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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* 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, P. R. China S Supporting Information *
ABSTRACT: Inspired by graphene, ultrathin two-dimensional nanomaterials with atomic thickness have attracted more and more attention because of their unique physicochemical 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 with HClO. For the first time, Rh-based nanostructures were used as the oxygen evolution reaction (OER) electrocatalyst in an alkaline medium. Surprisingly, the as-prepared Rh2O3-NSNSs displayed extremely improved catalytic activity and durability for the OER compared with those of the commercial Ir/C catalyst and most recently reported Ir-based electrocatalysts. The result indicated Rh-based nanostructures that have great promise to become a potential candidate for efficient OER electrocatalyst because of the similarity of Rh and Ir prices. These experimental results demonstrated the reasonable morphological control of Rh2O3 nanostructures could significantly improve their catalytic activity and durability during heterogeneous catalysis. KEYWORDS: two-dimensional nanomaterials, oxidation, Rh2O3, catalysis, oxygen evolution reaction
1. INTRODUCTION
Among various noble metals, rhodium plays an important role in various catalytic applications, including CO oxidation,25,26 H2O2 oxidation,27 N2O decomposition,28 etc. Unfortunately, in some cases, the catalytic mechanism of rhodium nanocrystals is still elusive. For example, the present dominant viewpoint is that CO oxidation takes place on the metal surface. However, recent investigations demonstrated that the rhodium surface could be oxidized to form oxides under realistic environmental conditions, which was responsible for CO oxidation.26,29−33 Similarly, recent reports also indicated that rhodium oxide rather than elemental rhodium was the active center for photocatalytic oxidation of methane,34 photocatalytic hydrogen production from methanol,35 and catalytic oxidation of NO.36 Although rhodium oxide has a wide range of applications in catalysis, the morphological control of rhodium oxide nanostructures has rarely been reported.27 Until now, the synthesis and application of 2D ultrathin rhodium oxide nanosheets have not been reported. On the basis of 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 a precursor. When used as an electrocatalyst for the oxygen evolution reaction (OER), the as-prepared Rh2O3-NSNSs displayed extremely
Like noble metal nanostructures, many noble metal oxide (such as RuO2, PdO, IrO2, Rh2O3, etc.) nanostructures also have wide applications in heterogeneous catalysis.1−3 In general, the catalytic reactivity and durability of metal/metallic oxide nanostructures strongly depend on their morphologies.4−8 Recently, the two-dimensional (2D) ultrathin noble metal (such as Ru,9 Ir,10 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 because of their particular 2D structure. On 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, the surface atoms at ultrathin nanosheets belong to the weakly coordinated defective atoms and have an unordinary electronic property because of the particular 2D ultrathin structure, which generally results in catalytic activity that is significantly improved compared with that of conventional nanostructures.9−11,16−22 At present, atomically thick transition metal oxide (such as SnO2, Co3O4, ZnO, TiO2, WO3, etc.) nanosheets have been obtained.23,24 However, until recently, the synthesis and catalytic activity of 2D ultrathin noble metal oxide nanosheets have rarely been investigated.9 © 2017 American Chemical Society
Received: April 6, 2017 Accepted: May 4, 2017 Published: May 4, 2017 17195
DOI: 10.1021/acsami.7b04874 ACS Appl. Mater. Interfaces 2017, 9, 17195−17200
Research Article
ACS Applied Materials & Interfaces
3. RESULTS AND DISCUSSION 3.1. Characterization of Rh-NSNSs. Rh2O3-NSNSs were obtained by oxidizing Rh-NSNSs with a HClO solution for 72 h at room temperature, as shown in Scheme 1. Herein, RhNSNSs were synthesized according our recently reported procedure with slight modifications.37 SEM, TEM, highresolution TEM (HRTEM), and AFM images show dendritic Rh-NSNSs are constructed of ultrathin Rh nanosheets with an ∼0.9 nm thickness (Figure 1A−D). The SAED pattern
improved catalytic activity compared with that of the commercial Ir/C catalyst.
2. EXPERIMENTAL SECTION 2.1. Chemicals. Poly(allylamine hydrochloride) (weight-average molecular weight of 5000) was purchased from Nitto Boseki Co., Ltd. (Tokyo, Japan). Rhodium(III) chloride hydrate (RhCl3·3H2O), a formaldehyde solution (HCHO, 40%), hypochlorous acid (HClO), and potassium hydroxide (KOH) were obtained from Aladdin Industrial Co. (Shanghai, China). Ir/C (20 wt % Ir) 1.5 nm in size was purchased from Premetek Co (Figure S1). All the reagents were used as received without further purification. 2.2. Synthesis of Rh2O3 Nanosheet Nanoassemblies (Rh2O3NSNSs). Rh2O3-NSNSs were obtained by oxidizing the elemental rhodium nanosheet nanoassemblies (Rh-NSNSs) with a HClO solution (Scheme 1). First, Rh-NSNSs were synthesized according
Scheme 1. Illustration of the Synthesis of Rh2O3-NSNSs
our recently reported procedure with slight modifications.37 Typically, RhCl3 (63 mg) and poly(allylamine hydrochloride) (200 mg) were added to 100 mL of water. After the solution pH had been adjusted to 7.0, 10 mL of a 40% HCHO solution was added to the mixture solution. Then, the mixture solution in a Teflon-lined high-pressure vessel was heated at 100 °C for 6 h, resulting in generation of RhNSNSs. Compared to our previous syntheses (pH 2 and 120 °C), the increase in the solution pH of the reaction system resulted in a lower synthesis temperature (pH 7 and 100 °C). After the mixture had cooled, 4 mL of a HClO solution was added to the obtained Rh-NSNS suspension and the mixture stirred for 72 h. After reaction, the obtained Rh2O3-NSNSs were separated by centrifugation. 2.3. Electrochemical Measurements. Linear sweep voltammetry (LSV), chronopotentiometry, and chronoamperometry tests were performed on an electrochemical analyzer (CHI 660 E) with a rotating disk electrode (Gamry RDE710) at 30 ± 1 °C, using a three-electrode assembly that includes 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 using a previously described method.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 the working electrode is ∼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 + 0.241 V + 0.0591pH. In all LSV curve, the iR drop was compensated at 95% through the positive feedback model using the CHI 660E electrochemical analyzer. 2.4. Instruments. Scanning electron microscopy (SEM) was performed with a model SU-8020 instrument. Transmission electron microscopy (TEM), selected area electron diffraction (SAED), and energy-dispersive X-ray (EDX) experiments were conducted with a TECNAI G2 F20 instrument. X-ray diffraction (XRD) patterns were obtained with a DX-2700 X-ray diffractometer. Atomic force microscopy (AFM) was performed on a Dimension Icon instrument. X-ray photoelectron spectroscopy (XPS) was performed on an AXIS ULTRA spectrometer, and the binding energy was calibrated with the C 1s peak at 284.6 eV as a standard value. The surface charge measurement was performed on a Nano ZS90 ζ potential 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 nanosheet debris after strong ultrasonic treatment. The inset in panel B shows the SAED pattern of Rh-NSNSs. The inset in panel C shows the magnified HRTEM image.
indicates Rh-NSNSs are polycrystalline (inset in Figure 1B). The magnified high-resolution HRTEM image shows a lattice spacing of 0.223 nm, corresponding to Rh(111) facets (inset in Figure 1C). Meanwhile, the ζ potential is measured to be 37 mV, indicating a few polyallylamine molecules still bind on the surface of Rh-NSNSs, in agreement with our previous report.37 3.2. Synthesis and Characterization of Rh2O3-NSNSs. The previously theoretical and experimental investigations indicated that the elemental rhodium surface could oxidize to generate oxide under realistic environmental conditions.29,39 Meanwhile, it is clear that the residual surfactant on the noble metal surface usually decreases the catalytic activity of noble metal nanostructures because of the decline of reaction sites.11,40,41 Thus, HClO with strong oxidizing capability (φHClO/Cl2 = 1.63 V) is used to tentatively oxidize RhNSNSs and simultaneously remove the residual polyallylamine. After HClO oxidation treatment, the chemical state of the Rh element was determined by XPS. The Rh 3d XPS spectrum presents a doublet peak with a spin separation with a 5 eV binding energy (Figure 2A). The Rh 3d3/2 and Rh 3d5/2 peaks are centered at 313.1 and 308.2 eV, respectively (Figure 2A), which are far from the standard value of metallic Rh (311.7 eV for 3d3/2 and 307 eV for 3d5/2)42 but match well with the standard value of Rh2O3 (313.2 eV for 3d3/2 and 308.5 eV for 3d5/2).42 This suggests that Rh atoms in the product exist in the form of an oxidation state rather than a metallic state. Compared with the Rh 3d XPS spectrum of the initial RhNSNSs, the Rh 3d binding energy of Rh2O3-NSNSs obviously positively shifts (Figure S2), confirming the metallic Rh has been oxidized successfully after HClO oxidation treatment. 17196
DOI: 10.1021/acsami.7b04874 ACS Appl. Mater. Interfaces 2017, 9, 17195−17200
Research Article
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nm. The SAED pattern shows the light-scattered diffraction rings (inset in Figure 3B), indicating Rh2O3-NSNSs are polycrystalline. EDX map measurements clearly show the existence of O element, and its pattern is very close to that of the Rh element (Figure 3C), again confirming the generation of Rh2O3. The detailed structure of Rh2O3-NSNSs was further investigated by HRTEM and AFM. The HRTEM image of the Rh2O3-NSNS 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 ∼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 (inset in Figure 3D). After strong ultrasonic treatment, the AFM height profile shows the thickness of the fallen Rh2O3 nanosheet debris is ∼0.9 nm (Figure 3E), in agreement with the HRTEM results (Figure 3D). So far, all physical investigations show the morphology, architecture, and size of Rh2O3-NSNSs are very similar to those of Rh-NSNSs, indicating HClO effectively oxidizes 2D ultrathin Rh nanosheets to Rh2O3 nanosheets and simultaneously creates a clean surface. Additionally, it is worth noting that the particularly dendritic architecture of Rh2O3-NSNSs can effectively restrain the restacking 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 atomic thickness have been reported. Prior to the practical application of Rh2O3-NSNSs, we first investigated 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 the SEM image shows the morphology of Rh2O3-NSNSs is maintained (Figure 4A). After oxidation
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 panel A stand for the standard Rh 3d5/2 values of Rh2O3 and metallic Rh, respectively.
Furthermore, XRD was used to identify the phase structure of the product (Figure 2B). The XRD pattern of the product is consistent with the standard diffraction pattern of the corundum phase Rh2O3 crystal (JCPDS Card 43-0009), implying the elemental rhodium in Rh-NSNSs oxidize to Rh2O3 by HClO. Meanwhile, XPS measurements show the N 1s peak intensity of Rh2O3-NSNSs is much lower than that of Rh-NSNSs (Figure S3), which can be attributed to the oxidation removal of polyallylamine by HClO. Meanwhile, the ζ potential of Rh2O3-NSNSs is measured to be −35 ± 2 mV, in contrast to the ζ potential (37 mV) of Rh-NSNSs, confirming the residual polyallylamine molecules are removed completely because of the strong oxidizing capability of HClO. 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 show Rh2O3-NSNSs have a threedimensionally dendritic morphology, which are constructed of two-dimensional nanosheet building blocks. The dendritic Rh2O3-NSNSs are monodisperse, and their average size is ∼100
Figure 4. SEM images of Rh2O3-NSNSs after (A) aqua regia treatment for 72 h and (B) oxidation treatment at a 1.5 V potential in a 1 M KOH solution for 2 h.
treatment at a 1.5 V potential in a 1 M KOH solution for 2 h, the SEM image shows the morphology of Rh2O3-NSNSs is also completely preserved (Figure 4B). These experimental results indicate that Rh2O3-NSNSs can withstand very harsh reaction environments, which will render them broadly applicable. 3.4. OER Activity and Durability of Rh2O3-NSNSs. The OER in the electrochemical device, a very important halfreaction for H2 production, CO2 electroreduction, and metal− air batteries, is attracting more and more attention because of 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
Figure 3. (A) SEM image, (B) TEM image, and (C) EDX maps of Rh2O3-NSNSs. (D) HRTEM image of the Rh2O3-NSNS edge. (E) AFM image of fallen Rh2O3 nanosheet debris after strong ultrasonic treatment. The inset in panel B shows the SAED pattern of Rh2O3NSNSs. The inset in panel D shows the magnified HRTEM image. 17197
DOI: 10.1021/acsami.7b04874 ACS Appl. Mater. Interfaces 2017, 9, 17195−17200
Research Article
ACS Applied Materials & Interfaces
Table 1. ηOER Values of Recently Reported Electrocatalysts at a 10 mA cm−2 Current Density in an Alkaline Solution
stability of Rh2O3-NSNSs under strong oxidation conditions (Figure 4A,B), we try to apply it in oxygen electrochemistry. Because of their three-dimensional interconnected architecture, Rh2O3-NSNSs can be used as a self-supported electrocatalyst without carbon support, which efficiently avoids the carbon support corrosion problem during the OER. To understand the inherent active sites of Rh-based nanostructures for the OER, the OER activity of Rh2O3-NSNSs was investigated by LSV using a rotating disk electrode and compared with that of RhNSNSs 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 Rh2O3NSNSs reveals a 60 mV negative shift compared with that of Rh-NSNSs (1.46 V vs 1.52 V), indicating the OER activity of the Rh2O3-NSNS catalyst is better than that of Rh-NSNSs (Figure S5). This fact not only demonstrates that rhodium oxide is the active center for the OER but also confirms that 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 that of the state-of-the-art Ir/C catalyst under the same experimental conditions. The EOER at Rh2O3-NSNSs reveals a 50 mV negative shift compared with that of the commercial Ir/C catalyst (1.46 V vs 1.51 V), indicating the improved OER kinetics (Figure 5A). The overpotential of OER (ηOER, the
catalyst
electrolyte
ηOER (mV)
ref
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
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
265 440 570 340 280 296 370 350 415 280
this work 53 54 55 56 57 58 59 60 61
SEM image shows commercial Rh2O3 nanoparticles seriously aggregate (Figure S7), which results in the low OER activity. Besides electrocatalytic activity, the durability of OER electrocatalysts is another critical parameter for their practical application. The durability of Rh2O3-NSNSs and a commercial Ir/C catalyst was assessed by performing chronopotentiometry measurements under a 10 mA cm−2 current density52 at 1600 rpm for a period of 3 h (Figure 5B). Within 3 h, a small increase in the overpotential rate (10 mV h−1) is observed at Rh2O3-NSNSs, which is 4.3 times slower than that (43 mV h−1) of the commercial Ir/C catalyst. These chronopotentiometry results clearly indicate that Rh2O3-NSNSs have a catalytic performance that is more stable than that of the commercial Ir/ C catalyst, originating from the extreme chemical stability of Rh2O3-NSNSs under harsh environmental conditions (Figure 4A,B). Indeed, the SEM image shows the morphology of Rh2O3-NSNSs is completely preserved after the chronopotentiometry test (Figure S8), which contributes to the excellent durability of Rh2O3-NSNSs for the OER. The durability of Rh2O3-NSNSs and the commercial Ir/C catalyst was further evaluated by a chronoamperometry technique in a N2-saturated 1 M KOH electrolyte at a 1.58 V potential (Figure S9). During the chronoamperometry measurements, Rh2O3-NSNSs show an OER current density higher than that of the commercial Ir/ C catalyst. At 3 h, the OER current density at Rh2O3-NSNSs is 1.8 times larger than that of the commercial Ir/C catalyst. The larger OER current density and slower decay rate confirm that Rh2O3-NSNSs are more active and durable as a catalyst for the OER than the commercial Ir/C catalyst is.
Figure 5. (A) LSV curves of (a) Rh2O3-NSNSs and (b) the commercial Ir/C catalyst in a 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) the commercial Ir/C catalyst in a N2saturated 1 M KOH electrolyte at a 10 mA cm−2 current density at 1600 rpm.
potential difference between a potential with a 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 the OER electrocatalyst. As observed, ηOER (265 mV) at Rh2O3-NSNSs reveals a 35 mV negative shift compared with that (300 mV) of the commercial Ir/C catalyst (Figure 5A). Besides, the 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. To understand OER activity enhancement, the catalytic activity of commercial Rh2O3 nanoparticles was investigated. ηOER (265 mV) at Rh2O3-NSNSs reveal a 165 mV negative shift compared with that (430 mV) of commercially available Rh 2 O 3 nanoparticles (Figure S6), indicating morphology-dependent OER activity. As already mentioned, the atomically thick 2D ultrathin nanosheets generally contain numerous weakly coordinated defective atoms and own the unordinary electronic property,9−11,16−22 which may be responsible for the extremely enhanced OER activity of Rh2O3-NSNSs. Additionally, the
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 nanosheets to 2D ultrathin Rh2O3 nanosheet but also effectively removed the residual surfactant. The as-prepared 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 those of the commercial Ir/C catalyst. At a current density of 10 mA cm−2, Rh2O3-NSNSs exhibited an ηOER value of only 265 mV, which outperformed various currently reported noble metal-based electrocatalysts (Table 1). Given the atomically thick ultrathin property of the 2D Rh2O3 nanosheet subunit, high chemical inertness, and particular self-supported architecture, Rh2O3-NSNSs might have wide applications in other heterogeneous catalyses. 17198
DOI: 10.1021/acsami.7b04874 ACS Appl. Mater. Interfaces 2017, 9, 17195−17200
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ACS Applied Materials & Interfaces
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04874. Additional information for SEM of commercial Rh2O3 and Rh2O3-NSNSs after a chronopotentiometry test, Rh 3d and N 1s XPS spectra of Rh2O3-NSNSs and RhNSNSs, LSV curves of Rh-NSNSs, Rh2O3-NSNSs, and commercial Rh2O3 nanoparticles, TEM images of Ir/C, and chronoamperometric curves of Rh2O3-NSNSs and the commercial Ir/C catalyst (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yu Chen: 0000-0001-9545-6761 Author Contributions
J.B. and S.-H.H. contributed equally to this work. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was sponsored by the National Natural Science Foundation of China (21473111) and Fundamental Research Funds for the Central Universities (GK201602002, GK201701007, and 2016TS063).
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DOI: 10.1021/acsami.7b04874 ACS Appl. Mater. Interfaces 2017, 9, 17195−17200
Research Article
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DOI: 10.1021/acsami.7b04874 ACS Appl. Mater. Interfaces 2017, 9, 17195−17200
