of Xiamen University and his research interests include electrochemical in situ FTIRS, electrocatalysis, fuel cells and nanomaterials. Na Tian Na Tian received her PhD degree in 2007 from Xiamen University. She is an associate professor at the Department of Chemistry of Xiamen Univer- sity and her research interests focus on electrocatalysis and nanomaterials. Chem Soc Rev Dynamic Article Links www.rsc.org/csr CRITICAL REVIEW Pu bl ish ed o n 09 M ay 2 01 1. D ow nl oa de d on 1 2/ 12 /2 01 3 01 :0 1: 08 . View Article Online / Journal Homepage / Table of Contents for this issue 4168 Chem. Soc. Rev., 2011, 40, 4167–4185 This journal is c The Royal Society of Chemistry 2011 {hkl} represents high-index planes with at least one Miller index larger than 1.8,9 For a metal oxide, the surface energy increases with increasing density of dangling bonds. Generally, high-energy surfaces have an open surface structure and possess exceptional properties. Long-term fundamental studies in surface science have shown that Pt high-index planes with open surface structure exhibit much higher reactivity than that of (111) or (100) low-index planes, because high-index planes have a large density of low-coordinated atoms situated on steps and kinks, with high reactivity required for high catalytic activity.10–12 More importantly, on high-index planes, there exist short-range steric sites (such as ‘‘chair’’ sites) that are considered as active sites and consist of the combination of several (typical 5–6) step and terrace atoms.13,14 Due to synergistic effect between step and terrace atoms, steric sites usually serve as catalytically active sites. Besides, open-structure surfaces also play a very important role in the charging/discharging process of lithium ion batteries. They can provide parallel channels, where Li+ ions are able to intercalate through the surface with the least resistance compared to other crystal plane orientations.15 This favors fast ion transfer between surface and interior. Normally, nanocrystals with low surface energy such as those formed under normal conditions usually have low catalytic activities. Those with high surface energies are known to possess enhanced catalytic properties. The goal here is to create nanocrystal catalysts which have high surface energy facets. Unfortunately, this presents a big challenging. When a crystal grows, different facets grow with different rates. High-energy facets typically have higher growth rates than low-energy facets. Overall, the final crystal shape is dominated by the slow-growth facets that have low surface energy.16 Taking Pt as an example, after the pioneering work of El-Sayed and co-workers who synthesized cubic and tetra- hedral Pt nanocrystals through chemical reduction of K2PtCl4 by H2 in the presence of polyacrylate in 1996, 17 various Pt nanocrystals with different shapes have been obtained by changing Pt precursors, reducing agents, stabilizing reagents and solvents.18–26 Nevertheless, the surface structure of the synthesized Pt nanoparticles is limited to low-index facets of {111} and {100}, and few Pt nanocrystals with high-index facets are produced by conventional wet chemistry synthesis.27–29 Remarkably, substantial progress has been made in overcoming the obstacle to form nanocrystals with high-energy facets in recent years.30–32 Although there are several excellent reviews about shape- controlled synthesis of metal nanocrystals, they mainly describe nanocrystals with low-energy facets.28,33,34 In this review, after a brief introduction of the relationship between surface structure and crystal shapes, we focus on the recent progress made in shape-controlled synthesis of metal nano- crystals with high-energy facets and open surface structure, including high-index facets and {110} facets, especially electrochemically shape-controlled synthesis of Pt-group metal nanocrystals. We then describe the synthesis of metal oxide nanomaterials (TiO2, SnO2, Li[Li1/3�2x/3MxMn2/3�x/3]O2, etc.) with exposed high-energy surfaces and their unique properties in photocatalysis, gas sensors and Li ion batteries. We finish by considering the challenging issues faced and future perspectives along this exciting new avenue of research.Jun-Tao Li Jun-Tao Li received his PhD degree in 2009 from Xiamen University and University of Pierre et Marie Curie. He currently works in Energy Research School, Xiamen University, focusing on electrochemical conversion and storage systems. Ian Broadwell Ian Broadwell received his PhD degree in 2003 from Hull University. Currently he works as EU Science and Technology and post-doctoral Fellow at Xiamen University. His research interests include biomedical application of nanoparticles; microfluidics for biofuel cells and the instru- mentation used in biophysics. Shi-Gang Sun Shi-Gang Sun received his Doctorat d’Etat in 1986 from Universite´ Pierre et Marie Curie and did one year Post- Doctoral research in the Laboratoire d’Electrochimie Interfacial du CNRS, France. He is presently professor of Chemistry in Xiamen Univer- sity. His research interests consist of: (1) Electrocatalysis and electrochemical surface science; (2) in situ FTIR spectroscopy; (3) Nano- materials and chemical power sources. He is president-elect of the Chinese Society of Electrochemistry, Fellow of Royal Society of Chemistry (UK) and Fellow of International Society of Electrochemistry. Pu bl ish ed o n 09 M ay 2 01 1. D ow nl oa de d on 1 2/ 12 /2 01 3 01 :0 1: 08 . View Article Online This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 4167–4185 4169 2. Relationship between surface structure and crystal shape For fcc metals (such as Pt, Pd and Au), a unit stereo- graphic triangle (Fig. 1) is often used to illustrate the coordinates of different crystal planes.14,35 Three vertexes in the triangle represent three low-index planes, i.e., (111), (100) and (110). Other planes lying in the sidelines and inside the triangle are high-index planes. The three sidelines represent [001], [01�1] and [1�10] crystallographic zones, and corres- ponding Miller indices can be expressed as {hk0}, {hkk} and {hhl}} (h a k a l a 0). The planes inside the triangle can be expressed as {hkl}. Fig. 1 also illustrates atomic arrangement models of several typical planes. Clearly, atomic arrangements are highly correlative with Miller indices. The (111) and (100) planes are atomically flat with closely packed surface atoms, and the CNs of surface atoms are 9 and 8, respectively. The (110) plane and high-index planes have open surface structure with low-coordinated step or kink atoms. Step atoms on {hkk}, {hhl} and (110) planes have the same CNs of 7, while kink atoms on {hk0} and {hkl} planes have the lowest CNs of 6. Generally, those surfaces possessing a high density of atoms with CNs of 6 and 7 have very high reactivity. By analogy with the unit stereographic triangle for crystal planes, there is a triangle of polyhedral nanocrystals bounded by corresponding crystal planes (Fig. 2).14,36 The shapes of nanocrystals situated at the three vertexes are octahedron with {111}, cube with {100} and rhombic dodecahedron with {110}. The polyhedral nanocrystals lying in three sidelines of the triangle are polyhedra bounded by 24 high-index facets: tetrahexahedra (THH) by {hk0}, trapezohedra by {hkk} and trisoctahedra by {hhl}. The polyhedra inside the triangle are hexoctahedra bounded by 48 {hkl} facets. Clearly, the polyhedra bounded by high-index facets have unconven- tional and complex shapes. For identifying them quickly, THH can be considered as a cube with each face capped by a square pyramid; trisoctahedron can be thought as an octahedron with each face capped by a trigonal pyramid, and so on. 3. Shape-controlled synthesis of metal nanocrystals of high surface energy and open surface structure We focus, in this section, on the progress made in shape- controlled synthesis of metal nanocrystals of high surface energy and open surface structure, i.e. high-index facets and {110} facets for fcc metals (Pt, Pd and Au) and {100} for Fe (bcc lattice). To synthesize nanocrystals of high surface energy, we need to develop new synthesis methods that can overcome the thermodynamics limitation, which results in a minimization of the total surface energy of nanocrystals during their growth. Using a surface-structure mediator (such as surfactant, oxygen and halide) that can selectively adsorb or stabilize the high-energy surfaces is one strategy, while another strategy is the more precise electrochemical method which has been developed recently by our group to synthesize metal nanocrystals of high surface energy. 3.1 Pt Pt nanocrystals due to their excellent activity and stability are widely used and are indispensible catalysts in fuel cells, auto- motive catalytic converters and petrochemical reforming.37–40 As Pt is very rare, a key issue is how to improve the intrinsic catalytic properties and utilization efficiency. Shape-controlled synthesis of Pt nanocrystals (NCs) with high-index facets offers a good approach for creating Pt nanocatalyst materials with much higher activities than currently available with commercial Pt catalyst. As mentioned, the synthesis of Pt nanocrystals with high- index facets is challenging. Conventional electrodeposition of metal nanocrystals made at a constant potential/current in plating solution only yields {111} and {100} low-index facets. Tian et al. in 2007 made a breakthrough in synthesis of Pt nanocrystals enclosed with high-index facets by developing an electrochemical method.30 As shown in Fig. 3a, they used a two-step synthesis process. The first step is electrodeposition of polycrystalline Pt nanospheres B750 nm in diameter on a glassy carbon (GC) electrode. After pre-deposition, a Fig. 1 Unit stereographic triangle of fcc metal single-crystal and models of surface atomic arrangement (modified with permission from ref. 14, copyright 2008, American Chemical Society). Fig. 2 Unit stereographic triangle of polyhedral nanocrystals bounded by different crystal planes (modified with permission from ref. 14, copyright 2008, American Chemical Society). Pu bl ish ed o n 09 M ay 2 01 1. D ow nl oa de d on 1 2/ 12 /2 01 3 01 :0 1: 08 . View Article Online 4170 Chem. Soc. Rev., 2011, 40, 4167–4185 This journal is c The Royal Society of Chemistry 2011 square-wave potential was applied to the Pt nanospheres in a solution containing 0.1 M H2SO4 + 30 mM ascorbic acid for 5–60 min. The lower (EL) and upper (EU) potential limits were ca. �0.10 and 1.20 V vs. saturated calomel electrode (SCE), respectively, and f = 10 Hz. Under this condition, the Pt nanospheres are partially dissolved at the EU, and provide low-concentrated Pt ions for the growth of new nanocrystals at the EL. Interestingly, nearly all of the growing Pt nano- crystals on the GC surface are tetrahexahedral Pt nanocrystals (THH Pt NCs). As shown in Fig. 3b–d, SEM images of the as-prepared Pt nanocrystals show good agreement with a geometrical model of THH (Fig. 3e). The average sizes of the THH Pt NCs can be changed from 20 to 220 nm by controlling the growth time, and the size distribution is relatively narrow, with relative standard deviation (RSD) ranging from 10% to 15%. Miller indices of exposed surfaces on the THH Pt NCs were identified as mainly {730} facets through the measurement of plane angles between two adjacent facets parallel to the [001] zone axis in a TEM image, as demonstrated in Fig. 4a–c. The Pt(730) plane is periodically composed of two (210) microfacets followed by one (310) microfacet (Fig. 4d), and has a density of step atoms as high as 5.1 � 1014 cm�2 (i.e. 43% of surface atoms are step atoms). More importantly, all surface atoms on the THH Pt NCs are arranged in such a way that they form active sites for catalysis. Therefore, THH Pt NCs exhibit high catalytic activity. It has been demonstrated that for formic acid electrooxidation, the catalytic activity of THH Pt NCs is 1.6–4.0 times higher than that of poly- crystalline Pt nanospheres, and 2.0–3.1 times larger than that of commercial Pt/C catalyst from E-TEK Co., Ltd. (Fig. 5a and b). For ethanol electrooxidation, the enhancement factor of the catalytic activity obtained on the THH Pt NCs varies from 2.0 to 4.3 relative to that of Pt nanospheres, and 2.5 to 4.6 relative to commercial Pt/C catalyst (Fig. 5c and d). The THH Pt NCs also exhibit much higher catalytic activity for NO electroreduction than polycrystalline Pt nanoparticles and bulk Pt electrode.41 Besides, the high-energy surfaces on the THH Pt NCs are thermally stable up to 815 1C, which is consistent with the theoretical prediction.42,43 One important question raised is why high-index facets can form under such electrochemical conditions. The presence of Fig. 3 (A) Scheme of preparation of THH Pt NCs from nanospheres by electrochemical square-wave method. (B) Low-magnification and (C, D, F) high-magnification SEM images of THH Pt NCs. (E) Geometrical model of an ideal THH (modified with permission from ref. 30, copyright 2007, American Association for the Advancement of Science). Fig. 4 (A) TEM image and (B) SAED of THH Pt NC recorded along the [001] direction. (C) High-resolution TEM image recorded from the boxed area marked in (A). (D) Atomic model of Pt(730) plane with a high density of stepped surface atoms (modified with permission from ref. 30, copyright 2007, American Association for the Advancement of Science). Fig. 5 Comparison of specific catalytic activity among THH Pt NCs, polycrystalline Pt nanospheres and 3.2 nm Pt/C catalyst. (A) Transient current curves recorded at 0.25 V and (B) steady-state current as a function of electrode potential for formic acid electrooxidation in 0.25 M HCOOH + 0.5 M H2SO4. (C) Transient current curves recorded at 0.30 V and (D) steady-state current as a function of electrode potential for ethanol electrooxidation in 0.1 M CH3CH2OH + 0.1 M HClO4 (modified with permission from ref. 30, copyright 2007, American Association for the Advancement of Science). Pu bl ish ed o n 09 M ay 2 01 1. D ow nl oa de d on 1 2/ 12 /2 01 3 01 :0 1: 08 . View Article Online This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 4167–4185 4171 ascorbic acid can be excluded, since THH Pt NCs with slightly imperfect shape can still be harvested in ascorbic acid-free solution. Compared with conventional synthesis of metal nanocrystals that only occurs in a reducing environment, the electrochemical square-wave potential method has one unique characteristic. That is, it can cause periodic adsorption/ desorption of hydrogen and oxygen on metal surfaces, as well as nucleation and growth/dissolution of the metals.30,44 It has been demonstrated that under periodic reduction and oxida- tion, high-index planes of Pt (e.g., Pt(210)) are more stable than low-index planes (e.g., Pt(111)).14,45–48 This indicates that the square-wave potential plays a key role in the formation of THH Pt NCs. As illustrated in Fig. 6, oxides or hydroxides (Oad, OHad), originating from the dissociation of H2O in solution, can readily form on the surface of Pt nanocrystals at +1.20 V vs. SCE. Low-index facets of {111} and {100} are smooth with highly coordinated atoms (CNs are 9 and 8, respectively), so oxygen atoms preferentially diffuse/invade into the lattice and replace Pt atoms.48,49 After desorption of oxygen atoms from the lattice at lower potential (e.g. �0.20 V), those displaced Pt atoms cannot always return to their original positions, so that the ordered surface structure will be destroyed, and the close packed structure will be opened. In contrast, since high-index facets contain many step/kink atoms with low CNs (e.g., 6 on Pt(730)), the oxygen atoms preferentially adsorb at such sites without replacing them, and the surface order is preserved. Further theoretical studies are still needed to reveal the detailed microscopic processes of oxygen adsorption on different Pt single crystalline surfaces.50,51 Although ascorbic acid was found not to be the key factor for the formation of high-index facets (as mentioned above), other additives in solution were found to affect the final surface structures of Pt nanocrystals during their formation. For example, if the ascorbic acid was replaced by sodium citrate during square-wave potential treatment, concave hexoctahedral P