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Article

Development of Core-Shell Rh@Pt and Rh@Ir Nanoparticle Thin Film Using Atomic Layer Deposition for HER Electrocatalysis Applications

by
Yiming Zou
1,†,
Ronn Goei
1,†,
Su-Ann Ong
1,
Amanda Jiamin ONG
1,
Jingfeng Huang
2 and
Alfred Iing Yoong TOK
1,*
1
School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore
2
School of Engineering, Republic Polytechnic, Singapore 738964, Singapore
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Processes 2022, 10(5), 1008; https://0-doi-org.brum.beds.ac.uk/10.3390/pr10051008
Submission received: 12 April 2022 / Revised: 10 May 2022 / Accepted: 16 May 2022 / Published: 18 May 2022
(This article belongs to the Section Environmental and Green Processes)
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Abstract

:
The efficiency of hydrogen gas generation via electrochemical water splitting has been mostly limited by the availability of electrocatalyst materials that require lower overpotentials during the redox reaction. Noble metals have been used extensively as electrocatalysts due to their high activity and low overpotentials. However, the use of single noble metal electrocatalyst is limited due to atomic aggregation caused by its inherent high surface energy, which results in poor structural stability, and, hence, poor electrocatalytic performance and long-term stability. In addition, using noble metals as electrocatalysts also causes the cost to be unnecessarily high. These limitations in noble metal electrocatalysts could be enhanced by combining two noble metals in a core-shell structure (e.g., Rh@Ir) as a thin film over a base substrate. This could significantly enhance electrocatalytic activity due to the following: (1) the modification of the electronic structure, which increases electrical conductivity; (2) the optimization of the adsorption energy; and (3) the introduction of new active sites in the core-shell noble metal structure. The current state-of-the-art employs physical vapor deposition (PVD) or other deposition techniques to fabricate core-shell noble metals on flat 2D substrates. This method does not allow 3D substrates with high surface areas to be used. In the present work, atomic layer deposition (ALD) was used to fabricate nanoparticle thin films of Rh@Ir and Rh@Pt in a core-shell structure on glassy carbon electrodes. ALD enables the fabrication of nanoparticle thin film on three-dimensional substrates (a 2D functional film on a 3D substrate), resulting in a significantly increased surface area for a catalytic reaction to take place; hence, improving the performance of electrocatalysis. The Rh@Pt (with an overpotential of 139 mV and a Tafel slope of 84.8 mV/dec) and Rh@Ir (with an overpotential of 169 mV and a Tafel slope of 112 mV/dec) core-shell electrocatalyst exhibited a better electrocatalytic performances compared to the single metal Rh electrocatalyst (with an overpotential of 300 mV and a Tafel slope of 190 mV/dec). These represented a 54% and a 44% improvement in performance, respectively, illustrating the advantages of core-shell thin film nanostructures in enhancing the catalytic performance of an electrocatalyst. Both electrocatalysts also exhibited good long-term stability in the harsh acidic electrolyte conditions when subjected to chronopotentiometry studies.

1. Introduction

Hydrogen gas production through electrochemical water splitting has been widely investigated as a sustainable energy source due to its high calorific value and zero-carbon footprint [1,2,3,4,5]. In a water splitting device, hydrogen gas is produced at the cathode while oxygen gas is produced at the anode. The separation of cathodic and anodic compartments, cell resistance, current density losses and the inherent high activation energy requirement of the reaction results in a high overpotential despite the theoretical thermodynamic requirement of the water splitting reaction of 1.23 V at 298 K and 1 atm [2,6]. There is a need for highly active and cost-effective stable electrocatalysts for the electrochemical water splitting reaction [7]. Noble metals such as Pt, Ru, Rh and Ir are benchmark choices of electrocatalysts for water splitting due to their high exchange current density and low overpotential. However, the use of noble metals, such as electrocatalysts, for the water splitting reaction has various limitations, as follows: (1) their poor structural stability leads to poor electrocatalytic performance and long-term stability; (2) the high cost of noble metal material [8,9,10,11]. Despite these limitations, noble metal electrocatalysts (particularly Pt-group elements) represent today’s state-of-the-art, and much research is focused on solving problems related to these electrocatalysts [12,13,14].
Rhodium (Rh), a member of the platinum group of metals, exhibits the lowest overpotential in the hydrogen evolution reaction (HER) [15,16,17,18]. However, its use as a single noble metal electrocatalyst is still limited by the aggregation of atoms caused by its inherent high surface energy, resulting in poor structural stability overall [19,20]. One commonly used technique to overcome this issue is to combine two dissimilar metals through the construction of a core-shell structure, as this prevents atomic aggregation, and, hence, improves the structural stability of the electrocatalyst [20,21]. The combined electrocatalyst exhibits a lower catalytic energy barrier and faster reaction kinetics, resulting in a superior electrocatalytic performance [7,16,22,23,24]. Furthermore, the modification of the electronic structure, optimization of adsorption energy and synergistic effect of the core-shell structure (due to the introduction of new active centers) may contribute to the enhanced electrocatalytic performance [19,25,26].
Core-shell nanoparticle structures of noble metals with tunable size and thickness have been fabricated using atomic layer deposition (ALD) in recent years [27,28]. By applying hundreds of cycles of alternate core precursors and co-reactants, the nanoparticles of core material were deposited. The subsequent ALD process of the shell materials was applied after the formation of the core. During the ALD process, the formation of nanoparticles was observed rather than the formation of thin film. This was because when the vaporized precursor was introduced into the reaction chamber, adsorption occurred at the random sites of the substrate. This resulted in the dispersed nucleation of the core material, followed by the subsequent diffusion and growth of nanoparticles [29,30].
Due to the self-limiting property of ALD [31,32,33,34,35], the growth of material could be precisely controlled by adjusting the number of cycles, and the deposited surfaces were highly uniform [36,37]. In addition, ALD is the dominant technique to grow nanoparticle thin films on a high aspect ratio, three-dimensional substrate, resulting in a structure with a high surface area of functional material [38,39], thus lowering the cost of the electrodes compared to bulk noble metal electrodes.
For noble metal ALD, reliable processes for metallic Rh [36,40], Pt [41,42,43] and Ir [44,45] have been developed; hence, there is potential to use ALD-deposited noble metal core-shell nanoparticles with the Rh core as an effective electrocatalyst. There have been few studies of noble metal ALD on glassy carbon electrodes (GCE), due to the lack of anchoring sites on the GCE surface [46]. As the ALD process is surface-dependent, adsorption of precursor molecules occurs when they bind to the favored groups. For most noble metal precursors, substrates with phenolic groups improve the attachment of the precursor to the substrate [31]. A graphitic-like surface contains very limited phenolic groups, making its adsorption inefficient. As a result, studies on noble metal ALD using GCE as the substrate are very limited. Some efforts have been devoted to pre-treating the surface of other substrates to create anchoring sites for noble metal precursors [36,45,47]. In this work, we explored the application of piranha solution to modify the GCE surface, creating anchoring sites for Rh nucleation and its subsequent growth.
In the present work, core-shell Rh@Pt and Rh@Ir nanoparticles were fabricated by sequential ALD deposition on a pre-treated GCE substrate. First, a layer of Rh nanoparticle thin film was deposited (core) followed by subsequent deposition of either a Pt or Ir nanoparticle thin film (shell). Ozone-based ALD deposition of the noble metals was used [36,37,48]. The use of ozone as the co-reactant eliminated the use of a more explosive H2 or O2 as a precursor and eliminated the need for an annealing agent. The proposed electrocatalyst consisting of a thin layer (~tens of nm) of noble metal offered a significant reduction in the quantity of catalyst material needed (thus a lower overall cost) when compared to the use of bulk catalyst. The potential electrocatalytic role in the hydrogen evolution reaction (HER) step of electrochemical water splitting in acidic conditions was investigated in terms of its overpotential and Tafel slope. The long-term stability of the electrocatalyst in the harsh acidic environment was also investigated.

2. Experimental Section

2.1. Preparation of Rh@Pt and Rh@Ir Bi-Layer Electrocatalysts

The following metallic precursors were procured from Sigma-Aldrich and used without further purification: rhodium(III) acetylacetonate, Rh(acac)3 (min, 97%); platinum(II) acetylacetonate, Pt(acac)2 (min, 97%); and iridium(III) acetylacetonate, Ir(acac)3 (min, 97%). The ozone generator (Nanofrontier XLK-G20, China) converted high-purity oxygen (99.999%) to ozone, providing stable ozone pulses. High purity nitrogen (99.999%) was used as the carrier gas. Glassy carbon electrode pieces, with dimensions 1 × 1 cm2, were used as the substrates for metallic growth. The substrates were rinsed in deionized water and 95% ethanol (with 5% methanol) in the ultrasonic bath to clean the surfaces. To create a higher density of OH adsorption sites for vaporized precursor molecules [36,47], piranha solution (H2SO4:H2O2 = 3:1) was prepared to provide the hydrophilic surface. The substrates were immersed in the piranha solution. After cooling to room temperature, the solution was ultrasonicated for 10 min. Subsequently, the substrates were washed by ethanol to clean off the residue solution and blown dry by a nitrogen gas stream.
Rh, Pt and Ir ALD processes were carried out in a custom-built ALD system with the operating parameters that are summarized in Table 1. A nitrogen gas flow of 150 sccm went through the system to maintain a basic pressure of 1 mbar. The ozone gas flow was set at 40 sccm. The powders of Rh(acac)3, Pt(acac)2 and Ir(acac)3 were loaded in three stainless steel bubblers held at 170, 150 and 185 °C, respectively, to obtain enough vapor pressure. The tube line was kept at 175, 150 and 185 °C, respectively, to prevent the condensation of the precursors. The reaction chamber was kept at 210, 140 and 180 °C for Rh, Pt and Ir deposition, respectively.
Rh(acac)3, Pt(acac)2 and Ir(acac)3 exhibited very low pressures even when they were heated. Thus, to ensure a sufficient amount of precursor could participate in the reaction, two precursor pulses were applied, followed by one ozone pulse. The schematic of two ALD cycles is shown in Figure 1. For the Rh@Ir thin film, 1000 and 500 cycles of Rh and Ir were applied, respectively. Similarly, for the Rh@Pt thin film, 1000 and 500 cycles of Rh and Pt were applied, respectively. For comparisons, 1000 cycles of Rh were applied to obtain Rh thin film.

2.2. Material Characterization

The crystal structures of the films were studied using a grazing-angle X-ray diffractometer (XRD, Bruker D8 discover, USA) with a Cu–Kα source (1.54 Å radiation). The grazing angle was specified as 1° for all the thin film measurements. The thickness of the film was measured by the X-ray reflectometry (XRR) function on the same diffractometer. The chemical compositions of the bi-layer thin films were obtained by X-ray photoelectron spectroscopy (XPS, Shimadzu Kratos Axis Supra, UK) equipped with 15 kV/15 mA Al Kα source. The raw XPS profiles were fitted using the corresponding software ESCApe. To appropriately fit the simulating profiles to the raw profiles in the metallic spectra, a mixed Gaussian–Lorentzian function with a 40% Gaussian peak profile was applied. The surface morphology of as-deposited bi-layer thin films were examined using field emission scanning electron microscopy (FESEM, JEOL 7600F, Japan) with 15 kV operating voltage. An EDX (Oxford Instrument, UK) was used to examine the atomic concentration of the metallic elements. The transmission electron microscopy (TEM) images were obtained using JEOL 2100F TEM (Japan) operating at a 200 kV accelerating voltage. The root mean square (RMS) roughness was determined over a 5 × 5 μm2 area by atomic force microscopy (AFM, Park System NX10, Korea).

2.3. HER Electrocatalytic Measurement

The HER electrocatalytic measurements were conducted in 1 M H3PO4 (Signa Aldrich) electrolyte using a three-electrode setup in a Metrohm Autolab Potentiostat 1470E (Netherlands). Ag/AgCl and Pt mesh were used as a reference and a counter electrode, respectively. The polarization curves were obtained using LSV (linear sweep voltammetry) with a scan rate of 10 mV s−1. All measured potential values (vs. Ag/AgCl) reported in this work were converted to the reversible hydrogen electrode (RHE) scales according to the Nernst equation (Equation (1)), as follows:
ERHE = EAg/AgCl + 0.059 × pH + E0Ag/AgCl
where E0Ag/AgCl = 0.1976 V was measured at 298 K. The thermodynamic electrical potential for HER is 0 V. The overpotential (η, an extra potential above the equilibrium required to achieve a reasonable reaction rate) values of each type of electrocatalyst were measured at a current density 10 mA cm−1 and used to compare the electrochemical activity of the electrocatalyst. The Tafel plots were obtained by plotting potential (vs. RHE) against the logarithm of current density. The values of the anodic Tafel slopes were obtained and were used to compare the performance of each electrocatalyst. In general, a good HER electrocatalyst should exhibit low overpotential and Tafel slope values. The durability of the electrocatalyst was tested using chronopotentiometry with a set current of 10 mA.

3. Results and Discussion

To verify the successful fabrication of each noble metal on the GCE, XPS analysis was carried out. From the wide scan spectra in the range 0–1200 eV (Figure 2a,d), signals from metallic Rh (3p, 3d), Pt (4p, 4d, 4f) and Ir (4p, 4d, 4f) were identified on the Rh@Pt/Rh@Ir thin films (Table 2), which verified the core-shell fabrication via ALD method. To further study the elemental state of the metals, the profiles of that Rh 3d, Pt 4f and Ir 4f spectra were fitted. In the Rh 3d spectra detected from both the Rh@Pt and Rh@Ir samples in Figure 2b,e, the wide peaks of Pt 4d5/2 and Ir 4d5/2 highly overlapped with Rh 3d3/2. In addition to these components, two pairs of doublets, corresponding to the Rh 3d5/2 and Rh 3d3/2 (Δ = 4.7 eV) of metallic Rh and its oxidation states with higher binding energy, were identified.
Figure 2c,f exhibits the Pt 4f spectrum of RhPt thin film and Ir 4f spectrum of RhIr thin film. Similarly, the curve fitting indicated that the profiles mainly consisted of metallic Pt and Ir and a small amount of related oxide products. The peaks at 72.0 and 75.3 eV were assigned to metallic Pt; the peaks at 61.3 and 64.5 eV were assigned to metallic Ir. It could be concluded from the XPS that the metallic Rh thin film could be deposited on the GCE when subjected to proper pre-treatment, and the subsequent deposition of other noble metals could be achieved on top of the Rh layer.
To identify the phases in the thin films, grazing incidence X-ray diffraction (GIXRD) with a 1° grazing angle was applied on the as-deposited thin films. An uncoated GCE piece with the above-mentioned pre-treatments was also scanned for comparison. The XRD patterns of both Rh@Ir and Rh@Pt are shown in Figure 3a, the peaks at 43.60 and 79.09° correspond to the (100) and (110) orientations of the graphitic domains in the glassy carbon [49,50,51]. The characteristic peaks at 41.17, 47.93, 70.04 and 84.34° are highlighted in the patterns of the as-deposited thin films, which correspond to (111), (200), (202) and (311) orientations of an FCC structure, respectively. As calculated by the Debye–Scherrer equation, the particle sizes of Rh@Ir and Rh@Pt were determined to be 11.18 nm and 9.58 nm, respectively.
For Rh@Ir, the peak positions were also identified as metallic Rh and Ir, when compared with previous studies. As the characteristic peaks of FCC Rh and Ir were so close they could hardly be distinguished, and the identification of both elements was confirmed with the XPS spectra. Even though XPS spectra identified the Pt components, the typical FCC Pt pattern was not observed in the Rh@Pt XRD pattern. To study the missing Pt pattern, a Pt thin film was deposited on glassy carbon. In Figure 3b, Pt was compared with the Rh@Pt pattern. The Pt peak represented the (111) orientation shift from 39.92° to 41.07° when Pt was deposited on Rh. This shift did not result from the alloying of Rh and Pt, as the deposition temperature was too low to trigger the alloying process. Instead, it was a consequence of the average diffraction of mass with different concentrations, which was reported in core-shell nanoparticle studies [51,52]. Therefore, the XRD pattern could imply the formation of an Rh@Pt core-shell structure.
The thickness of the deposited film was measured using the XRR technique and is presented in Figure 4. The thickness of Rh@Ir and Rh@Pt thin films were determined to be approximately 16 nm. The thickness of the deposited film grew linearly with the number of layers deposited.
Figure 5a shows FE-SEM images of Rh nanoparticles deposited on the GCE. Compared with previous Rh ALD processes in which a particle size of ~50 nm was obtained [36,40], the nanoparticle size was kept around 20 nm, even when 1000 cycles were applied. The narrow nanoparticle size distribution and small diameter on average indicated that glassy carbon could have many more anchoring sites than the Al2O3 substrate, after they were both pre-treated to create more phenolic groups on surface. The morphologies of Rh@Pt-GCE and Rh@Ir-GCE are shown in Figure 5b,c. The nanoparticle sizes of both samples did not change after the subsequent Pt/Ir depositions were applied. The distribution of Rh@Pt nanoparticles was denser, while Rh@Ir nanoparticles exhibited a dispersed distribution. This could be due to the difference in mobility of Pt and Ir atoms at different deposition temperatures [29].
Further evidence of the formation of a core-shell nanoparticle structure was obtained from the observation through the TEM. The representative Rh@Pt samples were observed with the TEM. The nanoparticle size of ~10 nm was observed, and it agreed with the results obtained from the XRD analysis (Figure 6a,b). The diffraction patterns presented in Figure 6c depict the presence of two sets of FCC structures, which belonged to Rh and Pt, respectively. This result confirmed the fabrication of a noble metal core-shell structure by the ALD method. This observation was in the agreement with results obtained from FESEM and XRR.
To further investigate the relationship between the morphology and the diffusion of Rh@Pt and Rh@Ir nanoparticles, the RMS roughness of the two were determined as 0.11 nm and 0.33 nm, respectively, by AFM over a 5 × 5 μm2 area (Figure 7). The higher roughness of Rh@Ir indicated a greater variation in height and lower uniformity in the nanoparticle. It is generally believed that a higher deposition temperature provides more kinetic energy for diffusion of the metal atoms. As such, due to the higher temperature used during the Ir deposition process as compared to the lower temperature during Pt deposition, the cluster of Ir atoms was easier to diffuse and aggregate to form a less uniform morphology.
Figure 8 shows the polarization curves (LSV) curves of the Rh, Rh@Pt and Rh@Ir electrocatalysts. The HER is highly dependent on the pH value. In acidic conditions, the generally accepted HER step mechanisms can be summarized as follow (Equations (2)–(4)) where two H+ ions are reduced to form H2 molecules [8,19,53]:
Volmer Step: H+ + M + e− ⇔ M−Hads
Tafel Step: 2 M−Hads ⇔ 2M +H2
Heyrovsky Step: M−Hads + H+ + e− ⇔ M + H2
where M−Hads represents a chemically adsorbed H atom on the active site of the catalyst. The HER starts with electrochemical hydrogen adsorption caused by proton-coupled electron transfer, known as the Volmer step. The Volmer step is followed either by the Tafel step (chemical desorption) or the Heyrovsky step (electrochemical desorption) that forms molecular hydrogen H2.
As summarized in the table nested in Figure 8, the Rh, Rh@Ir and Rh@Pt electrocatalysts exhibited HER overpotential values of 300, 169 and 139 mV, respectively. The high activity of the Rh@Pt electrocatalyst could be attributed to the thermo-neutral reaction at the surface of the catalyst [8]. According to Sabatier’s principle, a good HER electrocatalyst should have an optimum binding energy between the electrocatalyst and the reactant, i.e., not too weak to ensure adsorption of the H atom on the surface of the electrocatalyst, but not too strong either to ensure the ease of H2 molecules released at the end of the reaction [54]. The Tafel slopes of the electrocatalysts were then evaluated from the LSV data to further evaluate the reaction kinetics. The Tafel slopes for Rh, Rh@Ir and Rh@Pt electrocatalysts were 190, 112, and 84.8 mV/dec, respectively. The Rh@Pt electrocatalyst exhibited the lowest Tafel slope value, indicating the fastest hydrogen production rate among the electrocatalyst [55,56]. The electrocatalytic performance of the Rh@Pt electrocatalyst was comparable to other reported Rh-based electrocatalysts used in similar operating parameters [57,58,59]. The stability of the as prepared electrocatalysts were studied and the results are presented in Figure 9. All the electrocatalysts exhibited a stable voltage output for a given set current of 10 mA. The results suggested performance stability of the Rh-based electrocatalysts prepared by the ALD method.

4. Conclusions

Core-shell structures of Rh@Pt and Rh@Ir nanoparticle thin films were successfully fabricated on glassy carbon electrodes using ALD with ozone as the co-reactant. The process required no combustible reactants, such as H2 and O2, commonly used as precursors or annealing agents. The formation of the core-shell structure was confirmed using a combination of GIXRD, XPS, FESEM and AFM characterization techniques. The investigation into the catalytic performance of the electrocatalyst for HER reactions found that the Rh@Pt combination exhibited the lowest overpotential of 139 mV and lowest Tafel slope of 84.8 mV/dec when compared to the single metal Rh electrocatalyst (overpotential of 300 mV and Tafel slope of 190 mV/dec). The overpotentials recorded for Rh@Pt and Rh@Ir (169 mV) showed improvements of 54% and 44%, respectively when compared to the single metal Rh. The core-shell Rh@Ir and Rh@Pt performed better than a single metal electrocatalyst, suggesting the synergistic catalytic properties of a core-shell nanoparticle thin film electrocatalyst. All the prepared electrocatalysts showed very good long-term stability when subjected to the chronopotentiometry test under acidic electrolyte conditions. The work highlighted a new process for developing low-cost noble metal core-shell functional thin films by ALD, resulting in improved structural stability and, hence, improved electrocatalytic performance and long-term stability. The novel pre-treatment of the GCE surface by piranha solution created anchoring sits for Rh nucleation and its subsequent growth. ALD also allowed a high surface area 3D substrate to be coated, thus increasing the surface area of the functional thin film coating for improved catalytic performance.

Author Contributions

Y.Z. and R.G. (conceptualization, data curation, investigation, analysis and writing, reviewing and editing of the draft); S.-A.O. and A.J.O. (data curation); J.H. (review of the draft), and A.I.Y.T. (project supervision, review of the draft and funding acquisition). All authors have read and agreed to the published version of the manuscript.

Funding

This research is supported by the Agency for Science, Technology, and Research (A*STAR) under A*STAR Project No: A1983c0032.

Acknowledgments

The authors acknowledge funding support from Singapore’s Agency for Science, Technology and Research (A*STAR) AME Individual Research Grant (IRG) (Award Number SERC A1983c0032) for this project.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The pressure in the reaction chamber as a function of time in two ALD cycles.
Figure 1. The pressure in the reaction chamber as a function of time in two ALD cycles.
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Figure 2. The (a) survey, (b) Rh 3d and (c) Pt 4f XPS spectra of Rh@Pt deposited on the GCE, and (d) survey, (e) Rh 3d and (f) Ir 4f spectra of Rh@Ir on the GCE.
Figure 2. The (a) survey, (b) Rh 3d and (c) Pt 4f XPS spectra of Rh@Pt deposited on the GCE, and (d) survey, (e) Rh 3d and (f) Ir 4f spectra of Rh@Ir on the GCE.
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Figure 3. (a) XRD patterns of glassy carbon, and as-deposited Rh@Ir and Rh@Pt thin films and (b) XRD patterns of Pt and Rh@Pt thin films deposited on the GCE.
Figure 3. (a) XRD patterns of glassy carbon, and as-deposited Rh@Ir and Rh@Pt thin films and (b) XRD patterns of Pt and Rh@Pt thin films deposited on the GCE.
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Figure 4. XRR spectra for Rh@Ir and Rh@Pt thin films.
Figure 4. XRR spectra for Rh@Ir and Rh@Pt thin films.
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Figure 5. The FE-SEM images of (a) Rh, (b) Rh@Pt and (c) Rh@Ir nanoparticles deposited on glassy carbon electrodes.
Figure 5. The FE-SEM images of (a) Rh, (b) Rh@Pt and (c) Rh@Ir nanoparticles deposited on glassy carbon electrodes.
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Figure 6. The TEM images (a,b) and diffraction pattern (c) showing the formation of Rh@Pt core-shell nanoparticles.
Figure 6. The TEM images (a,b) and diffraction pattern (c) showing the formation of Rh@Pt core-shell nanoparticles.
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Figure 7. The AFM images of (a) Rh@Pt-GCE and (b) Rh@Ir-GCE over a 5 × 5 μm2 scanning area.
Figure 7. The AFM images of (a) Rh@Pt-GCE and (b) Rh@Ir-GCE over a 5 × 5 μm2 scanning area.
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Figure 8. Polarization curves (LSV curves) of the as prepared electrocatalysts. Insets exhibit the Tafel plot and summary of the HER electrocatalysis parameters.
Figure 8. Polarization curves (LSV curves) of the as prepared electrocatalysts. Insets exhibit the Tafel plot and summary of the HER electrocatalysis parameters.
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Figure 9. Stability test of the as prepared electrocatalyst. Inset exhibits the set current for the stability test.
Figure 9. Stability test of the as prepared electrocatalyst. Inset exhibits the set current for the stability test.
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Table
Table 1. The sublimation, tube line and deposition temperature for Rh, Pt and Ir ALD.
Table 1. The sublimation, tube line and deposition temperature for Rh, Pt and Ir ALD.
PrecursorSublimation (°C)Tube Line (°C)Deposition (°C)
Rh(acac)3170175210
Pt(acac)2150150140
Ir(acac)3185185180
Table
Table 2. Binding energy (BE) of Rh, Pt and Ir components in Rh@Pt and Rh@Ir.
Table 2. Binding energy (BE) of Rh, Pt and Ir components in Rh@Pt and Rh@Ir.
Spin OrbitBE (eV)Spin OrbitBE (eV)Spin OrbitBE (eV)
Rh 3d5/2308.0Pt 4f7/272.0Ir 4f7/261.3
Rh 3d3/2312.7Pt 4f5/275.3Ir 4f5/264.5
RhOx 3d5/2309.0PtOx 4f7/272.9IrOx 4f7/262.2
RhOx 3d3/2313.7PtOx 4f5/276.2IrOx 4f5/265.4
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Zou, Y.; Goei, R.; Ong, S.-A.; ONG, A.J.; Huang, J.; TOK, A.I.Y. Development of Core-Shell Rh@Pt and Rh@Ir Nanoparticle Thin Film Using Atomic Layer Deposition for HER Electrocatalysis Applications. Processes 2022, 10, 1008. https://0-doi-org.brum.beds.ac.uk/10.3390/pr10051008

AMA Style

Zou Y, Goei R, Ong S-A, ONG AJ, Huang J, TOK AIY. Development of Core-Shell Rh@Pt and Rh@Ir Nanoparticle Thin Film Using Atomic Layer Deposition for HER Electrocatalysis Applications. Processes. 2022; 10(5):1008. https://0-doi-org.brum.beds.ac.uk/10.3390/pr10051008

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Zou, Yiming, Ronn Goei, Su-Ann Ong, Amanda Jiamin ONG, Jingfeng Huang, and Alfred Iing Yoong TOK. 2022. "Development of Core-Shell Rh@Pt and Rh@Ir Nanoparticle Thin Film Using Atomic Layer Deposition for HER Electrocatalysis Applications" Processes 10, no. 5: 1008. https://0-doi-org.brum.beds.ac.uk/10.3390/pr10051008

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