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Syntheses, Plasmonic Properties, and Catalytic Applications of Ag-Rh Core-Frame Nanocubes and Rh Nanoboxes with Highly Porous Walls Yun Zhang, Jaewan Ahn, Jingyue Liu, and Dong Qin Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00602 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on February 26, 2018

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Chemistry of Materials

Revised MS #cm-2018-00602b

Syntheses, Plasmonic Properties, and Catalytic Applications of Ag-Rh Core-Frame Nanocubes and Rh Nanoboxes with Highly Porous Walls

Yun Zhang,†,‡ Jaewan Ahn,† Jingyue Liu,¶ and Dong Qin†,*

†

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia

30332, United States ‡

College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, Gansu 730000,

P. R. China ¶

Department of Physics, Arizona State University, Tempe, Arizona 85287, United States

*Corresponding author: [email protected]

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Abstract

We report a simple and general method for the production of Ag-Rh bimetallic nanostructures with a unique integration of plasmonic and catalytic properties exemplified by these two metals, respectively. When a Rh(III) precursor is titrated into a polyol suspension of Ag nanocubes held at 110 oC in the presence of ascorbic acid and poly(vinylpyrrolidone), Rh atoms are generated and deposited on the nanocubes. When the amount of Rh(III) precursor is relatively low, the Rh atoms tend to nucleate from the edges of the Ag nanocubes and then follow an island growth mode because of the relatively low temperature involved and the high cohesive energy of Rh. The Rh islands can be maintained with an ultrafine size of only several nanometers, presenting an extremely large specific surface area for catalytic applications. As the amount of Rh(III) precursor is increased, the galvanic replacement reaction between the Rh(III) and Ag nanocubes will kick in, leading to the formation of increasingly concaved side faces and increase in surface coverage for the Rh islands. Meanwhile, the resultant Ag+ ions are reduced and deposited back onto the nanocubes, but among the Rh islands. By simply controlling the amount of Rh(III) precursor, we observe the transformation of Ag nanocubes into Ag-Rh core-frame and then AgRh hollow nanocubes with a high porous surface. Upon selective removal of Ag by wet etching, the hollow nanocubes evolve into Ag-Rh and then Rh nanoboxes with highly porous walls. While the Ag-Rh core-frame nanocubes show a unique integration of the plasmonic and catalytic properties characteristic of Ag and Rh, respectively, the Rh nanoboxes show remarkable activity toward the catalytic degradation of environmental pollutants such as organic dyes.

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INTRODUCTION Manipulating the elemental composition of noble-metal nanocrystals provides a powerful means to tailor their properties for a variety of applications, including those related to catalysis,1,2 plasmonics,3-5 and biomedicine.6,7 To this end, bimetallic nanocrystals received ever increasing interest in recent years owning to the large number of possible variations in terms of elemental combinations and their spatial distributions. As a major advantage over the monometallic system, the involvement of two metals offers many opportunities to enhance their catalytic performance and optical properties, in addition to the creation of new capabilites.8-12 For example, one can tailor the localized surface plasmon resonance (LSPR) properties of Pd nanoplates by forming core-shell plates with controllable thickness for the Ag shell.13 Others demonstrated the transformation of Pd nanocubes to Pd-Ag bimetallic nanocrystals in dimeric structure with unique LSPR properties.14 Both the electrochemical reduction and oxidation reactions catalyzed by Pd-Pt bimetallic nanocrystals are substantially enhanced relative to either Pd or Pt nanocrystals and the activities can be further optimized by tuning the electronic coupling between these two metals.15-17 Although remarkable progress has been made in recent years, it is still challenging to generate bimetallic nanocrystals for some combinations of metals due to their marked difference in chemical reactivity and thus the possible involvement of galvanic replacement. Taking Rh as an example, its nanoparticles have been extensively used to catalyze a large number of reactions that include reduction,18 oxidation,19 cross coupling,20 and hydroformylation.21 Although shapecontrolled synthesis of Rh nanocrystals has been explored to a certain extent,22-24 there are only a very limited number of reports on M-Rh bimetallic nanocrystals, with the majority of them based on the Pd-Rh system. For example, with the use of Pd nanocrystals as seeds, Xia and coworkers have demonstrated the synthesis of Pd-Rh nanocrystals with either a core-frame or a core-shell structure.25,26 Tsung and co-workers also demonstrated the overgrowth of Rh on Pd nanocrystals with different shapes, as well as their subsequent transformation into Rh-based nanoboxes.27,28 Like Rh, however, Pd does not have LSPR in the visible region, excluding the use of Pd-Rh nanocrystals for LSPR-related applications such as surface-enhanced Raman scattering (SERS). A highly desirable replacement for Pd would be Ag, one of the least expensive noble metals with spectacular performance in applications related to both LSPR and SERS.

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Herein, we report the facile synthesis of Ag-Rh core-frame nanocubes and Rh nanoboxes with highly porous walls for plasmonic and catalytic applications. Specifically, we disperse Ag nanocubes in ethylene glycol (EG, a solvent and reductant) containing ascorbic acid (H2Asc, a co-reductant) and poly(vinylpyrrolidone) (PVP, a capping agent and colloidal stabilizer) and then titrate a solution of Na3RhCl6 in EG at an elevated temperature. In this system, Rh atoms can be produced from the Rh(III) precursor through chemical reduction by EG and H2Asc, as well as the galvanic replacement reaction with Ag nanocubes. When a relatively small amount of the Rh(III) precursor is titrated into the reaction system, we observe the preferential deposition of Rh atoms (mainly derived from the chemical reduction route) as small islands of only a few nanometers in size on the edges or {110} facets of the nanocubes, leading to the formation of Ag-Rh coreframe nanocubes. The core-frame nanocubes embrace a unique combination of the catalytic and plasmonic properties inherent to Rh and Ag, respectively, for in situ monitoring the degradation of Rhodamine 6G (R6G) by SERS. On the other hand, if a relatively large amount of the Rh(III) precursor is titrated into the reaction system, Ag atoms can also be removed from the side faces or {100} facets of the nanocubes owing to the initiation of galvanic reaction, generating cavities on the side faces partially covered with small Rh islands. Concurrently, the released Ag+ ions are reduced by EG and H2Asc, followed by their deposition back onto the nanocubes among the Rh islands, for the generation of Ag-Rh hollow nanocubes with small Rh islands on the edges and side faces. When Ag is removed by wet etching, the hollow nanocubes evolve into Rh nanoboxes with highly porous walls, which show a remarkable activity toward the catalytic degradation of R6G in an aqueous system.

EXPERIMETNAL SECTION Chemicals and Materials. Ethylene glycol (EG) was obtained from J.T. Baker. Sodium hexachlororhodate(III)

(Na3RhCl6),

silver

trifluoroacetate

(CF3COOAg,

98%),

poly(vinylpyrrolidone) with an average molecular weight of 55,000 (PVP-55), sodium hydrosulfide hydrate (NaHS·xH2O), aqueous hydrochloric acid (HCl, 37%), L-ascorbic acid (H2Asc, 99%), 2,6-dimethylphenylisocyanide (2,6-DMPI, ≥98.0%), Rhodamine 6G dye (R6G), sodium borohydride (NaBH4), iron(III) nitrate (Fe(NO3)3), nitric acid (HNO3, 70%) and hydrogen peroxide (H2O2, 30 wt.% in H2O) were all purchased from Sigma-Aldrich. All

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chemicals were used as received. Deionized (DI) water with a resistivity of 18.2 MΩ·cm at room temperature was used throughout the study.

Synthesis of Ag nanocubes. We followed the protocol developed by Xia and co-workers for the synthesis of Ag nanocubes with an average edge length of 39.4 ± 0.8 nm.29 The as-obtained Ag nanocubes were washed with acetone and water once each and then re-dispersed in water for further use.

Synthesis of Ag-Rh core-frame nanocubes. In a typical process, we introduced 4 mL of EG into a 50-mL three-neck flask containing 0.1 g of PVP-55, followed by the addition of 3 mL of H2Asc (100 mM, in EG), and 35 µL of the aqueous suspension of Ag nanocubes (approximately 1.8 ×1011 particles in total) under magnetic stirring and heated at 110 °C for 1 h. Next, we titrated 0.1 mL of aqueous Na3RhCl6 (2 mM, in EG) into the solution at a rate of 4.0 mL/h with the aid of a syringe pump. After reaction for 10 min, the flask was immersed in an ice-water bath to terminate the reaction. The products were washed with acetone and water three times through centrifugation and re-dispersion, and then re-dispersed them in water for further use.

Synthesis of Ag-Rh hollow nanocubes. We followed the protocol for the Ag-Rh core-frame nanocubes except that 0.3 mL of 2 mM Na3RhCl6 was titrated into the reaction solution. Synthesis of Rh-Ag and Rh nanoboxes. The as-prepared Ag-Rh hollow nanocubes (1.8 × 1011 particles) were collected by centrifugation at 6000 rpm for 18 min. When H2O2 was used as an etchant for the generation of Rh-Ag nanoboxes, the collected particles were mixed with 1 mL of 10% H2O2 and incubated at room temperature for 5 h. The resultant product was washed twice with water and re-dispersed in 0.2 mL of water. Alternatively, when Fe(NO3)/HNO3 was used as an etchant for the generation of Rh nanoboxes, we mixed Fe(NO3) (1 mM) with HNO3 (3 mM) at 1:1 volume ratio. The collected nanoparticles were dispersed in 100 µL water in a centrifuge tube, followed by the addition of 0.5 mL the Fe(NO3)/HNO3 aqueous mixture of. After the centrifuge tube was vortexed for a few seconds, the mixture was allowed to sit for 1 h. The solid product was collected by centrifugation at 13000 rpm for 18 min, washed twice with water, and then redispersed in water for further use.

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Instrumentation and characterization. We used an Eppendorf 5430 centrifuge for the collection and washing of all samples. The UV-vis spectra were recorded on a Cary 50 spectrometer (Agilent Technologies, Santa Clara, CA). Quantitative measurements of Rh and Ag contents were performed using an inductively-coupled plasma mass spectrometer (ICP-MS, NexION 300Q, PerkinElmer, Waltham, MA). A HT7700 microscope (Hitachi, Tokyo, Japan) operated at 120 kV was used to take transmission electron microscopy (TEM) images. Scanning electron microscopy (SEM) images were captured using a Hitachi SU-8230 microscope operated at 20 kV. The high-resolution HAADF-STEM imaging and STEM-EELS mapping were conducted on a Hitachi HD2700C STEM operated at 200 kV and equipped with a probe aberration corrector.

SERS characterization of the Ag-Rh core-frame nanocubes using the 2,6-DMPI molecular probe. The Ag-Rh core-frame nanocubes prepared using 0.02, 0.05, and 0.1 mL of 2 mM Na3RhCl6 solution were functionalized with 2,6-DMPI (10-5 M in ethanol) for 1 h. After washing with water twice, the functionalized nanoparticles were re-dispersed in 1 mL of water to achieve a similar concentration of 1011 particles per mL. In a typical SERS measurement, 20 µL of the solution was added into a polydimethylsiloxane (PDMS) cell. The Raman spectra were recorded using a Renishaw inVia Raman spectrometer coupled with a Leica microscope with a 100× objective lens. The laser excitation wavelength was 532 nm, together with a notch filter based on a grating of 2400 lines/mm. The laser power output was set to 50 mW and the collection time was set to 10 s.

SERS monitoring of the degradation of R6G. The Ag-Rh core-frame nanocubes prepared with 0.01 mL of 2 mM Na3RhCl6 solution were mixed with 1 mL of R6G (10-6 M) and incubated for 1 h. Afterwards, the particles were collected by centrifugation at 5000 rpm for 18 min, washed with water twice, and then re-dispersed in 1 mL of water. After mixing 200 µL of the R6Gmodified nanoparticles and 200 µL of aqueous NaBH4 (0.1 mg/L), we withdrew 20 µL of the reaction solution every several minutes, placed in a PDMS cell to monitor the reaction progress by SERS. The Raman spectra were recorded using a Renishaw inVia Raman spectrometer coupled with a Leica microscope with a 5× objective lens. The laser excitation wavelength was

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532 nm, and the notch filter was based on a grating of 2400 lines/mm. The laser power output was set to 100 mW, together with a collection time of 30 s. For the control experiment, the protocol was the same except for the use of Ag nanocubes to replace the Ag-Rh core-frame nanocubes.

Catalytic degradation of R6G using Ag-Rh and Rh nanoboxes. In a typical process, 2 mL of aqueous R6G (1 mM), 5 mL of water, and 1 mL of aqueous NaBH4 (10 mg/mL, freshly prepared, ice cold) were mixed in a 23-mL glass vial under magnetic stirring. Upon the addition of the RhAg or Rh nanoboxes (~1010 particles), we monitored the reaction by withdrawing 1 mL of the reaction solution to record the UV-vis spectra at different time points.

RESULTS AND DISCUSSION We prepared the Ag nanocubes with an average edge length of 39.4 ± 0.8 nm by following a previously published protocol (Figure S1).29 We then dispersed the Ag nanocubes in a mixture of EG, H2Asc, and PVP held at 110 oC, followed by the titration of Na3RhCl6 solution in EG using a syringe pump. Figure 1 outlines a proposed pathway to account for the deposition of Rh on a Ag nanocube under the polyol condition. Because of the selective binding of PVP to the side faces of the Ag nanocube in the EG system,12 the surface free energies of the low-index facets decrease in the order of γ(110) > γ(111) > γ(100).12 As a result, it is anticipated that the Rh atoms derived from the reduction of the Rh(III) precursor should be preferentially deposited on the edges for the generation of a Ag-Rh core-frame nanocube. At a small titration volume for the Na3RhCl6 solution, the concentration of Rh(III) in the reaction system is relatively low, and as a result, the Rh atoms are mostly produced through the chemical reduction by H2Asc and EG. When the synthesis is continued with the titration of more Na3RhCl6, the galvanic replacement reaction between Rh(III) and Ag will be activated. Under this circumstance, the added Na3RhCl6 can be reduced by the Ag nanocube via galvanic replacement, as well as by H2Asc and EG via chemical reduction. At the same time, the deposition of Rh atoms should still mainly occur on the edges of the nanocube. Due to a relatively low reaction temperature of 110 oC and the very high cohesive energy of Rh, the Rh atoms are expected to follow an island growth mode.30 On the other hand, the Ag+ released from the side faces of the nanocube will be reduced by H2Asc and EG for the generation of new Ag atoms, followed by their deposition back onto the

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nanocube. Given that Ag and Rh are immiscible and they tend to form segregated clusters of each element even in the liquid phase at 2000 oC,31 it is not unreasonable to expect that the Ag and Rh atoms will not be co-deposited in the form of an alloy. Figure 2A–C shows TEM images of the products obtained at different titration volumes of Na3RhCl6 when the standard protocol was used. We also used ICP-MS analysis to determine the Rh contents in the as-obtained solid products. At a titration volume of 0.02 mL, Figure 2A shows that the morphology of the products remained essentially the same as that of the Ag nanocubes, with a Rh content in the solid products as low as 2.84 wt.%. When the titration volume was increased to 0.05 mL, Figure 2B indicates the appearance of nanoparticles on the edges of the nanocubes, suggesting the deposition of Rh atoms as small islands on the {110} facets of the Ag nanocubes. The Rh content was increased to 6.70 wt.%. With the further increase of titration volume to 0.1 mL, Figure 2C shows the generation of nanocubes with somewhat concaved side faces and roughed edges. In this case, the Rh content was increased to 16.34 wt.%. Because PVP is able to selectively passivate the side faces on the Ag nanocubes suspended in EG,12 it is reasonable to observe the selective deposition of Rh on the edges and corners of Ag nanocubes in the initial stage, leading to the creation of somewhat concaved side faces on the {100} facets. When the concertation of the Rh(III) precursor reached to a certain level, it could also go through the galvanic replacement reaction with the Ag atoms.32 Both the dissolved Ag+ ions and the added Rh(III) precursor could be co-reduced by H2Asc and EG to produce Ag and Rh atoms, respectively, followed by their deposition on the Ag nanocubes. Because the side faces are involved in the dissolution of Ag via the replacement reaction, the deposition of Ag and Rh atoms would be more favored for the edges and corners. After the removal of Ag in the core, Figure S2 shows the formation of nanoframes, confirming the preferential deposition of Rh atoms on the edges and corners of the Ag nanocubes. We also used UV-vis spectroscopy to monitor the transformation of Ag nanocubes into AgRh core-frame nanocubes by following the changes to the LSPR of the Ag nanocubes before and after the titration of different volumes of the Na3RhCl6 solution. As shown in Figure 2D, the major LSPR peak of the Ag nanocubes was shifted from 442 to 448, 451, and 457 nm at titration volumes of 0.02, 0.05, and 0.1 mL, respectively, together with gradual decrease in peak intensity and slightly broadening in peak width. The red-shift of the LSPR peak can be largely attributed to the appearance of cavities on the side faces of the nanocubes.32 The increased deposition of Rh

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on the surface of the Ag nanocubes was responsible for the decrease in peak intensity. Both the TEM and UV-vis results supported our argument that the Ag nanocubes were transformed into Ag-Rh core-frame nanocubes with slightly concaved side faces when the titration volume of the Na3RhCl6 solution was kept below 0.1 mL. When the titration volume of the Na3RhCl6 solution was increased to 0.3 mL, Figure 3, A and B, shows TEM and SEM images of the resultant products. In this case, we observed the transformation of Ag nanocubes into Ag-Rh hollow nanocubes with Rh islands on the surface. We also collected both the solid products and supernatant by centrifugation to determine the Ag and Rh contents using ICP-MS analysis. As shown in Table S1, there was very little Ag+ ions remaining in the supernatant while the Ag content in the solid was close to the original value of the Ag nanocubes. We noticed that only 27% of the added Na3RhCl6 was converted into Rh atoms for their deposition on the Ag nanocubes. This data validated our hypothesis that the Ag atoms dissolved from the side faces of the nanocubes in the form of Ag+ ions were completely reduced by H2Asc and EG for the generation of new Ag atoms, followed by their deposition back onto the nanocubes. When the as-prepared Ag-Rh hollow nanocubes were etched with aqueous H2O2, Figure 3C shows the formation of Rh-Ag nanoboxes. Our ICP-MS analysis indicated that the nanoboxes had an elemental composition of Rh/Ag=54/46 by atomic ratio. Interestingly, when the etchant was switched from H2O2 to Fe(NO3)3/HNO3, we were able to remove almost all the Ag, leading to the formation of Rh nanoboxes with an atomic ratio of Rh/Ag=91/9. Figure 3D shows a TEM image of the Rh nanoboxes with highly porous walls. Based on these etching results, we argue that the newly formed Ag atoms were deposited among the Rh islands, making it feasible to dissolve essentially all the Ag with Fe(NO3)3/HNO3. If the Ag atoms were codeposited with Rh atoms as an alloy, it would be very difficult to completely dealloy the Ag with Fe(NO3)3/HNO3 to reduce the Ag content to a level as low as 9%. For example, in our recent study, we could only use Fe(NO3)3/HNO3 to etch Ag@Pt core shell nanocubes for the generation of nanoboxes with a composition of Pt42Ag58.33 Figure 4, A and B, shows high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of the Rh nanoboxes at two different magnifications, confirming the formation of a highly open, porous structure. Figure 4, C and D, shows the corresponding EDX mapping, confirming that Rh atoms were uniformly distributed in the nanoboxes with a very limited amount of Ag, consistent with the ICM-MS data.

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We used 2,6-dimethylphenyl isocyanide (2,6-DMPI) as a probe to evaluate the SERS activity of the Ag-Rh core-frame nanocubes. Based on our recent results on the use of 2,6-DMPI for monitoring the deposition of Pt on the edges of Ag nanocubes, where the SERS hot spots are located,34 we argue that the stretching frequency of the NC bond would be different when the isocyanide group binds to the Ag and Rh atoms, respectively, making it feasible to detect Rh atoms deposited on the edges of the Ag nanocubes. Figure 5 shows the SERS spectra of the 2,6DMPI adsorbed on the Ag nanocubes before and after they had reacted with different amounts of Na3RhCl6. Before adding Na3RhCl6, the two peaks located at 2170 and 645 cm-1 can be assigned to the NC stretching, νNC(Ag), and C-NC stretching, νC-NC(Ag), bands, respectively. The remaining peaks can be ascribed to the ring-associated bands of 2,6-DMPI.35 At a titration volume of 0.02 mL, we observed two new peaks at 2109 and 664 cm-1, with their assignments to νNC(Rh) and νCNC(Rh)

bands, respectively.36 At this titration volume, the intensity of the νNC(Ag) peak was

significantly reduced, together with a shift of peak position from 2170 to 2190 cm-1, while the νC-NC(Ag) band disappeared. The overall SERS activity was comparable to that of Ag nanocubes when the Rh content was at 2.84 wt.%. As the Rh content was increased to 6.70 and 16.34 wt.% at the titration volumes of 0.05 and 0.1 mL, respectively, we still detected the νNC(Rh) and νC-NC(Rh) peaks but not those peaks associated with νNC(Ag). Also, the SERS signal intensity showed more deterioration as more Rh atoms were deposited on the Ag nanocubes because of the weak coupling of the free electrons in Rh with visible light, in agreement with previous findings.33,37 We further demonstrated the use of the as-obtained Ag-Rh core-frame nanocubes with 2.84 wt.% Rh content for in situ monitoring the catalytic reduction of R6G by NaBH4. In a typical measurement, we functionalized the Ag-Rh core-frame nanocubes with R6G and then collected SERS spectra at different time intervals after the introduction of NaBH4 (see the experimental session for details). Figure S3 shows the time-dependent SERS spectra of R6G. Figure 6A shows the SERS spectra after subtracting the fluorescence background. According to the vibrational bands of R6G listed in Table S2,38 we observed the signature peaks of R6G at 612 cm-1 (in plane xanthene ring deformations), 775 (in plane xanthene ring deformations), 1364 (xanthene ring stretch), 1510 (xanthene ring stretch), and 1642 cm-1 (xanthene ring stretch) at t = 0 min. At t = 2 min, we noticed the decrease in peak intensity for all these five characteristic bands. As the reaction progressed to 6 min, the peak intensities continuously decreased by almost 70%. By t = 8 min, we could no longer detect the characteristic peaks of R6G, indicating the completion of 10


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degradation reaction. In comparison, when the Ag nanocubes were used as a catalyst while keeping all other experimental parameters unaltered, Figure 6B shows no changes to the SERS spectra over a period up to 30 min. Taken together, we believe that the Rh located on the edges of the Ag nanocubes played an essential role in catalyzing the degradation reaction of R6G. Finally, we benchmarked the catalytic activities of the Rh nanoboxes with two different Rh/Ag atomic ratios using the reduction of R6G by NaBH4 as a model reaction. In a typical measurement, we collected UV-vis spectra of R6G at different time intervals upon the introduction of NaBH4 into the aqueous solution containing R6G (50 mg/L) and the as-prepared nanoboxes. As shown in Figure 7, A and C, R6G were completely degraded in 4 and 46 min in the presence of the Rh nanoboxes and Rg-Ag nanoboxes, respectively. By monitoring the absorption peak at 527 nm, Figure 7, B and D, plots ln(At/A0) as a function of reaction time. In both cases, the reduction reaction exhibited first-order kinetics, and as a result, we were able to obtain the rate constant through curve fitting. The rate constant of the Rh nanoboxes with highly porous walls was 0.77 min-1, about 12 times greater than that of Rh-Ag nanoboxes. Taken together, we believe that Rh-based catalysts with larger amount of Rh and more open structures would become better catalysts for the degradation of R6G in water.

CONCLUSIONS In summary, we have demonstrated a facile route to the synthesis of Ag-Rh core-frame nanocubes and hollow nanocubes by titrating Na3RhCl6 into a mixture of EG, H2Asc, PVP, and Ag nanocubes at an elevated temperature. At a relatively low titration volume for the Rh(III) precursor, Rh atoms were mainly produced via chemical reduction by H2Asc and EG, followed by their nucleation on the edges of the Ag nanocubes in an island growth mode to generate Rh nanoparticles with extremely large specific surface areas. We further demonstrated that the asobtained Ag-Rh core-frame nanocubes embraced both SERS and catalytic activities for in situ monitoring the Rh-catalyzed degradation of R6G by NaBH4. On the other hand, at a relatively large volume of Na3RhCl6, Rh atoms could be generated through the galvanic replacement with Ag, as well as by H2Asc and EG via chemical reduction, followed by their deposition on the edges and then side faces through island growth. Concurrently, Ag atoms dissolved from the side faces of the nanocubes due to galvanic replacement with Rh(III) could be reduced by H2Asc and EG for their re-deposition onto the nanocubes, but among the Rh islands. As a result, the Ag

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nanocubes were transformed into Ag-Rh hollow nanocubes covered by Rh islands on the surface. Interestingly, when the as-prepared hollow nanocubes were subject to etching with a mixture of (FeNO3)3 and HNO3, they evolved into Rh nanoboxes with highly porous walls. The Rh nanoboxes with highly porous walls showed a remarkable catalytic activity toward the reduction of R6G by NaBH4, making them promising for the degradation of various organic pollutants commonly encountered in environmental remediation.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. TEM image of Ag nanocubes and TEM image of resultant structures after etching as-obtained Ag-Rh nanocubes with 3% aqueous H2O2, a table of ICP-MS results, and a table of Raman shifts of R6G.

Acknowledgement

We acknowledge the support from the National Science Foundation (CHE-1708300), start-up funds from the Georgia Institute of Technology (GT), and a 3M non-tenured faculty award. We acknowledge the use of the characterization facility at the Institute of Electronics and Nanotechnology at GT. Y.Z. acknowledges the International Postdoctoral Exchange Fellowship from the China Postdoctoral Council. J. L. acknowledges the support of NSF (CHE-1465057) and the use of facilities in the John M. Cowley Center for High Resolution Electron Microscopy at ASU.

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Figure 1. Schematic illustration of a proposed pathway for the polyol synthesis of Ag-Rh coreframe nanocubes and Ag-Rh hollow nanocubes when different volumes of Na3RhCl6 (in EG) solution is titrated into a mixture of EG, H2Asc, PVP, and Ag nanocubes.

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Figure 2. TEM images of the Ag-Rh core-frame nanocubes that were obtained by Ag nanocubes reacting with different volumes of the Na3RhCl6 solution: (B) 0.02, (C) 0.05, and (D) 0.1 mL, respectively. (D) UV-vis spectra recorded from aqueous suspensions of the Ag nanocubes before and after reacting with different volumes of the Na3RhCl6 solution.

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Figure 3. (A) TEM and (B) SEM image of Ag-Rh hollow nanocubes. TEM images of (C) Ag-Rh nanoboxes and (D) Rh nanoboxes with highly porous walls. The Ag-Rh hollow nanocubes were prepared with the titration of 0.3 mL of the Na3RhCl6 solution. The Ag-Rh and Rh nanoboxes were obtained by etching the Ag-Rh hollow nanocubes with H2O2 and Fe(NO3)3, respectively.

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Figure 4. (A) HAADF-STEM image of the Rh nanoboxes with highly porous walls. (B) HAADF-STEM image of an individual nanobox at a higher magnification than in (A). (C, D) STEM-EELS elemental mapping of Rh (purple) and Ag (green) of the nanobox in (B). The Rh nanoboxes were obtained by etching the Ag-Rh hollow nanocubes prepared with 0.3 mL of the Na3RhCl6 solution using an aqueous mixture of 0.5 mM Fe(NO3)3 and HNO3.

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Figure 5. SERS spectra recorded from aqueous suspensions of 2,6-DMPI-functionalized Ag-Rh core-frame nanocubes that were prepared by reacting Ag nanocubes with different volumes of the Na3RhCl6 solution.

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Figure 6. SERS spectra obtained by removing the fluorescence background from the as-recorded time-dependent spectra of R6G. The original data sets were collected before and after the introduction of NaBH4 solution into an aqueous suspension containing (A) R6G-functionalized Ag-Rh core-frame nanocubes and (B) R6G-functionalized Ag nanocubes, at an excitation wavelength of 532 nm. The Ag-Rh core-frame nanocubes were prepared with the titration of 0.02 mL of the Na3RhCl6 solution.

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Figure 7. UV-vis spectra recorded at different time intervals for the reduction of R6G by NaBH4 at room temperature in the presence of (A) Rh-Ag nanoboxes with Rh/Ag atomic ratios of 54/46 and (C) Rh nanoboxes with Rh/Ag atomic ratios of 91/9 as catalysts. The two samples were prepared by titrating 0.3 mL of the Na3RhCl6 solution, followed by etching with H2O2 and Fe(NO3)3, respectively. (B, D) Plots of ln[A0/At] versus time for the absorption peak located at 527 nm in (A) and (C), respectively.

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