IrOx

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Highly Active IrOx Nanoparticles/Black Si Electrode for Efficient Water Splitting with Conformal TiO2 Interface Engineering Miao Kan, Xufang Qian, Taiyang Zhang, Dongting Yue, and Yixin Zhao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02850 • Publication Date (Web): 28 Sep 2017 Downloaded from http://pubs.acs.org on September 30, 2017

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ACS Sustainable Chemistry & Engineering

Highly Active IrOx Nanoparticles/Black Si Electrode for Efficient Water Splitting with Conformal TiO2 Interface Engineering Miao Kan, Xufang Qian, Taiyang Zhang, Dongting Yue, and Yixin Zhao* School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China, [email protected].

ABSTRACT: Here we report a highly active electrochemical water splitting electrode fabricated from colloidal IrOx nanoparticles and nanoporous Si stabilized by conformal TiO2. The colloidal amorphous IrOx nanoparticles are highly active for oxygen evolution reaction (OER). However, their application for water splitting has a dilemma that the traditional annealing process could lead to low activities but the nanoparticles based electrode without annealing usually exhibited low stability. This nanostructured water splitting electrode exhibited both the high activities as the colloidal IrOx nanoparticles and comparable stability as traditional thermal annealing fabricated electrode. The impedance study revealed that conformal TiO2 significantly inhibit the interface oxidation and maintain the high activity of colloidal IrOx nanoparticles catalysts. The conformal TiO2 interface engineering combined with nanocatalysts would be a promising strategy to achieve balanced activities and stability for water splitting.

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KEYWORDS: Water splitting, IrOx nanoparticles, Nanoporous silicon, Conformal TiO2 INTRODUCTION Renewable and environmental-friendly hydrogen energy generated from water splitting is one of most promising renewable energy candidates in future.1-2 Whatever electrochemical water splitting or solar driven water splitting to produce hydrogen, the oxygen evolution reaction (OER) has always been the limiting step in the overall water splitting because of the challenging four electron reaction in the OER. To overcome the difficulty of high overpotential and instability of anodes, tremendous research efforts have been investigated in OER catalysts with high activity and stability.3 The dimensionally stable anode (DSA) has been regarded as a great breakthrough to solve both two problems above and has been commercially used.4-5 Recently, some novel configuration such as metal-oxides-semiconductor (MOS) structure has also demonstrated their high stability and activities for OER. These DSA and MOS configuration usually adopted the bulk or single layer catalysts, their performance should be further enhanced if they can fabricated into nanostructure with higher surface area and short electron transfer distance.6 The DSA IrO2 electrode is usually fabricated via thermal decomposition (TD) of iridium salt, the TD-IrO2 are the main active OER catalysts but they are usually micron size due to the high temperature annealing process during the fabrication, such DSA is highly stable in acidic condition but their activities are lower than IrOx nanoparticles.7 In contrast, the hydrolyzed IrOx nanoparticles are highly active for OER and can be anodic deposited as thin films,8-9 but these hydrolyzed IrOx nanoparticles deposited films are low in stabilities. Furthermore these IrOx nanoparticles catalysts have been proven more active with amorphous state and would become less active after high temperature annealing.10-12 It would be ideal to fabricate the stable and highly active IrOx nanoparticles based OER electrode at ambient conditions without annealing.13


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In order to obtain higher activities and decrease usage amount of IrOx nanoparticles, an ideal electrode should adopt a conductive substrate with higher surface areas to expose these active IrOx nanoparticles.14 Ti substrate has been widely used in the DSA.15-16 but the high temperature annealing is necessary to forming a good contact, which would impact the IrOx nanoparticles activities. Another popular substrate candidate is the conductive silicon and the planar Si substrate has demonstrate to be stable with metal oxides or forming MOS structure.17-19 Recently, the black Si (b-Si) with nanopores

technique has been adapted to fabricate

electrochemical electrodes and has also been used for solar cells or photoelectrochemical water splitting.20-22 However, black Si with more active sites could induce serious surface oxidization and corrosion.23 The TD-IrO2/b-Si, which is prepared by high temperature annealing to form a compact IrO2 layer on b-Si similar to DSA electrode, has demonstrated be stable to avoid corrosion but has much lower activities than colloidal IrOx.24 Some oxides such as SiO2 and TiO2 are supposed to prevent silicon from corrosion.25-26 The conformal amorphous TiO2 layer fabricated via ALD has been ulitized in protecting electrodes for both water reduction and oxidation.27-30 Here we report a conformal TiO2 stabilized highly active OER electrode based on IrOx nanoparticles and black Si, which is noted as TiO2/IrOx/b-Si. This electrode exhibit the high electrochemical activities comparable to state of art colloidal IrOx nanoparticles but with enhanced stability comparable to the TD-IrO2/b-Si electrode. EXPERIMENTAL SECTION Chemicals Potassium hexachloroiridate (IV) (K2IrCl6, Aladdin, 99.99%) was used as received. And Si wafers (orient 100, p type, 525 µm) were purchased from Harbin turbo Technology Co. Ltd. Materials synthesis


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Synthesis of IrOx (IrOx): The blue IrOx colloidal solution were obtained by basic hydrolysis of [IrCl6]2- as following procedure.8 0.1 mM K2IrCl6 was dissolved in 0.6 mM 80 mL NaOH solution with strong stirring. The solution was heated up to 90 oC and kept for 20 min. The blue solution was then cool down by ice bath. The resulting IrOx solution was then stored in a refrigerator at 2 oC. Preparation of nanostructured silicon substance (b-Si): The nanoporous silicon electrodes were obtained by a metal-assisted chemical etching process. In a typical procedure, the silicon substance (p type (100), 0.01-0.02 Ω·cm) was washed by acetone, alcohol and deionized (DI) water. The cleaned Si with native oxide was first etched in 5 wt% HF for 1 min. After being controlled by polyimide tape, the exposed Si surface was deposited with Ag nanoparticles by soaking in the solution of 1 mM AgNO3 and 1 wt% HF for 60 s, followed by a DI water rinse and N2 dry. Then it was transferred into an etching solution of 50 wt% HF, 30 wt% H2O2, and DI water with volume ratio of 25:6:370 for 5 min, followed by rinsing with DI water and drying with N2. The prepared black Si wafer was then soaked in a 35 wt% HNO3 solution for 6 min to remove the Ag residues. The obtained nanostructured Si was denoted as b-Si. The b-Si electrodes with different thickness were fabricated by etching time of 3min and 8min were also performed but they did not exhibited better performance. Thermal deposition of IrO2/b-Si (TD-IrO2): A IrO2 film was deposited on black Silicon (b-Si ) by the thermal decomposition of 10µL 86mM IrCl4 precursor aqueous solution on b-Si at 450 oC for 2h in air with a heating rate of 5 oC/min. A IrO2/b-Si electrode prepared by thermal decomposition of a precursor and denoted TD-IrO2/b-Si.

17, 31

The best loading amount of Ir on

TD-IrO2/b-Si was calculated as 0.86 μm/cm2 as shown in Figure S7.


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Fabrication of IrOx/b-Si: The IrOx/b-Si electrodes were fabricated by drop-coating 10μL/cm2 IrOx solution on the b-Si substance followed by drying in an 80 oC oven.32 This process was repeated for several times to make the amount of deposited IrOx controllable. Deposition of conformal TiO2 layers: The TiO2 layers were deposited on IrOx/b-Si by atomic layer deposition system (MNT ,ALD f-100-4). In detail, 27 Pa as the basic vacuum situation of a heated reaction chamber with 150 oC was used with 200 ms titanium tetrakis and 15 ms water with waiting time of 5s for the ALD deposition of TiO2. Each cycle would deposit ~ 0.625Å thick TiO2 layer as confirmed by Variable Angle Spectroscopic Ellipsometer (W-VASE with AutoRetarder). Characterization The UV-vis spectra were recorded on an Agilent Cary 60 Scan microscope. TEM images were performed with an FEI microscope. XRD measurements were performed on a Lab XRD-6100 Xray diffract meter. SEM and EDS mapping were performed on a Sirion 200 Field-emission Scanning Electron Microscopy. Electrochemical measurement The p+-Si wafer was back scratched with a diamond cutter and contact to a Pt sheet electrode holder and exposed ares were controlled by gelling with Lectite 9460 and taping with Polyimide tape. The electrochemical activity of the IrOx catalysts was measured by rotating disk electrode (RDE) in phosphate buffer with different pH value using Ag/AgCl as reference and Pt wire as counter. The oxygen evolution reactions (OER) and stability of the IrOx/b-Si electrodes were studied by linear polarization (I-V) with a scanning rate of 10 mV/s, contant voltage measurement (I-T) with 100s duration at 1.7 vs RHE and constant current measurement (V-T) at 5 mA/cm2 for 4h in 1 M HClO4. Liner polarization curves and tafel plots were measured between


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0.4 V and 1.6 V vs. RHE at a scan rate of 10 mV/s, and all curves were measured by Zahner Instrument with resistance correction. Electron Impendence Spectrum (EIS) were done from 500K to 0.1 Hz with 1.5 V vs. RHE. For convenience, the potential of Ag/AgCl was converted into RHE by the following Equation. E(vs.RHE)=E(vs.Ag⁄AgCl)+EAg⁄AgCl (reference)+0.0591×pH

(1)

EAg/AgCl(reference)=0.1976 V vs. NHE at 25℃

(2)

RESULTS AND DISCUSSION The morphology and activity of IrOx nanoparticles: The IrOx solution via basic hydrolysis of [IrCl6]2- is a blue colloidal suspension with two characteristic absorption peaks at 314 nm and 585 nm as shown in Figure 1a.8 The peak at around 585 nm is a typical absorption of IrOx·nH2O nanoparticles, and such blue IrOx·nH2O nanoparticle is supposed to be an effective catalysts for the oxygen evolution reaction.33 The other peak locating at 313~318 nm could be ascribed to the Ir monomeric complexes. This monomeric anions would be not only beneficial for high electrochemical activities but also could help the IrOx nanoparticle to link with different substance.9 TEM images of these blue IrOx nanoparticles are given in Figure 1b, these IrOx nanoparticles are pretty small and the size distribution are in the range of 1 nm to 3 nm. Furthermore, the SAED pattern of these aggregated particles exhibits a weak ring without obvious crystal lattice character, suggesting that these IrOx are amorphous state and was further confirmed by their XRD pattern (see Supporting Information (SI), Figure S1). The amorphous structure of the IrOx nanoparticles should be due to the low temperature synthesis process while the IrO2 crystalized particles were obtained by high temperature synthesis.11 The electrochemical activities of IrOx nanoparticles was shown in Figure 1c. Two characteristic redox peaks related to O2+/O- or Ir (III/IV) between 0.2 V and 0.6 V are observed, which is consistent with previous


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reported amorphous IrOx catalysts with high activities. 12, 24, 34 The CVs of the IrOx nanoparticles films deposited glassy carbon electrode in different pH value are listed in Figure 1d. The average overpotential for OER is ~ 0.23 V measured with a standard of 0.5 mA/cm2 current density. Such overpotential value is close to the state of art value for IrOx catalyst. The onset potential with constant current density for OER is pH dependent, which is linear to pH value as shown in SI, Figure S2. These results reveal that our IrOx nanoparticles show excellent catalytic activities for OER.

Figure 1. a) UV-vis spectra of IrOx solution; b) TEM image of IrOx nanoparticles with SAED pattern; c) Cyclic voltammetry curves of IrOx nanoparticles solution measured by modified glassy carbon electrodes (GCE) at a rate of 50 mV/s; d) Cyclic voltammetry curves of IrOx nanoparticles modified GCE measured by RDE system in different pH value of 0 (black), 3.5 (magenta), 7 (blue), 10.5 (green) and 13.5 (cyan) at a rate of 20 mV/s


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The structure of Ir-cat/Si electrodes: Then IrOx modified Si electrodes were fabricated by drop coating, which can precisely control the Ir loading. However, these colloidal IrOx nanoparticles are difficult to uniformly deposit onto planar Si. The top-view SEM image of these highly active IrOx nanoparticles deposited planar silicon (IrOx /pl-Si) was given in Figure 2a. It is difficult to deposit a uniformed IrOx nanoparticles film on pl-Si. Figure 2a show some uncovered area without IrOx nanoparticles. Unlike the planar Si, the nanoporous b-Si exhibits a rough surface consisted of nanopores as shown in the SEM image in Figure 2b. These nanopores has the diameter of around 50 nm and 500 nm depth as shown in SI, Figure S3a, making a rough surface. The rough surface with larger surface area may make it favor for nanoparticles deposition.21 As shown in Figure 2c, the IrOx/b-Si shows the nanoporous structures without obvious IrOx nanoparticles agglomerates. Only several accumulated particles are sparsely dispersed in the edges of some of these nanopores. To further confirm the distribution of catalysts on b-Si, the IrOx/b-Si was then characterized by SEM image and EDS mapping of elemental Ir (SI, Figure S3). And there is only small amount (

IrOx

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