Review pubs.acs.org/cm
Hybrid Semiconductor−Metal Nanoparticles: From Architecture to Function Uri Banin,* Yuval Ben-Shahar, and Kathy Vinokurov Institute of Chemistry and the Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel ABSTRACT: Hybrid nanoparticles combine two or more disparate materials on the same nanosystem and represent a powerful approach for achieving advanced materials with multiple functionalities stemming from the unusual materials combinations. This review focuses on recent advances in the area of semiconductor−metal hybrid nanoparticles. Synthesis approaches offering high degree of control over the number of components, their compositions, shapes, and interfacial characteristics are discussed, including examples of advanced architectures. Progress in hybrid nanoscale inorganic cage structures prepared by a selective edge growth mechanism of the metal onto the semiconductor nanocrystal is also presented. The combined and often synergistic properties of the hybrid nanoparticles are described with emphasis on optical properties, electronic structure, electrical characteristics, and light induced charge separation effects. Progress toward the application of hybrid nanoparticles in photocatalysis is overviewed. We conclude with a summary and point out some challenges for further development and understanding of semiconductor−metal hybrid nanoparticles. This progress shows promise for application of hybrid nanoparticles in photocatalysis, catalysis, optical components, and electronic devices. KEYWORDS: hybrid nanoparticles, semiconductor−metal interface, assembly, photocatalysis, nanoelectronics
1. INTRODUCTION Hybrid nanoparticles (HNPs) combine disparate materials onto a single nanosystem, thus providing a powerful approach for bottom-up design of novel architectures.1−5 Beyond the fundamental development in synthesis, the interest in HNPs arises from their combined and often synergetic properties exceeding the functionality of the individual components. These ideas are well demonstrated in hybrid semiconductor− metal nanoparticles, which are the focus of this review. The development of such HNPs evolved following the extensive progress in synthetic control achieved for nanocrystals of semiconductors and metals, yielding monodisperse systems with well-defined shapes and compositions.6,7 In the case of colloidal semiconductor nanocrystals, this allowed for high level understanding of quantum confinement effects dictating their unique optical and electronic properties.8,9 For colloidal metal nanocrystals, plasmonic effects were also intensively studied.10,11 These achievements provided the background for the development of semiconductor−metal HNPs. An early example of such systems was presented through growth of metal islands on semiconductor nanoparticles such as Au, Ag, Cu, and Pt on ZnO nanostructures12 or in TiO2−Au and TiO2−Pt composites.13−15 The role of the metal nanoparticles influencing the energetics of the system leading to photoinduced charge separation was further studied and established in photocatalytic reactions. Yet, in these early examples there was limited control of the semiconductor particle shape and size and the metal particle size and location, and mostly polydisperse samples were initially available. Further © 2013 American Chemical Society
synthetic efforts aimed to achieve high degree of control over these aspects. In this context, the selective growth of metal tips on the apexes of semiconductor nanorods was achieved via a facile solution reaction, as exemplified in the growth of Au tips on CdSe nanorods.1,16 In this system, the metal tips provide anchor points for electrical connections and for self-assembly of the semiconductor component. Moreover, a synergetic effect of light induced charge separation at the nanoscale semiconductor−metal junction17 opened the path for studies on the utilization of such HNPs in demanding photocatalytic applications.18 Since this pioneering study, significant advances have been made in expanding this concept to diverse materials systems, with different compositions and morphologies. These unique compositions manifest synergetic properties with potential applications in diverse fields, including optics, electronics, catalytic reactions, and more. Several main growth mechanisms have been developed and used to form HNPs, addressing the inherent challenge of combining the two different materials types on one nanoparticle in a controlled manner. The parameters that should be brought into consideration during the synthesis are the lattice constant mismatch and different crystal structures, the presence of polar facets, the interfacial energy among the materials, the Special Issue: Celebrating Twenty-Five Years of Chemistry of Materials Received: July 1, 2013 Revised: August 22, 2013 Published: September 3, 2013 97
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Figure 1. Various hybrid nanoparticles showing TEM images and corresponding schematic architecture. (a) Cu2S−Au at tilt angles of −30, 0, and 30° and their corresponding schematic model.25 (b) Ru nano inorganic caged copper sulphide nanoparticles. (c) “Microphone-like” AgCdSe−Au nanorods with a mean size of 24 nm for the AgCdSe parts and a mean length of 41 nm for the Au rods.26 (d) CoO tipped CdSe/CdS nanorods obtained after oxidation of the Co tips.27(e) ZnO-Au hexagonal nanopyramid-like structure.28 (f) CdS−Au core−shell nanorods with CdS shell thickness of ∼4.5 nm. (g) CdSe/CdS/ZnS−Au@hollow SiO2 yolk/shell nanospheres.29 (h) Single Au tipped CdS nanorods. (i) CdS−Au body decorated nanorods. (Reproduced with permission from: (a) ref 25, copyright 2010 American Chemical Society; (c) ref 26, copyright 2012 American Chemical Society; (d) ref 27, copyright 2012 American Chemical Society; (e) ref 28, copyright 2011 American Chemical Society; (f) ref 87, copyright 2011 Wiley; (g) ref 29, copyright 2012 Springer).
2. SELECTIVE SYNTHESIS OF ADVANCED HYBRID SEMICONDUCTOR−METAL NANOPARTICLES The synthesis of HNPs continues to develop rapidly. The progress as described below is directed toward several directions. Control of location of the metal component using selective reactivity of different facets of the semiconductor nanoparticles is developing further (section 2.1). The adoption of advanced strategies developed for growth of metal nanoparticles is also now allowing control of the morphology of the metal component (section 2.2). More complex hybrid nanoparticle architectures are being developed, with two different metallic components grown onto the semiconductor segment (section 2.3). Another development is the discovery of additional growth modes, specifically edge selective growth, which allowed for the synthesis of hybrid nanoscale inorganic cages (section 2.4). 2.1. Site-Selective Metal Deposition onto the Semiconductor Component. Among the synthesis mechanisms that have been identified in the growth of HNPs, the selective surface growth of one of the components on the other is widely used. The lower energy barrier for heterogeneous nucleation compared with the homogeneous nucleation process of the secondary component allows the formation of crystalline phases of both components on the same HNP. Moreover, selective deposition of the second phase may be dictated by the morphology and crystal structure of the first phase. The crystal morphology and surface capping provide different chemical reactivities for different facets of the nanocrystal, which can lead to specific growth of the second phase material, on the more reactive facets of the nanocrystals of the first phase material. Systems, on which different chemical reactivities are obtained, include, in particular, anistropic rod structures which have been widely developed in recent years. This includes semiconductor seeded rods which utilize a small quantum dot as a seed for directed rod growth of the same or another semiconductor, as is the case for CdS rods and CdSe/CdS seeded rods.30,31 The
materials miscibility, the presence of surface defects, and surface accessibility/reactivity. Among the successful HNPs synthesis routes, facet selective growth exploits the well-defined crystal structure of the semiconductor component to provide preferential nucleation and growth sites for the metal component.1,19 Furthermore, the absorption of the semiconductor component can be used for light induced growth of the metal component.20−22 Surprisingly, other material combinations yielded a pathway for diffusion of the metal component into the semiconductor, providing new HNP morphologies resembling core/shell architectures23 and opening a path for aliovalent doping of semiconductor nanocrystals via a facile solution reaction.24 These previously successful strategies for HNP synthesis and the resultant properties are discussed in several earlier review papers on HNPs.2−5 This review focuses on advanced semiconductor−metal HNPs characterized by a high degree of control over the size, position, composition, and shape of the different components, which further expand the selection of such architectures and enrich the reaction pathways for HNPs synthesis (Figure 1). In section 2, selective synthesis routes for advanced HNPs are described, showcasing systems with control over the metal domain shapes, HNPs with three or more components, hybrid nanocage architectures, and more. Section 3 describes assemblies of HNPs prepared by different strategies, which manifest the hybrid architecture also on a larger ensemble scale. In section 4, studies of the synergistic optical, electrical, and charge separation properties of HNPs are presented. Section 5 discusses the potential applications of semiconductor−metal HNPs in photocatalysis and light-assisted catalysis, followed by demonstrations of using HNPs for electrical connectivity toward printed electronics. 98
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absorption properties of the two different semiconductor regions (CdSe smaller gap than CdS) accompanied by trapping of the electrons on surface defects in that area. Another approach for site-selective metal growth control uses the semiconductor nanocrystal morphology. The specific morphology of the conelike CdSe/CdS tetrapod was found to have an increased selectivity toward single Au tipped deposition compared with regular tetrapods with cylinder-like arms, related to the tapered arms structure.44 This behavior was attributed to an enhanced intraparticle electrochemical Ostwald ripening process, as has been demonstrated previously in the growth of matchstick-like CdS-Au HNP.16 This site specific growth on the tip of one tetrapod arm was further used in a second step for selective growth of two different materials on different tips of the same tetrapod. As illustrated in Figure 2, a
selective facet growth was demonstrated for Cd-chalcogenide nanorods with double or single Au tips1,32 and for Au deposition on PbS nanocrystals.33 Crystal defect sites are also high energy surface sites that may promote growth of the metal component on the semiconductor nanocrystal.21,32 However, they can be substantially suppressed by effective surface ligands capping. Another mechanism that can transform multiple metal islands growth into a single metal island on a semiconductor nanostructure is provided by electrochemical Ostwald ripening.16 Accompanying the usual driving force for Ostwald ripening of dissolution of small islands along with further growth of the larger island, the electrochemical ripening requires oxidation/reduction of the metal components participating in the reaction. Another approach for suppressing the multiple Au islands growth along the CdSe rod was achieved by postsynthesis intraparticle Ostwald ripening via combination of both atomic and cluster diffusion induced by thermal annealing. At high temperature under vacuum, the smaller Au islands migrated to the apex of the CdSe nanorod, forming a larger single Au tip at that site. Moreover, the high temperature treatment for the CdSe-Au HNPs led to formation of a high quality epitaxial interface between the semiconductor and metal domains.34 This was attributed to overcoming the energy barrier, which allows reaching to the thermodynamically most stable configuration. This epitaxial relationship between the two components across the semiconductor−metal interface is of importance with respect to charge transfer and electronic properties of the HNPs. Further exploitation of the semiconductor nanocrystal surface characteristics provides a knob for advanced selective deposition. Site-selective deposition has been demonstrated previously on CdSe, CdS, and CdSe/CdS core/shell nanorods with deposition of various components, including Au,1,16,21,22,32 Pt,35−38 PtCo,35 PtNi,35,39 Pd, Ag2S,40 Co,41 PdO, and Pd4S.42 Either thermal or photochemical induced metal growth methods can be used, leading to different deposition patterns dependent on surface coating, reaction temperature, metal type, precursors, and concentrations. For example, photodeposition at room temperature yielded growth of multiple Pt metal islands along CdS nanorods deposited on defect sites, related to localization of electrons used for the metal reduction.36 Facet selectivity was achieved for Pt growth on anisotropic CdS nanorods at high temperature, yielding single Pt metal tips deposited at one of the apexes assigned to the highly reactive sulfur-rich facet.35 On the other hand, single Au tipped CdS nanorods were prepared by photodeposition,21,22 while thermal growth yielded either a single Au tip or multiple Au islands on the CdS rod surface, depending on the precursor concentration.40 In heterostructured nanocrystals, such as seeded CdSe/CdS nanorods, selective light induced metal growth on the surface near the CdSe core location was observed, due to the tendency for localization of the electron near the core region, which induces site-selective metal precursor reduction.32 Further extension of this selective deposition methodology was demonstrated in photodeposition of Pd nanoparticles on CdS0.4Se0.6 nanorods which have CdSe- and CdS-rich domains on opposite apexes of the rod. A dependence on irradiation wavelength was observed, allowing for preferred growth at the CdSe-rich end upon irradiation in the red, or on both CdSerich and CdS-rich regions upon blue irradiation.43 This partial site-selectivity was explained by the varying band gaps and
Figure 2. (a) Reaction scheme of CdSe/CdS tetrapods with conelike arms with Au at one tip and Ag2S at the other three tips. (b,c) HRTEM images of the different tetrapod arm tips showing the lattice fringes of the Au (111) (left) and Ag2S (1̅21) (right) planes. (d) High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image of a hybrid CdSe/CdS-Au/Ag2S tetrapod with conelike arms, with the different tips labeled. (Reproduced with permission from ref 44, copyright 2012 American Chemical Society.)
sequential growth of Au followed by reaction with Ag ions provided tetrapod structures with a Au tip specifically on one arm, while, on the other arms, Ag2S particles were grown as analyzed by the high resolution transmission electron microscopy (HRTEM) images. This was enabled since the Ag2S deposition occurs via cation exchange of Cd with Ag, which commences from the tapered tip of the bare arms, while this process is suppressed by the presence of the Au nanoparticle domain on one of the tetrapod arms. 2.2. Control of the Morphology of the Metal Component. Controlling the morphology of the metal domain is another advanced desired characteristic in the synthesis of HNPs. The control of the metal domain size and shape provides an important knob for modifying the surface facets with relevance for catalysis45 and photocatalysis, for tuning the plasmonic properties,10,46 and for modifying other electronic characteristics. With respect to metal nanoparticle shape, plasmonic features in metal nanoparticles such as Au and Ag NPs are affected by the shape, with a single plasmon feature observed for spherical nanoparticles in the visible spectrum, in comparison to different plasmon peaks observed in anisotropic shapes such as rods, triangular prisms, and cubes. For example, triangular Ag nanoparticles develop two distinct quadrupole plasmon resonances and scatter light in the red, while spherical 99
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2.3. Semiconductor−Dual Metal Hybrid Nanoparticles via Selective Deposition. Combining two different metals on the same semiconductor−metal hybrid system through core/ shell structures or by interfacial alloys can lead to new and improved chemical and physical characteristics. This idea follows the change of properties observed previously in bimetallic nanoparticle systems,57 including electric field enhancement effects, modification of surface plasmon properties, magnetic functionality, or enhanced catalytic activity. An elegant example of the latter is the improved methanol electrooxidation catalytic performance observed for Pt/Ni and Pt/Ru/Ni alloy nanoparticles compared with pure Pt nanoparticles.58 Habas et al. reported the synthesis of HNPs with alloy metal domains with compositions of CdS−PtNi and CdS−PtCo by using two different metal precursors in the growth solution, following a synthesis procedure for colloidal NixPt1−x NPs.35 Control of the bimetallic tip composition was obtained by modifying the ratio of the two metal precursors. More recently, Chan et al. demonstrated light induced control of bimetal deposition. CdSe/CdS−Au hybrid nanorods were used as precursors for the deposition of the second metal. Upon UV irradiation and formation of excited electron−hole pairs in the semiconductor, charge separation occurred due to the already existing Au tip that serves as an electron sink. The accumulated electrons on the Au metal domain were used for the surface chemical reduction reaction of the second metal ions in the solution, in this case Pd or Fe ions. This strategy led to formation of HNPs with an alloy Pd/Au tip, or Au/FexOy core/ hollow shell structure that revealed interesting magnetic properties.59 HRTEM images in Figure 4e,f show the dual metallic nature of the synthesized Au/FexOy tip on CdSe/CdS rods. A different mechanism of exploiting the favorable heterogeneous growth on a pre-existing metal tip serving as a substrate, over homogeneous nucleation of free-standing metal nanoparticles in a solution, was reported by Pyun et al. Successive selective deposition of Co and CoxOy on Pt tipped CdSe/CdS core/shell nanorods was obtained via kinetically controlled syntheses (scheme in Figure 4a).27 Time dependent experiments of Pt growth on CdSe/CdS nanorods showed a majority of single tip deposition at early times (
