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Aqueous Synthesis of Concave Rh Nanotetrahedra with Defect-Rich Surfaces: Insights into Growth, Defectand Plasmon-Enhanced Catalytic Energy Conversion Chin-Sheng Kuo, Chen-Rui Kao, Wei-Jie Chen, Ming-Yen Lu, David A. Cullen, Brian T Sneed, Yu-Chun Chuang, Ching-Ching Yu, and Chun-Hong Kuo Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02003 • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018

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Chemistry of Materials

Aqueous Synthesis of Concave Rh Nanotetrahedra with Defect-Rich Surfaces: Insights into Growth, Defect- and Plasmon-Enhanced Catalytic Energy Conversion Chin-Sheng Kuo,† Chen-Rui Kao,†,§ Wei-Jie Chen,†,‡ Ming-Yen Lu,¶ David A. Cullen,|| Brian T. Sneed,⊥ Yu-Chun Chuang,△ Ching-Ching Yu,§ and Chun-Hong Kuo*,†,□ †

§

Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan

Department of Chemistry and Biochemistry, National Chung Cheng University, Chiayi 62102, Taiwan ‡

¶

Department of Chemistry, National Taiwan Normal University, Taipei 10610, Taiwan

Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan ||

Materials Science and Technology Division and ⊥Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6496, United States △National

□

Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan

Institute of Materials Science and Engineering, National Central University, Jhongli 32001,

Taiwan

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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

ABSTRACT Control of morphology in the synthesis of Rh nanocrystals can be used to precisely tailor the electronic surface structure; this in turn directly influences their performance in catalysis applications. Many works bring attention to the development of Rh nanostructures with low-index surfaces, but limited effort has been placed on the study of high-index and surface-defect-enriched nanocrystals as they are not favored by thermodynamics due to the involvement of high-energy surfaces and increased surface-to-volume ratios. In this work, we demonstrate an aqueous synthesis of concave Rh nanotetrahedra (CTDs) serving as efficient catalysts for energy conversion reactions. CTDs are surface-defect-rich structures that form through a slow growth rate and follow the four-step model of metallic nanoparticle growth. By tuning the surfactant concentration, the morphology of Rh CTDs evolved into highly excavated nanotetrahedra (HETDs) and twinned nanoparticles (TWs). Unlike the CTD surfaces with abundant adatoms and vacancies, HETDs and TWs have more regular surfaces with layered terraces. Each nanocrystal type was evaluated for methanol electrooxidation and hydrogen evolution from hydrolysis of ammonia borane, and the CTDs significantly showed the best catalytic performance owing to the defect-enrichment, which benefits the surface reactivity of adsorbates. In addition, both CTDs and HETDs have strong absorption near the visible light region (382 and 396 nm), for which they show plasmon-enhanced performance in photocatalytic hydrogen evolution under illumination of visible light. CTDs show better photoactivity than that of HETDs, likely due to more pronounced LSPR hot spots. This facile aqueous synthesis of high surface area, defect-rich Rh nanotetrahedra is exciting for the fields of nanosynthesis and catalysis.

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INTRODUCTION Among catalytic materials, Rh has a significant commercial impact for its wide applications in chemical syntheses,1-6 in auto-exhaust and catalytic energy conversion,7-10 and in optical sensing.11-17 Unfortunately, the rare abundance (0.0002 ppm on earth) renders it an expensive material comparable with Au (0.0011 ppm) and Pt (0.003 ppm) metals. For optimum utilization, morphology-controlled synthesis, or morphosynthesis, and surface modification of Rh nanocrystals has become an emerging research interest. Morphosynthesis of metallic nanocrystals tailors the physical and chemical attributes by the precise control of crystal size and shape, which directly influences the nanocrystal performance in various applications.18-20 For example, increasing the number of active sites for catalysis is accomplished by reducing the particle size, and tuning the absorption range of localized surface plasmon resonance (LSPR) for efficient solar energy conversion is done by modulating the particle shape.21-22 Much effort has been directed at the development of Rh nanostructures with low-index surfaces, such as cubes with {100} faces,23-25 plates and tetrahedrons with {111} faces.26-27 In contrast, work is limited on the study of high-index Rh nanocrystals because they are not favored by thermodynamics due to the involvement of high-energy surfaces and increased surface-to-volume ratios. However, more open surfaces of a metallic nanocrystals containing increasingly less coordinated surface atoms causes the upshift of its d-band center to higher energy, and thus can benefit the interaction between the surface and the molecular adsorbates.28-29 Very recently, nanocrystals with defect-rich surfaces, i.e twin boundaries, concave, and terraced faces, have been demonstrated with significantly improved activities and unique properties different from those enclosed by more closed, flat surfaces.13, 18, 30-31 In this work, we demonstrate a facile strategy for the synthesis of concave Rh nanotetrahedra (CTDs) with defect-rich surfaces and visible-light-active properties in the aqueous phase. The 3



structure of concave nanotetrahedra was carefully analyzed and interpreted based on the results acquired by synchrotron XRD and ac-HAADF-STEM imaging. The growth kinetics of the Rh CTDs was investigated revealing the existence of a rate-determining step from the reduction of Rh3+ to Rh+. By tuning the experimental parameters, i.e. the concentration of CTAB, the morphology of Rh nanocrystals could be modulated, by which highly excavated nanotetrahedra (HETDs) and twinned nanoparticles (TWs) were prepared and tested for catalytic energy conversion, in addition to the CTDs. In the three samples, CTDs and HETDs were found to have defect-rich structure owing to their concave faces and etched surfaces. Furthermore CTDs and HETDs had strong absorption peaks of LSPR close to the range of visible light while TWs did not. Based on further characterization, CTDs and HETDs are more active catalysts with defect- and plasmon-enhanced catalytic performances in both methanol electrooxidation and hydrogen generation from hydrolysis of ammonia borane. The CTDs in particular gave the best performance because of their small sizes and abundant atomic-scale defects on the surface, which serve as hot spots for LSPR enhancement. EXPERIMENTAL SECTION Materials. Rhodium(III) bromide hydrate (RhBr3·xH2O, Alfa Aesar), formic acid (HCOOH, 98%, Sigma-Aldrich), hexadecyltrimethylammonium bromide (CH3(CH2)15N(Br)(CH3)3, CTAB, 98%, TCI), hexadecyltrimethylammonium chloride (CH3(CH2)15N(Cl)(CH3)3, CTAC, 95%, TCI), sodium bromide (NaBr, 99.5%, J. T. Baker), methanol (CH3OH, 99.9%, Fluka), potassium hydroxide (KOH, ≥85%, Sigma-Aldrich), and borane-ammonia complex (NH3BH3, 97%, Aldrich) were used without further purification. Deionized water (DI H2O,18.2 MΩ·cm, Sartorius arium pro) was used as solvent in all experiments. Synthesis of Rhodium Nanocrystals. Before the synthesis, an oil bath was preheated to 90 °C and set stirring at a rate of 350 rpm (Fisher Scientific IsotempTM) for a while to ensure the stability of reaction temperature. To synthesize concave Rh tetrahedral nanocrystals, a precursor solution was prepared mixing 9.325 mL of DI water, 0.6 mL of 0.02 M RhBr3·xH2O, 0.1 mmol of CTAB, and 0.075 mL of 98% formic acid in a 20 mL glass vial. The concentrations of CTAB and formic 4


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acid in the ultimate solution were 0.01 and 0.2 M, respectively. The solution was then heated in an oil bath at 90 °C without stirring for 18 hours. Afterward, the formed Rh nanocrystals were collected by centrifuging at 11,000 rpm for 20 minutes. They were redispersed in 10 mL of DI H2O for washing and collected at 11,000 rpm again. Finally, the collected Rh nanocrystals were dispersed in 0.2 mL of DI H2O for further use. In control experiments, the final concentration of CTAB was varied from 0.005 to 0.075 M and that of formic acid was adjusted from 0.05 M to 0.4 M. Highly excavated Rh tetrahedral nanocrystals and twinned Rh nanoparticles were synthesized with 0.005 and 0.075 M CTAB, individually. Characterization. To prepare samples for characterization, rinsing and concentrating by centrifuging had to be carried out. Typically, all products were collected at 11000 rpm for 20 min followed by removal of the supernatant. After adding fresh DI water to reach the original volume, centrifugation was performed again to rinse the dispersions. The washing step was repeated twice. Finally, the collected products were re-dispersed to 0.2 mL and stored in a 1.5 mL centrifuge tube. To prepare samples for SEM and TEM, 5 µL of sample solutions were dropped onto silicon wafers in the size of 0.3 × 0.3 cm2 or carbon-coated copper grids, respectively, allowed to dry at room temperature. Centrifuging steps were done using an Eppendorf Centrifuge 5804 and Thermo Scientific Heraeus Pico 17. SEM images were recorded by a Zeiss Ultra Plus equipped with an Oxford EDX detector, operated at the accelerating voltage of 10 kV. TEM and HRTEM bright-field images were taken by a JEOL JEM-2100F microscope operating at 200 kV. Aberration-corrected HAADF-STEM imaging was conducted on a JEOL 2200FS-AC STEM and a JEOL JEM-ARM200FTH-AC STEM both operated at 200 kV. UV–Vis absorption spectra were measured on a HITACHI U-3310 spectrophotometer. The X-ray diffraction experiments were performed at BL01C2 in National Synchrotron Radiation Research Center (NSRRC). The diffraction data were collected using 18 keV X-rays (0.68888 Å in wavelength) and Mar345 image plate detector with Debye-Scherrer geometry. The patterns were converted by GSAS-II program and the angle calibration was performed according to LaB6 (SRM 660c) standard. The surface chemical states of 5



Rh nanocrystals were measured by PHI Quantera SXM (Ulvac-Phi, Inc.) using single optical scanning device (scanning monochromated Al anode) as X-ray source. Catalyst mass and Rh ion concentrations in solutions were measured by inductively coupled atomic emission spectroscopy (ICP-OES) analyses performed on Varian 720-ES (Agilent Technologies). Electrocatalytic Methanol Oxidation. All electrochemical measurements were carried out in a standard three-electrode cell using an electrochemical workstation (CHI 705E). To prepare the catalyst-loaded working electrodes, 1.5 µg of nanocrystals (determined by ICP-OES) was dropped on a bare glassy carbon electrode (GCE). Then, 2 µL of 0.05 % Nafion aqueous solution was spread on the catalysts and thoroughly dried. A KCl-saturated Hg/Hg2Cl2 electrode was used as the reference electrode and a Pt wire as the counter electrode, respectively. The blank scan of the catalyst-loaded GCE for catalyst surface cleaning was run in cyclic voltammetry (CV) mode at a scanning rate of 100 mV s−1 in a 25 mL of 0.5 M KOH electrolyte solution (Ar pre-purging for 30 minutes) within the range of −1.0 to 0.3 V at room temperature until the CV curve became stable. Next, the cleaned GCE was exposed to a CO purging flow at −0.6 V in a fresh 0.5 M KOH electrolyte solution for CO adsorption followed by CO stripping to determine the electrochemically active surface area (ECSA). To carry out CO stripping, the CO-adsorbed GCE was soon transferred to another fresh 0.5 M KOH electrolyte solution and run at a scanning rate of 50 mV s−1 from −1.0 to 0.3 V. To calculate the ECSA, the value from integration of the total charge collected from the CO stripping peak was divided by the charge per area (0.42 mC/cm2) and further normalized by the catalysts mass. For methanol electrooxidation, sweeping in the CV mode within the range of −1.0 ∼ 0.3 V at a scan rate of 50 mV s−1 was conducted in the 1 M methanol/0.5 M KOH electrolyte at room temperature. The durability test was carried out in the chronoamperometric i-t mode by sweeping at −0.1 V for 1 hour. Catalytic H2 Evolution. Hydrolysis of NH3BH3 as the following reaction: NH3BH3(aq) + 2H2O(l) → NH4+(aq) + BO2‒(aq) + 3H2(g) was carried out to examine the catalytic performances of different Rh catalysts by monitoring the amount of H2 production. In a typical run, a 20 mL of 6


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aqueous solution containing 1 mg/L of Rh catalysts and 23.2 mg of NH3BH3 was prepared in a reaction flask (50 mL) for hydrolysis in dark at four different temperatures of 25, 30, 35, and 40 °C. During the reaction, the flask was sealed by a septum and the reaction medium was agitated by a magnetic stirrer operated at 350 rpm. All amounts of evolved H2 gas was measured by a gas chromatograph (Agilent 7890B) equipped with a GS-CARBONPLOT column (0.32 mm in diameter, 30 m in length, and 3 µm in the thickness of the inner-wall coating) and operated at 50 °C in the oven temperature. The activation energies of the hydrolysis reaction with no catalyst and Rh catalysts were calculated referring to the Arrhenius equation: k = Ae-Ea/RT, where k is the rate coefficient, A is a constant, Ea is the activation energy (kJ/mol), R is the universal gas constant (8.314 × 10‒3 kJ/mol·K), and T is the temperature (in Kelvin). To further investigate the plasmon-enhancement effect in the catalytic activity, a Xe lamp providing visible light irradiation was used as the light source outputting the light power density of 68.6 mW/cm2. During the light-irradiated reaction, the temperature was constantly kept at 27 °C. RESULTS AND DISCUSSION Structural Analysis of Defect-Rich Concave Rh Nanotetrahedra (CTD). In order to build nanostructures with high surface area frameworks and monodispersity, precise control of the kinetics of crystal nucleation and growth is required.32-33 More specifically, modulation of the reduction rate of the metallic precursor in a bottom-up synthesis largely influences the quality of resulting nanocrystals.34-38 In this work, a one-step and one-pot procedure with mild heating in the aqueous phase was utilized for investigation and optimization of Rh nanocrystal growth. In the aqueous synthesis, RhBr3 acts as the metallic precursor and the ionic surfactant, CTAB, in low concentration, was chosen for particle size confinement, shape directing, and ease of removal after synthesis. For reduction of Rh precursors, formic acid (FA) was used instead of multi-carbon reagents such as ascorbic acid. FA is a well-known sustainable liquid fuel widely studied for use in fuel cell systems. As a reducing agent, the formate can either contribute electrons (ideally 2e− per formate molecule) to the Rh3+ ion after coordinating to the Rh complex or be dehydrated by heating 7



to form CO which is also able to reduce the Rh3+ ion (ideally 2e− per CO). These direct (formate) and indirect (CO) pathways, both result in CO2 as the byproduct. However, it is possible to obtain Rh+ rather than Rh0 from Rh3+ reduction relying on the coordination states of formate and CO in a Rh complex or from charge transfer.39-42 The indirect pathway was confirmed in one of our control experiments where a 0.2 M aqueous solution of formic acid was heating in a sealed glass vial at 90 °C for 18 hours. The gas above the solution surface was taken for GC analysis and found to consist of CO and CO2 (Figure S1a). Figure 1a and 1b show the TEM images of the triangle-like nanocrystals that were obtained after reaction. Face-centered-cubic Rh nanocrystals were generated and confirmed by synchrotron X-ray diffraction shown in Figure 1c. After calculating the edge lengths of at least 100 nanocrystals, the size distribution histogram is given in Figure 1d, where the average size is 17 nm with a 7.6% size deviation. To distinguish the structures of the triangle-like Rh nanocrystals, single-particle aberration-corrected (ac-) HAADF-STEM images were acquired at a tilted angle (Figure 1e) and along a [111] axis (Figure S2a). The triangular nanocrystals were observed to be concave Rh nanotetrahedra (CTDs), rather than triangular nanoplates. Single-crystalline regular nanotetrahedra are encased by four {111} faces with four {111} vertices and exhibit higher catalytic activity than that of spherical Rh nanoparticles.5 Notably, the selected-area electron diffraction (SAED) pattern of the single Rh CTD in Figure S2a reveals a single-crystalline structure in spite of the cavities observed on the faces (Figure S2b). Moreover, from high-resolution ac-HAADF-STEM images projected along the [111] and the [211] axes, we can observe the crystal planes of the concave side and edge faces are parallel to the (220) and (111) lattices, respectively (Figure S2c and S2d). This suggests that the CTD is a structure enclosed by a mix of {110} and {111} facets, corresponding with the conclusion demonstrated in Xia’s work.30 Figure 1f is the lattice-resolved image of the bottom-left arm of the Rh CTD in Figure 1e. It shows that the growth of the arms comes from growth of {111} crystal planes. The surfaces of the arms are extremely rough, suggesting an abundance of surface defects such as adatoms and vacancies resulting from the synthesis (Figure 1g). 8


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Formation Mechanism of Defect-Rich CTD. From Figure 1 and S2, the structure of a Rh CTD is a single crystal encased by a mix of {110} and {111} facets and possesses a rough, defect-rich surface. To understand details of its formation, we first focus the discussion on the reduction kinetics of the Rh precursor. Figure 2a is the plot of the ratio of [Rh3+ + Rh+]t/[Rh3+]0 vs time, where [Rh3+]0 and [Rh3+ + Rh+]t denote the initial and the time-dependent concentrations of Rh ions in the reaction solution, respectively. The plot suggests a rapid reduction of Rh ions in the first hour driven by heating, when Rh complexes with low bromide coordination are mostly converted to Rh0 atoms followed by instant nucleation. However, a sluggish drop occurs during the 2nd hour followed by a speedy decrease. The lagging drop is possibly caused by slow reduction of Rh+ complexes, and re-oxidation of formed atoms and nuclei (discussed later). After 6 hours, the reduction slows down gradually. The inset plot of ln([Rh3+ + Rh+]t/[Rh3+]0) vs time suggests reduction of Rh ions follows a first-order reaction at the rate of 5 × 10‒4 min‒1 under heating at 90 °C. In our control experiments, 90 °C was found to be the minimum threshold for efficient reduction of Rh ions. When 80 °C was applied, the resulting solution is almost colorless containing an extremely low amount of aggregated Rh nanocrystals (Figure S3a). When raising the heating temperature to 100 °C, a dichotomy resulted of several smaller spherical Rh nanoparticles along with Rh CTDs, suggesting overlapping of the nucleation and growth phases in this condition (Figure S3b). Figure 2b is the time-dependent histogram recording the evolved percent yields of CTD (blue) and non-CTD (red) nanocrystals by carefully counting over 2,000 particles in every sample. We use “TCTD” here to indicate corner-truncated CTDs that were often observed during the synthesis. It is worth noting that the rate of the increasing yield of CTD/TCTD nanocrystals elevates in the period of the 3rd to the 6th hour, leading to a turnover between the two yields of non-CTD and CTD/TCTD nanocrystals. The result corresponds with the rapid drop in the concentration of Rh ions during the same period of time (Figure 2a). When dissolving the precursor RhBr3 in water, the hydrated rhodium bromide complexes [RhBrx(H2O)6-x]3-x form, the proportions of which change with time and depend on the 9



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concentration of bromide. In the typical synthesis of Rh CTDs, the total amount of bromide ions from the precursor (0.036 mmol) and the CTAB (0.1 mmol) is much more than that of Rh ions (0.012 mmol). This gives high proportions of bromide-rich Rh complexes such as [RhBr6]3− that are stable after reaching equivalence.43-44 In the 1st hour, the rapid drop of Rh ions is a result of the swift reduction of bromide-poor [RhBrx(H2O)6-x] 3-x complexes to Rh0 atoms by formate and CO. This leads to the burst of Rh nuclei from atom aggregation. Referring to the LaMer mechanism of nucleation, the burst of nuclei continues in first few minutes and soon leads to a saturated particle concentration in the solution, followed by nuclei coalescence that generates stable clusters with max free energy ∆G.33, 45-46 These clusters coalesce and their depletion removes the supersaturation condition where growth and structure evolution follow. This estimation was verified by examining the intermediate particles obtained at the reaction times of 5, 10, and 30 minutes. In the sample at the 5th minute, three kinds of particles were observed and classified as oval, trapezoid, and branched shape cross-sections by their projected images in Figure S4a. Both oval and trapezoidal particles are about 7 nm serving as the seeds for the formation of CTDs. Figure S4b shows the atom-resolved TEM image of an oval particle. In the image, (200) and (111) lattices are clearly recorded and the FFT pattern reveals that the particle is viewed along the direction. The results

suggest

that

the

single-crystalline

oval

particle

originates

from

a

platonic

rhombicuboctahedron (RC) after non-equivalent facet growth (inset scheme). Such heterogeneous facet growth is perhaps determined by the favored adsorption of CO and formate on {111} crystal faces. This phenomenon has been studied in the case of noble fcc metals.47 This conclusion also accounts for the formation of a trapezoidal particle. For the branched particles, we believe these arise from the cluster coalescence to form a large cluster with facets, which are a rarely encountered morphology. According to all observed data, the Rh growth mechanism follows a four-step model of metallic nanoparticle growth demonstrated by the Turkevich method.48-49 The four-step nanoparticle growth include (1) reduction of metallic ions (birth of atoms) and nuclei formation from atom aggregation, (2) cluster (seed) formation by nuclei coalescence, (3) adsorption and 10


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reduction of metallic ions in the electric double layers (EDLs) of clusters, and (4) finally reduced metal atoms grow onto the clusters. Checking the samples of the 10th (Figure S4c and S4d) and 30th min (Figure S4e and S4f), the particles show an increasing yield of triangular cross-sections in the projected images. The triangles in fact denote the projected image of the truncated tetrahedral particles shown as the crystal model in Figure S4d. We believe these result from the growth of oval and trapezoidal seeds. Figure S5a to S5e are intermediate particle species collected at reaction times of 1, 2, 3, 6, and 11 hours. It is obvious that the morphology purity and the particle size increase with time. However, there are still many small nuclei (< 2 nm, arrows) along with the crystal seeds in the sample by the end of the 1st hour. We interpret this as a very slow nucleation process possibly caused by two factors, the disturbance of the reduction of Rh3+ to Rh+, and/or the oxidative etching of Rh atoms and nuclei in the acidic solution. Both could explain the sluggish drop of Rh ions (Figure 2a). As aforementioned, the high amount of bromide in the reaction solution brings about high proportions of bromide-rich [RhBrx(H2O)6-x]

3-x

complexes. The bromide coordination

stabilizes the Rh center (lower reduction potential) and forms a negative-charged complex repulsive to formate and CO. This results in a slow reduction of Rh3+ to Rh+, rendering the process a rate-determining step in the Rh nanoparticle growth. The generated Rh+ ions are soon converted to Rh0 assisted by Rh clusters via surface-catalyzed reduction, a known process widely investigated in the case of Au-CTAB.50-51 Noticeably, the “seed-mediated growth” process corresponds with the final stage in the four-step model of metallic nanoparticle growth. The other factor to consider in mechanistic assignment of the drop of Rh ions is in owing to an oxidative etching process as:

2Rh0 + 3O2 + 12Br— + 6H+ → 2[RhBr6]3‒ + 6H2O

This process occurs together with nanoparticle growth. Due to the aqueous conditions, oxygen atmosphere, and the low pH value in the reaction solution (pH = 2.18), oxidation of Rh0 is active and competing with reduction of Rh ions. In the beginning of the reaction, oxidation is minor due to 11



rapid reduction of bromide-poor Rh complexes. However, it becomes significant when the reduction is the rate-determining step and hence results in a decrease of Rh ions. The oxidative etching process is also a key driving force in the formation of surface-defect-rich Rh CTDs. At the end of Rh CTD growth, the supply of Rh atoms almost depletes owing to the shortage of Rh complexes. Meanwhile, the oxidation gradually dominates and begins to etch off the surface atoms of Rh CTDs during their surface reconstruction. Ultimately, this leads to abundant defects on the surfaces of Rh CTDs. After the reaction time of 6 hours, well defined CTD and TCTD are obtained beyond a percent yield of 60%. In the inset panel of Figure S5e, the side-view and the tilted-view images of a Rh TCTD reflect its concave structure. Figure S5f displays a top-view image of a TCTD validating the truncation at the corner. The concave TCTD morphology, as aforementioned, comes from the selective passivation of {111} faces by CO and formate. Accordingly, it is concluded that the capping effect is always active from the stage of oval clusters (seeds) formation. Hence, the CO (formate)-capped nanoparticle growth can be schematically summarized (Figure 2c). In it, there are four stages corresponding with the four-step model of nanoparticle growth. Stage I is the formation of RC nuclei after a burst of Rh0 atoms. Stage II is the CO (formate)-induced non-equivalent facet growth of the RC nuclei, which results in the formation of oval/trapezoid clusters. Based on the shaped clusters, Stage III is “seed-mediated growth” triggered via seed-catalyzed reduction of Rh+ ions. However, most reduced atoms attached to the truncated corners of oval/trapezoid clusters followed by surface diffusion (yellow arrows) and redeposition (due to the ox-red process, pink asteroids) of Rh atoms onto the edges. Stage IV is the repeating cycle of the processes that eventually result in the concave Rh nanotetrahedra with abundant surface defects. The Role of CTAB in Rh nanocrystal Growth. Next we considered the surfactant’s role in the growth of the Rh nanostructures. CTAB consists of a CTA+ ion with the aliphatic long chain and a bromide ion. CTA+ usually plays a major role in size confinement via forming the soft template on the surface of a growing nanocrystal. More importantly, it is an auxiliary in nanocrystal shaping led by the first EDL on a nanocrystal which is the key role to direct the surface capping. In other words, 12


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the nanocrystal morphology is shaped by a synergistic effect of surfactant (CTA+) and EDL ions. Hence, modulating the ionic components in the system of nanoparticle growth alters the surface capping by surfactants and therefore the resulting particle morphology. To investigate the influence of CTAB on Rh nanocrystal growth, syntheses with a series of different CTAB concentrations were carried out. Figure S6 is a collection of TEM images of Rh nanocrystals obtained with 25 mM, 50 mM, and 75 mM CTAB in the solutions. With increasing concentrations, the yield of CTDs decreases while that of nanoparticles with round cross-sections elevates. In the condition of 75 mM CTAB, the products merely contain a majority of twinned and a minority of single-crystalline nanocrystals in irregular shapes (Figure S6c and S6d). The histogram of their size distribution reveals an average size of 12.1 nm with a 17% deviation (Figure S7a), smaller than that of CTDs. This indicates effective size and shape confinement by CTAB. Characterized by ac-HAADF-STEM imaging, the twinned structure (TW) of a round nanoparticle is confirmed (Figure S7b and S7d). It has a much smoother appearance and boundary than that of a CTD despite layered step-terraces (Figure S7c). When a low concentration of CTAB was applied, the concave nanotetrahedra were still obtained yet the four faces of them were highly excavated. Figure 3 and S8 collects the TEM images of the mixed products of highly excavated nanotetrahedra (HETD) in regular and hierarchical (fractal) shapes. The regular HETDs are minor (< 20%) while the hierarchical ones are major. Their average size is 36.2 nm with a 19% deviation, and the corresponding size distribution is shown as Figure 3e. According to the XRD pattern in Figure 3d, the HETDs are fcc Rh. For the regular HETDs, they have a size smaller on average (20−25nm in edge). Figure 3b and 3c show a regular HETD (25 nm) with a clear and intact tetrahedral skeleton. The regular HETD is validated single-crystalline referring to its SAED pattern (the inset in Figure 3d); nevertheless, it has abundant surface defects as those of a CTD. Figure S8a show a single hierarchical HETD with branching or additionally growing pods based on the original one. The branches typically grow into tiny tetrahedral skeletons as well, giving a fractal-type appearance. The hierarchical HETDs have their sizes in a wide range of 30−50 nm. The bigger hierarchical HETD usually possesses the 13



smoother surface (Figure S8b). Accordingly, it is estimated that the defect-rich surfaces of either CTDs or regular HETDs are the result of incomplete surface reconstruction. With either low or high capping by surfactants, the surfaces of Rh nanocrystals experience sufficient reconstruction to reach thermodynamically stability or are otherwise protected by CTA+ and other ions from severe etching. For highly excavated structures, this is understood through measuring the evolved CO amounts in the syntheses because the concave faces result from the CO/formate capping. As shown from Figure S1b to S1d, they are 12.03 µmol/L for Rh TWs, 12.34 µmol/L for Rh HETDs, and 12.30 µmol/L for Rh CTDs, respectively. A small, but still significant increase in the CO amount is generated in the synthesis of the more open structures, especially the HETDs with least interruption of CTAB, bringing about the largely excavated faces of tetrahedra. This phenomenon was also confirmed using low and high FA amounts instead of the original in the synthesis of Rh CTDs. Figure S9a displays the result of a low FA concentration (50 mM). Most nanocrystals in this case form in truncated tetrahedral shape with unclear concave contrast, revealing the slow growth process was ceased by the limited amount of formate and CO. In contrast, Figure S9b shows that similar nanostructures to HETDs were obtained with more FA, but all concave faces were not intact. This is likely due to restriction of growth on the {111} faces by excess formate and CO. Although nanoparticle shaping is a result of a synergistic effect of aliphatic surfactants and ions, the latter becomes especially important in morphology control. We have shown that the concentration of CTAB has been a significant factor to induce and guide the formation of HETDs, CTDs and TWs. However, it is useful to clarify the specific roles of CTA+ and bromide anions. Figure S10 shows the TEM images of the Rh nanocrystals obtained adding an extra 5 and 10 mM NaBr in the synthesis of HETDs. Here the influence of bromide to the nanocrystal faces under the strong interruption of CO and formate can be investigated and decoupled from the contribution of CTA+. In both results, their products give mixtures of single-crystalline, twinned nanocrystals, and a minority of HETDs. Characterized by atom-resolved TEM imaging and SAED, the single-crystalline nanocrystal is found to be a Rh nanocube enclosed by six {100} crystal faces. In 14


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carrying out the synthesis using 10 mM CATC instead of CTAB, nanocrystals in a spiny, sea-urchin-like shape resulted (Figure S11). Figure S11d and S11e reveal that all arms form owing to the dominant growth of {111} crystal planes. These observations correspond well with those demonstrated in much of the previous works,52-55 where chloride was recognized a factor enhancing the growth of single-crystalline nanoparticles with exposed {111} faces but bromide assisted the formation of those with exposed {100} faces or twinned boundaries. This strongly confirms the critical role of halide ions, playing an ionic switch for modulating nanocrystal morphology. Defect‒ and Plasmon‒Enhanced Catalysis. It has been known that surface defects of nanocrystals can benefit their catalytic performance due to the adsorbate-favored upshift of the d-band centers.28-29 To explore the enhancement effect, CTD, HETD and TW nanocrystals were prepared as catalysts for electrocatalytic oxidation of methanol and hydrogen generation from catalytic hydrolysis of ammonia borane. Figure 4a collects the blank-scanning CVs of TWs, CTDs, and HETDs on a GCE in a 0.5 M KOH electrolyte solution to show their constant red-ox behaviors in the basic environment, and Figure 4b are those for methanol oxidation. The electrochemically active surface areas (ECSAs) gained by CO stripping are 5.17 m2/g for CTDs, 5.04 m2/g for HETDs, and 5.29 m2/g for TWs (Figure S12), based on which the J−V plot of methanol oxidation with the Rh catalysts is obtained in Figure 4b, where J denotes current density. Figure 4c is a summary column chart where the blue columns display the specific activities of 7.42 mA/cm2 for CTDs, 6.31 mA/cm2 for HETDs, and 5.07 mA/cm2 for TWs, and the red columns represent the mass activities of 0.38 A/mg for CTDs, 0.32 A/mg for HETDs, and 0.27 A/mg for TWs. The CTDs generate the highest values in both specific and mass activities. This suggests that surface defects on CTDs improve the interaction between catalyst surfaces and methanol. In the chronoamperometric test, all the three catalysts quickly reached stable curves in 10 minutes, indicating their robustness (Figure 4d). It’s a surprising result as defective surfaces generally experience a process of reconstruction to become smooth (minimize their surface energy), leading to less stability. To clarify what change taking place on the surfaces of CTDs and HETDs, the particles post-reaction were taken down from 15



the GCEs in a mixed solution of isopropanol and H2O by ultrasonication. Figure S13 collects the HAADF-STEM images of the CTDs and the HETDs after their chronoamperometric tests for an hour. The results reveal that CTDs and HETDs with defective surfaces persist very well (Figure S13a and S13c). However, the roughness of their surfaces look not much diminished, inferring that the reconstruction seems very slow. A possible reason is due to the stabilization by surface-binding oxygen molecules on the low-coordinated surface Rh atoms, which results in amorphous oxide surface layers (Figure S13b and S13d). It was confirmed by the corresponding XPS spectra of CTDs and HETDs as-synthesized, shown in Figure S14. The estimated components on their surfaces (in the depth of 5 nm) indeed include RhOx (Rh2O3 as the major) in addition to Rh0. For hydrogen generation, hydrolysis of ammonia borane with Rh catalysts at four different temperatures of 25, 30, 35, 40 °C was first carried out for estimating activation energies (Figure S15). The calculation of activation energies was done referring to the Arrhenius equation: k = Ae−Ea/RT, where k is the rate coefficient, A is a constant, Ea is the activation energy (kJ/mol), R is the universal gas constant (8.314 × 10‒3 kJ/mol·K), and T is the temperature (in Kelvin). The equation can be transformed into the function of a natural logarithm: lnk = lnA + (−Ea/R)/T, where the slope in the plot of lnk vs 1/T represents the value of −Ea/R. Figure 5a shows the plots of lnk vs 1/T with no catalyst and the three Rh catalysts. The values of their activation energies are 37.0 kJ/mol for CTDs, 42.5 kJ/mol for HETDs, 44.7 kJ/mol for TWs, and 61.3 kJ/mol for no catalyst. It is clear that hydrogen evolution from ammonia borane is actually a difficult reaction without assistance of Rh catalysts. More importantly, the CTDs show the lowest barrier (the best activity), not only corresponding with that observed in methanol electrooxidation but also verifying the existence of an enhancement effect by surface defects. In addition, localized surface plasmon resonance (LSPR) of Rh nanocrystals is also a factor to induce enhancement in their catalytic performance under irradiation of light. The LSPR is a general property of metallic nanocrystals, by which the light energy is transferred into catalytic reactions to enhance performance.21 Interestingly, the energy range of LSPR is size- and shape-dependent, suggesting the absorption of light is tunable by 16


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modulation of nanocrystal morphology. Figure 5b shows the UV-vis spectra of LSPR absorption for Rh CTDs, HETDs and TWs. The absorption of TWs is inside the UV region and disturbed by other possible charge-transfer absorption of ions.12, 14-15 Both of CTDs and HETDs possess strong and broad LSPR absorption peaks close to the region of visible light (382 and 396 nm), similar to those observed in previous works.56-57 This indicates that a possible transfer of visible light into the catalytic reactions can be carried out with the two Rh tetrahedral catalysts. Figure 5c and S16 show the results of hydrogen evolution without catalyst and catalyzed by the Rh catalysts at 27 °C in dark and under illumination of visible light. In the blank reaction with no catalyst, the reaction rate is not significantly changed under illumination of visible light. This means that the heat interruption from light illumination can be disregarded. For the case of TWs, the specific reaction rates (r) in dark and under light are 40.13 and 41.92 mmol g−1 s−1, quite constant and leading to an enhancement factor of 1.05 (rL/rD). This is a predictable result according to their weak absorption in the region of visible light. However, for HETDs and CTDs, the enhancement factors are 1.28 and 1.68, respectively. The differences in the enhancement of specific reaction rates are summarized in Figure 5d. We conclude that the two kinds of tetrahedral catalysts are able to harvest visible light energy and further transfer it into hydrolysis of ammonia borane via LSPR. However, it has to be pointed out that there is a difference between the values of the enhancement factors by HETDs and CTDs. The difference might be caused by two reasons. First is the large difference in their sizes. As aforementioned, the HETD sample is a mixture of regular and hierarchical HETD morphologies that lead to a very broad size distribution and a large average size of 36.2 nm. Compared with those of CTDs (17.1 nm), this indicates that a lower concentration of particles from the HETD sample would be used for catalysis by taking the same weights of catalysts. Second, the defect sites on catalyst surfaces are potential hot spots of LSPR, providing additional enhancement on surface reactivity. In this regard, CTDs which own higher surface roughness and particle numbers can have relative better plasmon-enhanced catalytic performance than that of HETDs. Since the rough particle surfaces play a key role to the performance of hydrogen production, we are curious about if 17



they are still observable post-reaction, like those in electrocatalysis. Figure S17 shows the HAADF-STEM images of CTDs and HETDs collected after hydrolysis of ammonia borane at ambient without light irradiation for an hour. From the results, it’s clear to see that both CTDs and HETDs are robust in their morphologies (Figure S17a and S17c). Meanwhile, obvious defects persist on their surfaces although the roughness significantly diminishes (Figure S17b and S17d). It fully corresponds with the phenomenon observed after electrocatalysis, and again reflects the existence of a slow surface reconstruction induced by surface-binding oxygen layers. CONCLUSIONS In short, the advantages of this work can be summarized into three points. (1) The aqueous synthesis done in one pot and one step is a benchmark for production of monodispersive, shape-controlled Rh nanocatalysts in water. (2) Concave nanotetrahedra with defect-rich surfaces are produced in the aqueous system and we have showed that the defects exhibit significant enhancement effects on catalytic energy conversion. (3) By modulating the particle morphology, we tuned the typical weak and in-UV-region absorption peak of Rh LSPR to the region close to visible light energy. This rendered the concave Rh nanotetrahedra as a promising energy converter, able to harvest visible light energy and transfer it into catalytic systems. We believe that the findings of the aqueously synthesized Rh nanotetrahedra with large defect- and plasmon-enhanced catalytic properties are exciting and will greatly impact the fields of nanosynthesis and catalysis.

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Figure 1. Bright-field (BF) TEM images of Rh concave tetrahedral nanocrystals (CTD) at (a) low and (b) high magnification. (c) The powder XRD pattern of the CTDs acquired by the synchrotron radiation with the X-ray energy of 15 keV. (d) The size-distribution histogram of the Rh CTDs measured in the edge length. (e) The ac-HAADF-STEM image of a Rh CTD recorded at a tilted angle from the top vertex, and (f) that of its bottom-left corner in high resolution. (g) The ac-HAADF-STEM image of the corner marked in Figure S2a in high-resolution.

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Figure 2. (a) The time-dependent evolution of the [Rh3+]t/[Rh3+]0 ratio in the supernatants of Rh CTD reaction solutions at different reaction times. The inset plot shows the corresponding evolution of ln([Rh3+]t/[Rh3+]0) with time which represents a first-order reduction rate of 5.34 × 10‒4 min‒1. (b) The change in percent distribution of CTD/truncated CTD nanocrystals with time (blue columns). (c) The proposed growth mechanism of a Rh CTD in the aqueous phase.

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Figure 3. (a) BF TEM image of the highly excavated Rh tetrahedral nanocrystals (HETD) obtained with 5 mM CTAB, and (b) that of a single Rh HETD. (c) The HAADF-STEM image of a single Rh HETD. (d) The PXRD and SAED patterns showing the Rh HETDs single-crystalline F.C.C. structure. (e) The size-distribution histogram of the Rh HETDs measured in the edge length. (f) The ac-HAADF-STEM image of the corner marked in Figure 3c in high-resolution.

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Figure 4. CVs of twinned Rh nanoparticles (TW), concave Rh nanotetrahedra (CTD), and highly excavated Rh nanotetrahedra (HETD) on a GCE in (a) 0.5 M KOH, and (b) 1 M MeOH/0.5 M KOH electrolyte solutions. (c) Column chart summary of their corresponding specific and mass activities. (d) The chronoamperometric curves of the Rh catalysts acquired at −0.1 V.

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Figure 5. (a) lnk vs 1000/T plots of blank and Rh nanocrystals in the catalyzed hydrogen generation from hydrolysis of NH3BH3. The activation energies were calculated by referring to the Arrhenius equation. (b) UV-vis spectra of Rh nanocrystals dispersed in water. Peaks of the Rh CTDs and HETDs come from their localized surface plasmon resonance (LSPR) absorption. (c) Time-dependent H2 evolution plots of blank and Rh nanocrystals in the same catalyzed reaction at ambient in dark and under visible light illumination. (d) The column chart summarizing specific reaction rates of blank and Rh nanocrystals. The values above columns are their enhancement factors calculated from rL/rD.

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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX. Particle-size distributions, TEM images, GC spectra, UV-vis spectra, plots of H2 generation, and electrochemical CVs. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Ming-Yen Lu: 0000-0003-1788-1425 David A. Cullen: 0000-0002-2593-7866 Brian T. Sneed: 0000-0002-5656-6180 Yu-Chun Chuang: 0000-0002-2879-5381 Ching-Ching Yu: 0000-0003-2103-6419 Chun-Hong Kuo: 0000-0001-6633-8985 Author Contributions †

C.-S. K., C.-R. K. and W.-J. C. contributed equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS We are grateful for the technical support from Ms. I-Hui Chen, the technician in the Advanced Nano/Micro-Fabrication and Characterization lab in Academia Sinica (AS) for TEM characterization and operation training. A portion of the electron microscopy was performed as part 24


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of a user project through Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences, which is a U.S. Department of Energy (DOE) Office of Science User Facility and using instrumentation provided by the U.S. DOE Office of Nuclear Energy, Fuel Cycle R&D Program, and the Nuclear Science User Facilities. We specially thank Ms. Mei-Ying Chung, the technician in the Institute of Chemistry (IOC) in AS, for carrying out SEM and ICP-OES analyses. All catalytic reactions were done with the instruments in the Center of Catalytic Facility in IOC, AS. This work is financially supported by the Ministry of Science and Technology, Taiwan (MOST 106-2113-M-001-030-MY2), Academia Sinica (Innovative Materials and Analytical Techniques), and Executive Yuan, Taiwan (Government Policy Allocation Plan for Key S&T Developments). REFERENCES (1) Vermisoglou, E. C.; Romanos, G. E.; Karanikolos, G. N.; Kanellopoulos, N. K. Catalytic NOx Removal by Single-Wall Carbon Nanotube-Supported Rh Nanoparticles. J. Hazard. Mater. 2011, 194, 144‒155. (2) Guo, W. S.; Pleixats, R.; Shafir, A.; Parella, T. Rhodium Nanoflowers Stabilized by a Nitrogen-Rich PEG-Tagged Substrate as Recyclable Catalyst for the Stereoselective Hydrosilylation of Internal Alkynes. Adv. Synth. Catal. 2015, 357, 89‒99. (3) Nguyen, L.; Liu, L. C.; Assefa, S.; Wolverton, C.; Schneider, W. F.; Tao, F. F., Atomic-Scale Structural Evolution of Rh(110) during Catalysis. ACS Catal. 2017, 7, 664‒674. (4) Jiang, Y. Q.; Su, J. Y.; Yang, Y. A.; Jia, Y. Y.; Chen, Q. L.; Xie, Z. X.; Zheng, L. S. A Facile Surfactant-Free Synthesis of Rh Flower-Like Nanostructures Constructed from Ultrathin Nanosheets and Their Enhanced Catalytic Properties. Nano Res. 2016, 9, 849‒856. (5) Park, K. H.; Jang, K.; Kim, H. J.; Son, S. U. Near-Monodisperse Tetrahedral Rhodium Nanoparticles on Charcoal: The Shape-Dependent Catalytic Hydrogenation of Arenes. Angew. Chem. Int. Ed. 2007, 46, 1152‒1155. (6) Guan, H. L.; Lin, J.; Qiao, B. T.; Yang, X. F.; Li, L.; Miao, S.; Liu, J. Y.; Wang, A. G.; Wang, X. D.; Zhang, T. Catalytically Active Rh Sub-Nanoclusters on TiO2 for CO Oxidation at Cryogenic 25



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