Letter pubs.acs.org/JPCL
Designing Bimetallic Co-Catalysts: A Party of Two Eran Aronovitch,† Philip Kalisman,‡ Shai Mangel,† Lothar Houben,§ Lilac Amirav,‡ and Maya Bar-Sadan*,† †
Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva 8410501, Israel Schulich Faculty of Chemistry, TechnionIsrael Institute of Technology, Haifa 32000, Israel § Peter Grünberg Institut 5 and Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany ‡
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S Supporting Information *
ABSTRACT: The enhanced catalytic properties of bimetallic particles has made them the focus of extensive research. We compare the photocatalytic activity for hydrogen production of core−shell structures of Au@Pd and Au@(Au/Pd alloy) on seeded rods of CdSe@CdS and show that Au@alloy was superior toward hydrogen production. Our finding reveals that the promotion effects of Au in Pd originate both from the alteration of the electronic structure by the Au core as well as by the atomic rearrangement of the surface. Long-term monitoring of the activity of the photocatalysts offered insights into the dynamic processes during the illumination showing that the tip morphology influenced the stability of the hybrid structures. The Au core served as a physical barrier, protecting the CdS rod against cation exchange reactions with the Pd. The coupling of these factors to achieve synergistic effects is therefore a prime aspect in the rational design of efficient cocatalysts.
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metal tip on the overall performance of the photocatalyst. Another driving force to explore bimetallic cocatalysts is the cost of the known catalytic metals, primarily Pt, which motivates their mixing with other metals or searching for more abundant metals.20−24 Here, we compare two bimetallic Au−Pd morphologies: Au@Pd and Au@(Au/Pd alloy) as reduction cocatalysts (see schematic representation in Figure 1) using CdSe@CdS as the photoactive charge carrier generation unit. We study the effects of the inner composition of the two metals on the H2 production and on the stability of the hybrid particle, which were all found to be sensitive to the tip morphology. We carried out our experiments for 50 h of continuous illumination, where the H2 production was regularly monitored in order to understand the dynamic processes over the illumination period. We found that CdSe@CdS capped with either bimetallic tip outperformed the monometallic ones. The CdSe@CdS with Au@alloy tips were superior, both in terms of photocatalytic activity and stability. The synthesis of CdSe@CdS was carried out according to published protocols (see the Supporting Information for detailed description of the synthetic procedures). Photodeposition was used in order to promote the deposition of only a single tip for each seeded rod: Au tips were grown by photodeposition in a modified protocol based on ref 25; Pd was photodeposited with slight changes based on ref 26. TEM
roducing considerable amounts of clean and renewable hydrogen fuel by water splitting is a promising route to supply the world needs for sustainable energy sources. An efficient photocatalyst is a hybrid system, composed of a multicomponent photoactive charge carrier generation unit, which is designed for maintaining charge separation, a sink to collect the charge carries, and finally a suitable substrate for catalyzing the chemical reaction. It is common to couple a semiconductor as the electron photoexcitation component1 with noble metal catalysts upon which the reaction takes place.2−7 Metals are suitable for trapping photoexcited electrons, prolonging their lifetime and enabling them to participate in reduction reactions.1,3 The use of bimetallic tips, where two metals form the catalytic site of the system, is known to enhance catalytic activity toward numerous reactions.8−15 Recently, a few reports have shown the advantage of using bimetallic tips for photocatalytic water splitting.10,16 Loading the metal particles on irregular porous substrates such as TiO2 or SiO2 prevents direct comparison of the different tips and their morphologies, thus hindering the drawing out of more general design rules. In contrast, seeded rods of CdSe within CdS (CdSe@CdS) can be grown with particular chosen dimensions. For CdSe@CdS, the influence of parameters such as the distance between the reaction sites and the degree of charge separation on the photocatalytic performance is well known3,17−19 and can be controlled by the nanorod length and by the relative band alignment by tuning the seed size. Depositing a bimetallic tip on CdSe@CdS facilitates the understanding of effects such as the internal structure of the © XXXX American Chemical Society
Received: August 4, 2015 Accepted: September 2, 2015
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DOI: 10.1021/acs.jpclett.5b01687 J. Phys. Chem. Lett. 2015, 6, 3760−3764
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The Journal of Physical Chemistry Letters
Figure 1. Schematic representation of the photocatalysts. CdS rod was grown on a CdSe core, and then photodeposited with metal tips. From the left: Au@alloy, Au@Pd, Au, and Pd tips.
images of the nanorods with their metallic tips are displayed in Figure 2. The morphology of the monometallic tips (see Figure 2A−B) was in accordance with literature findings: Au tips were quasispherical; Pd tips resembled matches with quasirectangular shape.27 Bimetallic samples of Au/Pd were produced either by a two-step process (first a Au core was grown, followed by a Pd shell) or by injecting both precursors simultaneously. In the latter case, the higher reduction potential of Au led to the formation of a Au core followed by a disordered shell comprising both metals. The CdS−Au interface gives the bimetallic tips (Figure 2C−D) the overall spherical appearance typical to Au tips (Figure 2A), rather than the typical match-like morphology of the Pd tip (see Figure 2B). Figure 3 presents the energy dispersive X-ray spectroscopy (EDS) mapping of the Au core (yellow) and Pd shell (blue). For the codeposition process, the Au core was resolved with overlapping signals of Au and Pd at the periphery of the structure (islands), indicating a mixture of Au and Pd, probably as an alloy. (Figure 3D−F). Au and Pd are a specific case of mixing a catalytically active noble metal (Pd) with a more catalytically inert noble metal (Au). Their alloy promotes various reactions, such as synthesis of vinyl acetate,28 CO oxidation,29 H2O2 synthesis,11 oxidation of alcohols using TiO230,31 and electro-oxidation of formic acid.14 The origin of enhanced catalytic ability of bimetallic Au−Pd over pure Pd was attributed to a few factors:11 (1) The new surface rearrangement which is provided by the alloys separates single Pd sites by Au and leads to a decrease in the adsorption strength thus enabling easier release of products. (2) The mixing between Au and Pd is energetically favorable, and some Pd−Pd repelling forces are then compensated for by the presence of Au atoms and the net transfer of electrons to the Au. (3) The Pd unit cell is smaller than Au by 4%, forcing a longer Pd−Pd bond that alters the d band and leads to the same desired effects (easier release of products and less selfpoisoning).11 The photocatalytic activity of CdSe@CdS with different bimetal compositions toward hydrogen production was examined. Solutions of CdSe@CdS suspended in water with isopropyl alcohol (10% by volume) acting as a hole scavenger (electron donor) were placed in a custom-built gas tight reaction cell purged with argon. The samples were then illuminated with a 455 nm LED, adjusted to 100 mW
Figure 2. TEM images of the monometallic and bimetallic seeded rods of Au (A,E), Pd (B,F), Au@Pd (C,G), and Au@alloy (D,H) before and after, respectively, the illumination period. The red arrows highlight areas where degradation and dissolution of the rods is observed.
(equivalent to a photon flux of 2.3 × 1017 photons/sec). The evolving hydrogen was analyzed in real time using an online gas chromatograph equipped with a thermal conductivity detector (see the Supporting Information (SI)). Operation in continuous flow mode allowed for direct determination of the evolved gas production rate. There were two sets of seeded rods: the first were capped with Au, Pd, and Au@Pd tips and a second set with Au and Au@alloy tips. Because geometric factors affect hydrogen production, we use here the seeded-rod Au tip of each experiment as an internal calibration and present the results relative to it. In addition to measuring the hydrogen production 3761
DOI: 10.1021/acs.jpclett.5b01687 J. Phys. Chem. Lett. 2015, 6, 3760−3764
Letter
J. Phys. Chem. Lett. Downloaded from pubs.acs.org by WEST VIRGINIA UNIV on 09/11/15. For personal use only.
The Journal of Physical Chemistry Letters
Figure 3. EDS elemental mappings of the bimetallic photocatalyst systems. All scale bars are 3 nm. The Au is in yellow and the Pd is in blue. The images to the left are overlays of the individual Au, Pd, and Cd maps. Upper panel: Au@Pd tip. Lower panel: Au@alloy tip.
at the end of the experiment, we followed the H2 accumulation over time, in order to understand the synergistic effects on both functionality and stability. Figure 4A provides the relative accumulated hydrogen that was obtained from the different samples relative to their Au-
tipped counterparts. The morphology effect of the alloy tip is striking: the Au@Pd showed moderate elevation in the structures’ efficiency, which we measure by the relative hydrogen production. However, the Au@alloy tips show over 5-fold increase in activity for H2 production. This confirms that the Au promotes the activity of the bimetallic tip not only due to electronic effects, which are expected to occur for both the Au@Pd and Au@alloy structures, but also by the creation of different active sites on the surface. Additional factors to be considered are the contributions of CdSe@CdS rods that were not deposited with a metal tip and of free metal particles. Previous reports showed that uncoated seeded rods of CdSe@CdS have significantly decreased activity toward hydrogen production than seeded rods decorated with metal tips.32 The initial concentration of free CdSe@CdS rods was
