Ultrathin Rhodium Oxide Nanosheet Nanoassemblies: Synthesis

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Ultrathin Rhodium Oxide Nanosheet Nanoassemblies: Synthesis, Morphological Stability, and Electrocatalytic Application Juan Bai, Shu-He Han, Rui-Li Peng, Jing-Hui Zeng, Jia-Xing Jiang, and Yu Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 04 May 2017 Downloaded from http://pubs.acs.org on May 8, 2017

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ACS Applied Materials & Interfaces

Ultrathin Rhodium Oxide Nanosheet Nanoassemblies: Synthesis, Morphological Stability, and Electrocatalytic Application Juan Bai,† Shu-He Han,† Rui-Li Peng, Jing-Hui Zeng, Jia-Xing Jiang, and Yu Chen* Key Laboratory of Macromolecular Science of Shaanxi Province, Shaanxi Key Laboratory for Advanced Energy Devices, Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710062, PR China. ABSTRACT: Inspired by graphene, ultrathin two-dimensional nanomaterials with atomic thickness have attracted more and more attention due to their unique physical/chemical properties and electronic structure. In this work, the atomically thick ultrathin Rh2O3 nanosheet nanoassemblies (Rh2O3-NSNSs) were obtained by oxidizing the atomically thick ultrathin Rh nanosheet nanoassemblies (Rh-NSNSs) with HClO. For the first time, Rh-based nanostructures were used as the oxygen evolution reaction (OER) electrocatalyst in the alkaline medium. Surprisingly, the as-prepared Rh2O3-NSNSs displayed extremely improved catalytic activity and durability for the OER compared with commercial Ir/C catalyst and most recently reported Ir-based electrocatalysts. The result indicated Rh based nanostructures highly promised to become a potential candidate for efficient OER electrocatalyst due to the similarity of Rh and Ir prices. The present experimental results demonstrated the reasonable morphological control of Rh2O3 nanostructures could significantly improve their catalytic activity and durability in heterogeneous catalysis. KEYWORDS : Two-dimensional nanomaterials, oxidation, Rh2O3, catalysis, oxygen evolution reaction

2D ultrathin noble metal oxide nanosheets are investigated rarely.9 Among various noble metal, rhodium plays an important role in various catalytic applications, including CO oxidation,25-26 H2O2 oxidation,27 and N2O decomposition,28 and so on. Unfortunately, in some cases, the catalytic mechanism of rhodium nanocrystals is still elusive. For example, the dominant viewpoints still believe that CO oxidation takes place on metal surface. However, the recent investigations demonstrated that rhodium surface could be oxidized to form oxides under realistic environmental conditions, which was responsible for the CO oxidation.26, 29-33 Similarly, recent reports also indicated that rhodium oxide rather than elemental rhodium was active center for photocatalytic oxidation of methane,34 photocatalytic hydrogen production from methanol,35 and catalytic oxidation of NO.36 Although rhodium oxide have wide application in catalysis, the morphological control of rhodium oxide nanostructures are rarely reported.27 Up to now, the synthesis and application of 2D ultrathin rhodium oxide nanosheets have not been reported. Based on our previous work, herein, we successfully developed a HClO oxidation method to synthesize the ultrathin Rh2O3 nanosheet nanoassemblies (Rh2O3-NSNSs) by using elemental Rh nanosheet nanoassemblies (Rh-NSNSs) as precursor. When used as electrocatalyst for the oxygen evolution reaction (OER), the as-prepared Rh2O3-NSNSs

1. INTRODUCTION Like noble metal nanostructures, many noble metal oxide (such as RuO2, PdO, IrO2, and Rh2O3, etc.) nanostructures have also wide applications in heterogeneous catalysis.1-3 In general, the catalytic reactivity and durability of metal/metallic oxide nanostructures highly depend on their morphologies.4-8 Recently, the two-dimensionally (2D) ultrathin noble metal (such as Ru,9 Ir10, Rh,11-15 Pd,16-18 and Pt19-22) nanosheets with atomic thickness have emerged as advanced catalytic materials in various heterogeneous catalytic reactions, including hydrogen evolution reaction,9 catalytic hydrogenation,11 formic acid oxidation reaction,16-19 oxygen reduction reaction,20 methanol oxidation reaction,21 ethanol oxidation reaction,22 and oxygen evolution reaction,10 owing to their particular 2D structure. On the one hand, the atomically thick ultrathin nanosheets efficiently maximize the utilization of noble metal atoms because nearly all atoms lie at the surface of nanosheets. On the other hand, the surface atoms at ultrathin nanosheets belong to the low-coordinated defective atoms and have the unordinary electronic property due to the particular 2D ultrathin structure, which generally result in significantly improved catalytic activity compared with conventional nanostructures.9-11, 16-22 At present, the atomically thick transition metal oxide (such as SnO2, Co3O4, ZnO, TiO2, and WO3, etc.) nanosheets have been obtained.23-24 However, until recently, the synthesis and catalytic activity of 1

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displayed extremely improved catalytic activity compared with commercial Ir/C catalyst.

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2.4. Instruments. Scanning electron microscopy (SEM) was measured by SU-8020. Transmission electron microscopy (TEM), selected area electron diffraction (SAED), and energydispersive X-ray (EDX) maps were conducted with TECNAI G2 F20 instrument. X-ray diffraction (XRD) patterns were obtained by a DX-2700 X-ray diffractometer. Atomic force microscopy (AFM) was performed on Dimension Icon instrument. X-ray photoelectron spectroscopy (XPS) was carried out on a AXIS ULTRA spectrometer, and binding energy was calibrated with C1s peak 284.6 eV as standard value. The surface charge measurement was performed on Nano ZS90 zeta potential analyzer.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Polyallylamine hydrochloride (weightaverage molecular weight: 5 000) was purchased from Nitto Boseki Co., Ltd (Tokyo, Japan). Rhodium (III) chloride hydrate (RhCl3·3H2O), formaldehyde solution (HCHO, 40%), hypochlorous acid (HClO), and potassium hydroxide (KOH) were obtained from Aladdin Industrial Co. (Shanghai, China). Ir/C (20 wt% of Ir) with 1.5 nm was purchased from Premetek Co (Figure S1). All the reagents were used as received without further purification. 2.2. Synthesis of Rh2O3 Nanosheet Nanoassemblies (Rh2O3-NSNSs). Rh2O3-NSNSs were obtained by oxidizing the elemental rhodium nanosheet nanoassemblies (Rh-NSNSs) with HClO solution (Scheme 1). Firstly, Rh-NSNSs were synthesized according our recently reported procedure with slight modifications.37 Typically, RhCl3 (63 mg) and polyallylamine hydrochloride (200 mg) were added into 100 mL of water. After adjusting solution pH to 7.0, 10 mL of 40% HCHO solution was added in the mixture solution. Then, the mixture solution in Teflon-lined high-pressure vessel was heated at 100 °C for 6 h, resulting in generation of Rh-NSNSs. Compared to our previous syntheses (pH 2, 120 oC), the increase in solution pH of reaction system resulted in a lower synthesis temperature (pH 7, 100 oC). After cooling, 4 mL HClO solution was added into the obtained Rh-NSNSs suspension and stirred for 72 h. After reaction, the obtained Rh2O3-NSNSs were separated by centrifugation.

3. RESULTS AND DISCUSSION 3.1. Characterization of Rh-NSNSs. Rh2O3-NSNSs were obtained by oxidizing Rh-NSNSs with HClO solution for 72 h at room temperature, as shown in Scheme 1. Herein, Rh-NSNSs were synthesized according our recently reported procedure with slight modifications.37 SEM, TEM, high-resolution TEM (HR-TEM), and AFM images show dendritic Rh-NSNSs are constructed of ultrathin Rh nanosheets with ca. 0.9 nm thickness (Figure 1A-D). SAED pattern indicates Rh-NSNSs are polycrystalline (Insert in Figure 1B). The magnified highresolution HR-TEM image shows the lattice spacing of 0.223 nm, corresponding to Rh(111) facets (Insert in Figure 1C). Meanwhile, zeta potential is measured to be +37 mV, indicating a few polyallylamine still bind on Rh-NSNSs surface, in agreement with our previous report.37

Scheme 1. Schematic illustration of the synthetic procedure of Rh2O3-NSNSs. 2.3. Electrochemical Measurements. Linear sweep voltammetry (LSV), chronopotentiometry, and chronoamperometry tests were carried out on a electrochemical analyzer (CHI 660 E) with a rotating disk electrode (Gamry RDE710) at 30 ± 1 oC, using a threeelectrode assembly including a 200 ml glass cell, a saturated calomel electrode (SCE) as the reference electrode, a Pt wire as the counter electrode, and a catalyst modified glassy carbon electrode (5 mm diameter) as the working electrode. The working electrode was prepared according to the previous work.38 The catalyst ink was prepared by dispersing 5 mg of the catalyst in 2.0 ml of water containing 25 μL of 5 wt % Nafion under strong sonication conditions. Then, 10 μl of the catalyst ink was carefully loaded on the surface of glassy carbon electrode and dried at room temperature. The metal loading density of catalysts on working electrode is ca. ~0.13 mg cm–2. In this work, all potentials measured were calibrated to the reversible hydrogen electrode (RHE) using the following equation: ERHE = ESCE + 0.241V + 0.0591pH. In all LSV curve, iR drop was compensated at 95% through the positive feedback model using the CHI 660E electrochemical analyzer.

Figure 1. (A) SEM and (B) TEM images of Rh-NSNSs. (C) HRTEM image of Rh-NSNSs on the edge regain. (D) AFM image of fallen Rh nanosheets debris after strong ultrasonic treatment. Insert in Figure 1B: SAED pattern of Rh-NSNSs. Insert in Figure 1C: the magnified HRTEM image. 3.2. Synthesis and Characterization of Rh2O3-NSNSs. The previously theoretical and experimental investigations indicated that elemental rhodium surface could oxidize to generate oxide under realistic environmental conditions.29, 39 Meanwhile, it is clear that the residual surfactant on noble metal surface usually decrease the catalytic activity of noble metal nanostructures due to the decline of reaction sites.11, 4041 Thus, HClO with strong oxidizing capability (φHClO/Cl2 =1.63 V) is used to tentatively oxidize Rh-NSNSs and simultaneously remove the residual polyallylamine. After 2


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HClO oxidation treatment, the chemical state of Rh element was identified by XPS. The Rh 3d XPS spectrum presents a doublet peak with a spin separation of 5 eV binding energy (Figure 2A). The Rh 3d3/2 and Rh 3d5/2 peaks are respectively centred at 313.1 and 308.2 eV (Figure 2A), which are far away from the standard value of metallic Rh (3d3/2: 311.7 and 3d5/2: 307 eV) 42 but match well with the standard value of Rh2O3 (3d3/2: 313.2 and 3d5/2:308.5 eV). 42 This suggests that Rh atoms in product exist in the form of oxidation state rather than metallic state. Compared with Rh 3d XPS spectrum of initial Rh-NSNSs, Rh 3d binding energy of Rh2O3-NSNSs obviously positively shift (Figure S2), confirming the metallic Rh has been oxidized successfully after HClO oxidation treatment. Furthermore, XRD was used to identify phase structure of the product (Figure 2B). The XRD pattern of the product is consistent with the standard diffraction pattern of corundum phase Rh2O3 crystal (JCPDS 43-0009), implying the elemental rhodium in Rh-NSNSs oxidize to Rh2O3 by HClO. Meanwhile, XPS measurements show N1s peak intensity of Rh2O3-NSNSs is much lower than that of Rh-NSNSs (Figure S3), attributing to the oxidation removal of polyallylamine by HClO. Meanwhile, the zeta potential of Rh2O3-NSNSs is measured to be −35 ± 2 mV, in contrast to zeta potential (+ 37 mV) of RhNSNSs, confirming the residual polyallylamine are removed completely due to the strong oxidizing capability of HClO.

Figure 3. (A) SEM image, (B) TEM image, (C) EDX maps of Rh2O3-NSNSs. (D) HRTEM image of Rh2O3-NSNSs edge. (E) AFM image of fallen Rh2O3 nanosheets debris after strong ultrasonic treatment. Insert in Figure 3B: SAED pattern of Rh2O3-NSNSs. Insert in Figure 3D: the magnified HRTEM image. The detailed structure of Rh2O3-NSNSs was further investigated by HR-TEM and AFM. HRTEM image of Rh2O3NSNSs edge shows two regions of dark and bright, which correspond to 2D Rh2O3 nanosheets perpendicular and parallel to the TEM grid, respectively (Figure 3D).10, 23 The widths of the dark wire-like regions are ca. 0.9 nm, indicating Rh2O3 nanosheets with a thickness of 0.9 nm. The magnified HRTEM image shows the lattice spacing of 0.259 nm, corresponding to Rh2O3 (114) facets (Insert in Figure 3D). After strong ultrasonic treatment, AFM height profile shows the thickness of the fallen Rh2O3 nanosheets debris is ca. 0.9 nm (Figure 3E), in agreement with HRTEM result (Figure 3D). So far, all physical investigations show the morphology, architecture and size of Rh2O3-NSNSs are very similar to theses of Rh-NSNSs, indicating HClO effectively oxidize 2D ultrathin Rh nanosheets to Rh2O3 nanosheets and simultaneously create a clean surface. Additionally, it is worth noting that the particularly dendritic architecture of Rh2O3NSNSs can effectively restrains the re-stacking of ultrathin Rh2O3 nanosheets,11 which is highly desirable for their practical applications. 3.3. Chemical Stability of Rh2O3-NSNSs. In the previous work, no such ultrathin Rh2O3 nanosheets with atomically thick are reported. Prior to the practical application of Rh2O3NSNSs, we first investigate their chemical stability. It is clear that metallic Rh has extreme resistivity for acids and oxidants.43-44 Like metallic Rh, Rh2O3-NSNSs also have extreme resistivity for acids and oxidants. After aqua regia treatment for 72 h at room temperature, Rh2O3-NSNSs still exist (Figure S4), and SEM image shows the morphology of Rh2O3-NSNSs still maintains (Figure 4A). After oxidation treatment at 1.5 V potential in 1 M KOH solution for 2 h, SEM image shows the morphology of Rh2O3-NSNSs also preserves

Figure 2. (A) Rh 3d XPS spectrum of Rh2O3-NSNSs and (B) XRD pattern of Rh2O3-NSNSs. The vertical red line and green line in Figure 2A stand for the standard Rh 3d5/2 values of Rh2O3 and metallic Rh, respectively. The morphology, size, structure, and chemical composition of Rh2O3-NSNSs were preliminarily investigated by SEM, TEM, and EDX analysis. Typical SEM (Figure 3A) and TEM (Figure 3B) images shows Rh2O3-NSNSs have threedimensionally dendritic morphology, which are constructed of two-dimensionally nanosheets building blocks. And, the dendritic Rh2O3-NSNSs are monodisperse and their average size is ca. 100 nm. SAED pattern shows the light scattered diffraction rings (Insert in Figure 3B), indicating Rh2O3-NSNSs are polycrystalline. EDX maps measurements clearly show the existence of O element, and its pattern is very close to Rh element pattern (Figure 3C), confirming the generation of Rh2O3, again.

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completely (Figure 4B). These experimental results indicate that Rh2O3-NSNSs can withstand very harsh reaction environments, which will render them broadly applicable.

Rh2O3-NSNSs reveal a 165 mV negative shift compared with that (430 mV) at commercially available Rh2O3 nanoparticles (Figure S6), indicating morphology-dependent OER activity. As already mentioned, the atomically thick 2D ultrathin nanosheets generally contain numerous low-coordinated defective atoms and own the unordinary electronic property,911, 16-22 which may be responsible for the extremely enhanced OER activity of Rh2O3-NSNSs. Additionally, SEM image shows commercial Rh2O3 nanoparticles seriously aggregate (Figure S7), which results in the low OER activity.

Figure 4. SEM images of Rh2O3-NSNSs after (A) aqua regia treatment for 72 h and (B) oxidation treatment at 1.5 V potential in 1 M KOH solution for 2 h.

Table 1. The ηOER of recently reported electrocatalysts at 10 mA cm−2 current density in an alkaline solution.

3.4. OER Activity and Durability of Rh2O3-NSNSs. The OER in electrochemical device, a very important half reaction for H2 production, CO2 electroreduction, and metal-air batteries, is attracting more and more attention due to the global energy crisis.10, 45-52 Unfortunately, the present OER electrocatalysts generally suffer from the slow reaction kinetics and weak durability. Considering that the extremely amazing stability of Rh2O3-NSNSs under strong oxidation conditions (Figure 4A and 4B), we try to apply it in oxygen electrochemistry. Due to their 3D interconnected architecture, Rh2O3-NSNSs can be used as a self-supported electrocatalyst without carbon support, which efficiently avoid the carbon support corrosion problem during the OER. In order to understand the inherent active sites of Rh-based nanostructures for the OER, the OER activity of Rh2O3-NSNSs was investigated by LSV using rotating disk electrode and compared with Rh-NSNSs under the same experimental conditions. The current density was normalized to the geometric area of the glassy carbon electrode. The onset potential of OER (EOER) at Rh2O3-NSNSs reveal a 60 mV negative shift compared with Rh-NSNSs (1.46 vs. 1.52 V), indicating OER activity of Rh2O3-NSNSs catalyst is better than Rh-NSNSs (Figure S5). This fact not only demonstrates the rhodium oxide is active center for the OER but also confirms HClO oxidation is an efficient strategy for removing residual polyallylamine on Rh-NSNSs. Considering the potentially practical application, the OER activity of Rh2O3-NSNSs is further compared with the state-ofthe-art Ir/C catalyst under the same experimental conditions. The EOER at Rh2O3-NSNSs reveal a 50 mV negative shift compared with commercial Ir/C catalyst (1.46 vs. 1.51 V), indicating the improved OER kinetics (Figure 5A). The overpotential of OER (ηOER, the potential difference between a potential with 10 mA cm−2 current density and the theoretical equilibrium potential of O2 generation10, 50-51) is another important criterion for estimating the catalytic performance of OER electrocatalyst. As observed, ηOER (265 mV) at Rh2O3-NSNSs reveal a 35 mV negative shift compared with that (300 mV) at commercial Ir/C catalyst (Figure 5A). Besides, ηOER value (265 mV) at Rh2O3-NSNSs is lower than those of most recently reported noble metal-based electrocatalysts (Table 1),53-61 further confirming the high OER activity of Rh2O3-NSNSs. In order to understand the OER activity enhancement, the catalytic activity of commercial Rh2O3 nanoparticles was investigated. ηOER (265 mV) at

Catalysts Rh2O3-NSNSs Pt–LiCoO2 Pt/CaMnO3 Ir–Cu nanoframes Ir/Ni oxide Mn/Ru oxide IrOx/Au CNTs-Au@Co3O4 Cu0.3Ir0.7Oδ IrOx/CoOx/Ni foam

Electrolyte 1 M KOH 0.1 M KOH 0.1 M KOH 1 M KOH 1 M KOH 1 M NaOH 0.1 M NaOH 1 M KOH 0.1 M KOH 1 M NaOH

ηOER (mV) 265 440 570 340 280 296 370 350 415 280

Ref This work 53 54 55 56 57 58 59 60 61

Besides electrocatalytic activity, the durability of OER electrocatalysts is another critical parameter for their practical application. The durability of Rh2O3-NSNSs and commercial Ir/C catalyst was assessed by performing chronopotentiometry measurement under 10 mA cm−2 current density52 at 1600 rpm for a period of 3 h (Figure 5B). Within 3 h, a small overpotential increase rate (10 mV h−1) is observed at Rh2O3-NSNSs, which is 4.3 times slower than that (43 mV h−1) at commercial Ir/C catalyst. This chronopotentiometry results clearly indicate that Rh2O3NSNSs have a more stable catalytic performance than the commercial Ir/C catalyst, originating from the extreme chemical stability of Rh2O3-NSNSs under harsh environmental conditions (Figure 4A and 4B). In deed, SEM image shows the morphology of Rh2O3-NSNSs preserves completely after chronopotentiometry test (Figure S8), which contributes to excellent durability of Rh2O3-NSNSs for the OER. The durability of Rh2O3-NSNSs and commercial Ir/C catalyst was further evaluated by chronoamperometry technique in N2saturated 1 M KOH electrolyte at 1.58 V potential (Figure S9). During the chronoamperometry measurements, Rh2O3-NSNSs show the higher OER current density than commercial Ir/C catalyst. At 3 h, OER current density at Rh2O3-NSNSs is 1.8 times bigger than that at commercial Ir/C catalyst. The bigger OER current density and and slower decay rate confirm that Rh2O3-NSNSs are more active and durable catalyst for the OER than commercial Ir/C catalyst.

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ACS Applied Materials & Interfaces This research was sponsored by the National Natural Science Foundation of China (21473111), Fundamental Research Funds for the Central Universities (GK201602002, GK201701007 and 2016TS063).

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Figure 5. (A) LSV curves of (a) Rh2O3-NSNSs and (b) commercial Ir/C catalyst in N2-saturated 1 M KOH electrolyte at a scan rate of 5 mV s−1 at 1600 rpm. (B) Chronopotentiometry curves of (a) Rh2O3-NSNSs and (b) commercial Ir/C catalyst in N2-saturated 1 M KOH electrolyte at a 10 mA cm−2 current density at 1600 rpm.

4. CONCLUSIONS In summary, we have developed a facile HClO oxidation strategy for the synthesis of Rh2O3-NSNSs. The strong oxidizing capability of HClO not only oxidized atomically thick 2D ultrathin Rh nanosheet to 2D ultrathin Rh2O3 nanosheet but also effectively removed the residual surfactant. The asprepared Rh2O3-NSNSs with extreme resistivity could effectively withstand harsh reaction environments. When used as the self-supported OER electrocatalyst, Rh2O3-NSNSs displayed very high catalytic activity and durability compared with commercial Ir/C catalyst. At the current density of 10 mA cm−2, Rh2O3-NSNSs only exhibited a 265 mV ηOER value, which outperformed currently reported various noble metal-based electrocatalysts (Table 1). Given their atomically thick ultrathin property of 2D Rh2O3 nanosheet subunit, high chemical inertness, and particular self-supported architecture, Rh2O3-NSNSs might have wide applications in other heterogeneous catalysis.

ASSOCIATED CONTENT Supporting Information. Experimental section and characterization details are available in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Additional information for SEM of commercial Rh2O3 and Rh2O3-NSNSs after chronopotentiometry test. Rh3d and N1s XPS spectra of Rh2O3-NSNSs and Rh-NSNSs. LSV curves of RhNSNSs, Rh2O3-NSNSs and commercial Rh2O3 nanoparticles. TEM images of Ir/C. Chronoamperometric curves of Rh2O3NSNSs and commercial Ir/C catalyst.

AUTHOR INFORMATION Corresponding Author * Email: [email protected] (Y. Chen) Author Contributions †

Dr J. Bai and S. H. Han contributed equally to this work.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT 5



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Superior Anode Materials for Oxygen Evolution Reaction. J. Power Sources 2015, 300, 285-293. 60. Sun, W.; Song, Y.; Gong, X. Q.; Cao, L. m.; Yang, J., An Efficiently Tuned d-orbital Occupation of IrO2 by Doping with Cu for Enhancing the Oxygen Evolution Reaction Activity. Chem. Sci. 2015, 6, 4993-4999. 61. Tae, E. L.; Song, J.; Lee, A. R.; Kim, C. H.; Yoon, S.; Hwang, I. C.; Kim, M. G.; Yoon, K. B., Cobalt Oxide Electrode Doped with Iridium Oxide as Highly Efficient Water Oxidation Electrode. Acs Catal. 2015, 5, 5525-5529.

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Ultrathin Rhodium Oxide Nanosheet Nanoassemblies: Synthesis

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