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Oxygen-Intercalated CuFeO2 Photocathode Fabricated by Hybrid Microwave Annealing for Efficient Solar Hydrogen Production Youn Jeong Jang, Yoon Bin Park, Hyo Eun Kim, Yo Han Choi, Sun Hee Choi, and Jae Sung Lee Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b00460 • Publication Date (Web): 16 Aug 2016 Downloaded from http://pubs.acs.org on August 19, 2016
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Chemistry of Materials
Oxygen-Intercalated CuFeO2 Photocathode Fabricated by Hybrid Microwave Annealing for Efficient Solar Hydrogen Production Youn Jeong Jang,a Yoon Bin Park,a Hyoeun Kim,b Yo Han Choi,c Sun Hee Choi,d Jae Sung Leeb* a
Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784 South Korea
b
School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798 South Korea c
Division of Advanced Nuclear Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784 South Korea d
Pohang Accelerator Laboratory (PAL), Pohang University of Science and Technology (POSTECH), Pohang 790-784 South Korea KEYWORDS: Delafossite CuFeO2, Photocathode, Solar Hydrogen Production, Hybrid Microwave Annealing, Oxygen Intercalation
ABSTRACT: Delafossite CuFeO2 is a promising photocathode material for solar hydrogen production, but its performance is low because of poor charge transport properties. When the prepared CuFeO2 electrode is annealed by hybrid microwave annealing (HMA), its photoelectrochemical water reduction activity increases by more than 4 times (-1.3 mA cm-2 @0.4 VRHE), while the conventional thermal annealing (CTA) improves the performance by only 2 times (-0.62 mA cm-2 @0.4 VRHE). The post annealing of the electrode intercalates extra oxygen into the CuFeO2 lattice to form CuFeO2+1.5δ, which increases the charge carrier density and thus improves charge transport properties. The oxygen intercalation with HMA takes place more uniformly over the whole solid and is more effective than CTA. In addition, HMA post-treated CuFeO2 is modified with a NiFe-layered double hydroxide/reduced graphene oxide electrocatalyst, which exhibits a high photoactivity of -2.4 mA cm-2 @ 0.4 VRHE, unprecedented for CuFeO2-based photocathodes.
INTRODUCTION Photoelectrochemical (PEC) water splitting is a promising technique to convert solar energy and water into a storable and transportable chemical energy (H2).1-3 Especially, a dual band gap solar water splitting system with electrically connected photocathode and photoanode in series is attractive because it does not need an expensive photovoltaic cell or additional external bias.4 While several ntype photoanodes (WO3, Fe2O3, BiVO4, etc.) have been developed that show excellent photoactivity, robustness in water, and cost-effective fabrication, it has been a challenge to find suitable p-type photocathode materials meeting such requirements.5-9 For example, in spite of the high photo-activity for solar water reduction, p-Si and copper oxides have serious problems of chemical instability and expensive fabrication cost. The copper-based chalcogenide materials like Cu(In,Ga)S2 and Cu2ZnSnS4 require a toxic n-type semiconductor layer (CdS) to promote band bending.10-13 Among many delafossite materials based on copper (CuMO2; M = Cr, Al, Fe, Ga, Rh), CuFeO2 in particular has attracted great attention as a promising photocathode material for solar fuel production in a pho-
toanode/photocathode dual-absorber/ four-photon (D4) tandem system because of a small band gap, high onset potential, earth abundant elements and relatively good durability in aqueous solution.14-20 However, it has a drawback of poor charge transport properties and as a result, its performance for photoelectrochemical water reduction is not satisfactory. In attempt to enhance the photo-activity of CuFeO2, here we have introduced a unique fabrication strategy, i.e. hybrid microwave annealing (HMA) as a post-treatment, in order to boost its crystallinity and introduce oxygen intercalation. Thus, HMA-treated CuFeO2 decorated with a NiFe-layered double hydroxide (LDH)/ reduced graphene oxide (RGO) electrocatalyst exhibited unprecedented high photoactivity of -2.4 mA cm-2 @ 0.4 VRHE in Arpurged NaOH electrolyte, the best among CuFeO2-based photocathodes and outperforming most of the reported photocathodes in the literature. The HMA treatment caused effective interstitial oxygen doping into CuFeO2 lattice to form CuFeO2+1.5δ uniformly over the whole film, thereby increased charge conductivity resulting in enhanced charge separation. Although the intercalation was also observed in case of the conventional thermal anneal-
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ing (CTA), it was less uniform and less effective than HMA. EXPERIMENTAL Synthesis of CuFeO2 photocathode. The CuFeO2 electrode was prepared by a simple spin coating method. Thus, 0.2 M of Cu(NO3)2 3H2O and Fe(NO3)3 9H2O were mixed in ethanol and then 0.45 M ethylene glycol was added to the solution. The fully dispersed mixture was spin-coated (Midas, Spin-1200D) onto F:SnO2 (FTO) glass (PECTM 8, 6-9Ω, Pilkington) at 3300 rpm for 1 min and then 600 rpm for 30 s. The spin-coated FTO was directly sintered on a hot plate at 450 oC in air for 1 h and the cycles were repeated for 6 times. Finally, the deposited electrode was transferred to a furnace and annealed for 10 h in Ar flow at different temperatures of 500, 600, and 700 oC. Post hybrid microwave annealing. The film was treated in a microwave oven (Daewoo Electrics, 2.45 GHz – 800 W) for different durations (1, 3, 5, 10, and 15 min). For effective microwave absorption, the film was transferred onto a silicon wafer (n-type, Silicon Technology Corporation) as a susceptor. The detailed procedure is illustrated in Scheme 1. For comparison, the film was loaded on a graphite-filled reactor with the graphite as the susceptor. Deposition of NiFe layered double hydroxide/reduced graphene oxide co-catalyst. NiFe layered double hydroxide and reduced graphene oxide composite (NiFe LDH/RGO) was synthesized by a modified solvothermal method.21,22 The graphene oxide (GO) was prepared by the Hummer's method23 and fully dispersed in distilled water (2 mg/ml). 4 mmol of Ni(OAc)2·4H2O and 0.8 mmol of Fe(NO3)3·9H2O were mixed in 48 ml distilled water, then 30 ml of N,N-dimethylformamide (DMF), 12 ml of GO solution and 20 μl of 65% hydrazine were added. The NiFe LDH/RGO composite was synthesized in a Teflon-lined stainless steel reactor at 120 °C for 18 h. As a reference sample, NiFe LDH was synthesized following the same procedure without GO and hydrazine. Characterization of the photoelectrodes. The morphology of the samples was characterized by high resolution scanning electron microscopy (SEM, JSM-7410 JEOL) and high resolution transmission electron microscopy (HR-TEM, JEM-2200FS JEOL with Cs-corrector) in the National Institute for Nanomaterials Technology, Korea. The X-ray diffraction (XRD, PW3040/60 X’pert PRO, PANalytical) using Cu Kα and X-ray photoelectron spectroscopy (XPS, ThermoFisher Machine using Al Kα source) characterized the films. The light absorption properties of the samples were measured by UV-vis diffuse reflectance spectroscopy (UV-vis DRS, UV-3600 Shimadzu). X-ray absorption fine structure (XAFS) analysis was applied to investigate the local structures of Cu and Fe in the CuFeO2 electrode. Measurements were conducted on 7D beamline of Pohang Accelerator Laboratory (PLS-II, 3.0GeV, 400mA), Korea. The radiation was mono-
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chromatized using a Si(111) double crystal monochromator. The spectra for the K-edges of Fe (E0 = 7112 eV) and Cu (E0 = 8979 eV) were taken in fluorescence mode at room temperature. To minimize higher-order reflections of the silicon crystals, the incident beam was detuned by 30% and 20% and its intensity was monitored using a Hefilled or a N2-filled IC Spec ionization chamber for Fe Kedge and Cu K-edge, respectively. The fluorescence signal from the sample was measured under an atmosphere of helium with a passivated implanted planar silicon (PIPS) detector. The obtained data were analyzed with ATHENA in the IFEFFIT suite of programs. Photoelectrochemical measurements. Photoelectrochemical measurements were made with a potentiostat (Gamry–Reference 600TM) in 3-electrode configuration with photocathode, Ag/AgCl reference electrode and Pt anode. The electrolyte was Ar-purged 1 M NaOH. All the PEC measurements were performed under the simulated 1 sun (100 mW cm-2) from a solar simulator (91160, Oriel) with an AM 1.5G filter and the intensity was calibrated by a reference guaranteed by National Renewable Energy Laboratories, USA. The applied potential was cathodically swept with a 5 mV s-1 and the evolved gases (O2 and H2) were detected by a gas chromatograph equipped with a thermal conductivity detector (HP 7890, molecular sieves 5 Å column, Ar carrier gas). Mott–Schottky (M–S) analysis was carried out by sweeping the potential range of 0.5-1.0 VRHE DC potential with 10 kHz AC potential frequency and 10 mV amplitude under dark conditions. The incident photon-to-current conversion efficiency (IPCE) was measured at 0.4 V vs. RHE by using a 300 W Xe lamp (66 905, Oriel Instruments) and a monochromator (74-004, Oriel Cornerstone 130 1/8 m) with a bandwidth of 5 nm. RESULTS AND DISCUSSION Fabrication and characterization of CuFeO2 photocathodes The fabrication and post treatment procedures of CuFeO2 are depicted in Scheme 1. The copper iron oxide films (CFO 450 Air) were deposited by the simple spin coating and thermal annealing at 450 ℃ in air to remove organic compounds in the precursor solution.
Scheme 1. Schematic process of the delafossite CuFeO2 films fabrication followed by post-treatment of hybrid microwave annealing (HMA) and conventional thermal annealing (CTA).
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Chemistry of Materials
Figure 1. X-ray photoelectron spectroscopy (XPS) of core-level Cu 2p and Fe 2p for CFO (A1 and A2), CTA (B1 and B2), and HMA (C1 and C2) films
Then the films were annealed under Ar flow at 500, 600, or 700 oC for 10 h in order to transform amorphous copper iron oxide into highly crystalline delafossite structure. The thermally annealed CuFeO2 samples were denoted as CFO 500 Ar, CFO 600 Ar and CFO 700 Ar. Above 700 oC, the film was destroyed resulting from breakdown of FTO substrate.24 As shown in X-ray diffraction (XRD) patterns in Figure S1A of Supporting Information (SI), amorphous copper iron oxide becomes crystalline delafossite structure as indicated by the peaks at 2θ = 31.4, 35.8 and 40.4 ° representing (006), (012), and (104) crystal planes of delafossite CuFeO2 (JCPDS no.01-075-2146). The peak intensity representing crystallinity increases with increasing annealing temperatures, i.e. CFO 450 Air
