Subscriber access provided by Nottingham Trent University
Article
Seeded rods with Ag and Pd bimetallic tips – spontaneous rearrangements of the nanoalloys on the atomic scale Eran Aronovitch, Lothar Houben, and Maya Bar-Sadan Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b01523 • Publication Date (Web): 12 Aug 2019 Downloaded from pubs.acs.org on August 13, 2019
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 8 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Chemistry of Materials
Seeded rods with Ag and Pd bimetallic tips – spontaneous rearrangements of the nanoalloys on the atomic scale Eran Aronovitch1, Lothar Houben2, Maya Bar-Sadan1* 1 Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva, Israel 2 Department of Chemical Research Support, Weizmann Institute of Science, Rehovot, Israel
ABSTRACT: Deposition of metal co-catalysts is a common practice to improve the activity of photocatalysts. The use of nano-alloyed nanoparticles allows the formation of diverse nanostructures, tailored for a specific application. Nevertheless, too often the spontaneous atomic scale phenomena interfere with the initial design to produce a modified structure with undesirable properties. Here, we demonstrate such a process for Pd, Ag or their combination as metal tips mounted on seeded rods of CdSe dot in a CdS rod (CdSe@CdS) that serve as hydrogen evolution photocatalysts. Spontaneous radial reconstruction at the metal tip brings both Pd and Ag atoms outwards even when a two-stage preparation process is applied to specifically produce a core-shell structure. The diffusion of Pd outwards enables hydrogen evolution even when the initial Pd tip is covered by a Ag shell, and in the opposite case, a Pd shell shows reduced activity compared with Pd-only tips, due to the surfacing of Ag atoms. In addition, we show that the tip reconstruction occurs already during synthesis; aberrationcorrected high-resolution electron microscopy also reveals other processes, such as cation exchange and small clustering around the seeded rods, all quite invisible using regular TEM techniques. In addition, we studied the size effect of Pd-tipped seeded rods and showed that the %QE of seeded rods with 2.2 Pd tips is as high as 91%. These results are significant in the understanding of the structure-function relationship, as it highlights one possible hidden reconstruction pathway of nanoalloys.
1. Introduction Although well-known, the changing phenomena that emerge with shrinking dimensions continue to pose a complicated challenge for those who wish to characterize and understand the structure-properties correlation in the nano-scale. The quantum size effect, for example, is well known, but beyond the nanoscale lies the atomic scale, where single atoms and atomic layers significantly determine the functionality of nanostructures. One such functionality is heterogenous catalysis, where the construction of the very last atomic layer of the solid is extremely important, since the identity of its constituent atoms is a key to achieve specific functionality by modifying the adsorption energies for the reactants, intermediates and products. In the extensively researched materials system of Pd and Ag, alloying offers attractive properties for hydrogen separation and purification.1 Although in the bulk form the two metals form a solid solution at every mixing ratio, for nanoparticles, theory predicts a strong tendency for phase segregation due to different surface energies.2-6 Furthermore, the ambient environment dictates the preference for one of the two metals at the surface, with Ag favored in vacuum and Pd favored in the presence of H2 or O2 partial pressures.2 The preference for a Pd overshell is partially attributed to the stronger binding of H to Pd than to Ag.3 The structural features of the exposed alloy surface, where the catalyzed chemical reaction takes place, dictate the activity and selectivity of the various reactions,4, 6-8 making the challenge of characterizing them an important topic of research.
Commonly used techniques for characterization at the nanoscale, such as imaging by high resolution electron microscopy, are not useful in this case since the main contrast mechanism is mass contrast and Pd and Ag are neighboring elements in the periodic table. EDS mapping of the surface of a 3D sphere is also limited in its resolution and detection. In a recent paper, atom probe tomography was used to directly image the segregated Pd layer, 1-10 atoms thick, surrounding the Ag core.6 This work demonstrated that the thin shells of Pd are advantageous due to the promotion effect of the Ag and the charge transfer between the metals.6 However, most of the currently available studies assume that the bimetallic nanoparticles are stable structures as prepared, usually comparing the starting materials with the obtained catalytic results. Under real operating conditions, it is known that hydrogen intake causes volume expansion of Pd and AgPd that results in mechanical instabilities.1 Moreover, our previous work has shown that similarly grown bimetallic Pd and AuPd tips on seeded rods of CdSe core – CdS rod (CdSe@CdS) used as photocatalysts, exhibit quite mobile Pd atoms that eventually degrade the structure of a spherical bi-metal tip over time, and bring the loss of the photocatalytic functionality.9-10 Another topic of interest is the deposition of the metal tip itself on the substrate, as opposed to the competing process of reduction and cation exchange, where the deposited metal cations substitute the metal ions of the substrate nanoparticle (which is usually another semiconductor).11-12 Previous work has shown that many parameters (such as the deposition protocol, the concentration of the substituting metal ions and
ACS Paragon Plus Environment
Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
their loading, the size of the substrate, the reaction temperature and the bound ligands) all play a delicate role in controlling the oxidation state of the deposited metal tip and its structure or morphology.13 Here we use Ag, Pd, and AgPd co-catalyst particles deposited on seeded rods of CdSe-CdS as probes to study the surface reconstruction of the AgPd nanostructures before and during the photocatalytic process. The ability of Ag to attract and store electrons is appealing for some catalytic applications14 but it possesses poor H adsorbing properties,15 preventing its use as a catalyst for the hydrogen evolution reaction. We used the capability of the seeded rods with the metallic tips (Pd, Ag or PdAg) to produce hydrogen to indicate the composition of the external metallic surface. We show that although the tips are prepared in a two-stage process, spontaneous mixing of the Ag and Pd occurs such that the surface of the bimetallic tip contains both atoms, regardless of the growth procedure. Consequently, the bimetallic seeded rods produce hydrogen even when the outer shell was designed to comprise only Ag, and in the parallel case, where the aim was to produce an outer shell of only Pd, its activity towards hydrogen production is much lower than monometallic Pd tips. We also show that similarly to Au and Ni, Pd also has an optimal tip size for hydrogen evolution, and that the analysis should take the tip size into account.
2. Experimental Section In order to allow direct comparison between the various structures, it is extremely important to control the dimensions of the CdSe seeds, the CdS rods and the metal tips. In addition, an adequate amount of material needs to be produced in order to enable the photocatalytic measurements. Therefore, each set of experiments consists of a large batch of CdSe@CdS rods that was divided for smaller portions for the metal deposition stages (for the full experimental procedures see the Supporting Information). To grow CdSe seeds, cadmium oxide (CdO), octadecylphosphonic acid (ODPA) and trioctylphosphine oxide (TOPO) were mixed and aged in a three-necked flask until homogenous. Selenium (Se) was dissolved in trioctylphosphine (TOP). The TOP:Se solution was injected into the main flask at 370°C. The heating mantle was removed 20sec after the injection time. The obtained CdSe nanoparticles with a diameter of 2.2 nm were used as seeds for the growth of the CdS rods. Synthesis of seeded nanorods of CdSe@CdS: CdO, ODPA, hexylphosphonic acid (HPA) and TOPO, were mixed together at 150°C in a three-necked flask. In a separate vial, sulfur (S) was dissolved in TOP. Once the Sulfur dissolved completely, the previously prepared CdSe seeds solution in toluene was injected into the flask. The TOP:S-CdSe solution was injected to the flask at 360°C and was maintained at that temperature for 8 min before the heating mental removal. The Pd deposition: For Pd deposition, palladium chloride (PdCl2) and didodecyldimethylammonium bromide (DDAB) were dissolved in 5 ml of toluene. In a separate container, a solution of octadecylamine (ODA) in toluene was prepared by dissolving ODA in toluene with mild heating. 0.2ml of methanol was added to the previously prepared CdSe@CdS as a hole scavenger and the suspension was stirred with a magnetic stirrer and illuminated with a 300W Xenon lamp equipped with a cutoff filter blocking radiation below 400nm. The Pd solution and the ODA solution were injected together in small amounts. Overall the solution was illuminated for 20-25 minutes before the xenon lamp was turned off. The Ag deposition: The silver stock solution was prepared by dissolving AgNO3 and DDA
(Didodecylamine) in ethanol. The rest of the process was similar to the Pd deposition except the addition of methanol which was unneeded since ethanol (which also acts as a hole scavenger) was part of the stock solution. The co-deposition: both Ag and Pd stock solutions were injected simultaneously. The metal precursors were injected in small volumes. The use of the illumination promotes the deposition of a single metal tip16-17 and the presence of amines promotes the reduction of the silver ions to Ag0.18 In addition, in order to further suppress the cation exchange reaction, slow addition of reactants and an overall low loading (about 5% of the existing Cd atoms in the rods) were used. First, one batch of the CdSe@CdS nanorods was photo-deposited with Pd tips of various sizes and another batch was deposited with Ag in various loadings. Then, a third batch was produced with either Pd, Ag, Pd core – Ag shell (Pd@Ag), Ag core – Pd shell (Ag@Pd) or a co-deposition of the two metals (Pd-Ag). Prior to the photocatalytic measurements, the particles went through ligand exchange to suspend them in water. The photocatalytic hydrogen evolution measurements utilized a closed system that includes an airtight cell, a light source, and a gas chromatography (GC) instrument. The evolved H2 molecules were carried continuously from the cell to the GC instrument and measured. The illumination wavelength was 455 nm, the pH value was maintained at 11 and isopropanol (25% by volume) was used as the hole scavenger. The high activity of the particles required the use of a rather high isopropanol fraction in order to allow long measurements without a significant fluctuation in the hole scavenger concentration. The measurements usually ran for 36 h. During that time the solvent partially evaporated and the reactor was filled with additional solvents after 18 h, and then again after additional 6 h. Samples for TEM were drop-casted on grids and gently dried at 35 °C under high-vacuum for a few days to remove organic residues. Characterization of the photocatalysts was done using state-of-the-art aberration corrected transmission electron microscopes at a low acceleration voltage of 80 kV. Compositional mappings were performed with high resolution energy dispersive X-ray spectroscopy (EDS).
3. Results and discussion Previous publications have pointed at the significant effect of the metal tip size on the photocatalytic activity.19-20 To understand the effect of the metal tip size on the photocatalytic activity, and to separate this effect from that of the PdAg composition, we first prepared seeded rods with mono-metallic Pd tips in different sizes. We chose to calibrate to the size of Pd tips rather than to the Ag size, since the Ag tips produced only negligible amounts of hydrogen, as will be shown in the photocatalytic studies. Figure 1 presents the elemental mapping of a seeded rod with a Pd tip, showing the rod area where the Cd:S ratio is 1:1 and the single tip of Pd at the rod’s edge. See the Supporting Information for additional TEM images (Figure S1), UV-Vis absorption spectra (Figure S2) and a summarizing table which includes size statistics (Table S1). The deposition of Ag on CdS rods can potentially result in a cation exchange reaction and the formation of Ag2S rather than Ag0.11-13, 18, 21 The tendency for cation exchange is enhanced when the loading of the Ag+ is above 30% of the Cd within the CdS structure.11-12 We therefore made sure to maintain low Ag+ concentration in the solution by slowly adding the Ag precursor. In addition, the use of illumination for the photo-deposition of the metals facilitates the reduction of Ag+ to Ag0, by making use
ACS Paragon Plus Environment
Page 2 of 8
Page 3 of 8 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Chemistry of Materials of the photo-generated electrons,16, 22 and in the presence of amines.18 Once a metal tip is formed, the capability of Ag to attract and store electrons could promote further growth of the metal particle on the expanse of the cation exchange. Nevertheless, the deposition of small clusters or single atoms/ions along the facets of the CdS rods is possible and such a deposition will not be reliably detected by electron microscopy. The cation exchange in this system can be detected by TEM: the Ag2S segments have a typically darker contrast compared with CdS, they occupy the diameter of the original rod, are spaced 12-14 nm apart, not only at the tip.11-12, 21, 23-24 Similar observations were also made for the deposition and cation exchange of Cd by Au, where the differences in atomic number between Au and Cd enabled direct imaging of the Au2S-CdS junction in atomic-scale resolution, since the cation exchange preserves the anionic lattice structure.16, 25
of all the constituting elements, including S, making the transformation of Ag2S to Ag0 a negligible phenomenon (See Figure S8 for details). Structures with 0.01 Ag loading and their EDS elemental maps are presented in Figure 2a-d. Only a small fraction of the rod was deposited with Ag, showing specific Ag locations on top of the rods, while Cd and S signals overlapped. We could not detect Ag in the CdS rod, such that if it exists, it is beneath the detection limit (⁓0.5%, see Figure S9a). The Ag tip did not show the lattice of Ag2S but some S content was present at the tip, in a ratio of 4:1 Ag:S, and no Cd was detected. We conclude that although most of tip is Ag, the presence of silver sulphide in a small fraction cannot be excluded. We found similar findings for the higher loading (Figure S9b); Ag was not present along the rod, and the tip contained S and also Cd, in a ratio of app. 1:1, possibly showing a CdS rod beneath the Ag tip.
Figure 1. HAADF STEM image of seeded rods with a 1.5nm Pd tip (a) and their elemental mapping (b). A single Pd tip per rod structure is presented (in green) as well as Cd (blue). S was also present in an approx. ratio of 1:1 with Cd across the length of the rods, indicating the CdS composition. Here, we have used loadings of Ag:Cd in the range of 0.01-0.40 (See Figure 2, Figure S3 and Table S2, and Figure S4 for the UV-Vis absorption spectra). We saw that there is no direct correlation between the loading and the product, although higher loadings generally produced a larger fraction of rods tipped with Ag, larger tips (varying from 2.2 nm to 5.1 nm, on average), and a secondary nucleation on the long facets of the rods or a second tip at the opposite edge (Figure 2a, d and Figure S3). Darker strips were not imaged for the full set of samples, only darker tips, supporting the suppression of cation exchange. XRD and XPS studies were not carried out since the material produced was needed for the photocatalytic measurements and previous studies have shown that in these small loadings (
