Rapid Flame-annealed CuFe2O4 as Efficient Photocathode for

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Rapid Flame-annealed CuFe2O4 as Efficient Photocathode for Photoelectrochemical Hydrogen Production SANGWOOK PARK, Ji Hyun Baek, Liang Zhang, Jae Myeong Lee, Kevin H Stone, In Sun Cho, Jinghua Guo, Hyun Suk Jung, and Xiaolin Zheng ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05824 • Publication Date (Web): 06 Feb 2019 Downloaded from http://pubs.acs.org on February 7, 2019

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ACS Sustainable Chemistry & Engineering

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Rapid

Flame-annealed

CuFe2O4

as

Efficient

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Photocathode for Photoelectrochemical Hydrogen

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Production

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Sangwook Park, a,‡ Ji Hyun Baek, a, b,‡ Liang Zhang, c Jae Myeong Lee, b Kevin Hunter Stone, d

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In Sun Cho, e Jinghua Guo, c Hyun Suk Jung, b, * and Xiaolin Zheng a, *

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a Department

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California 94305, United States

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b School

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Seobu-ro, Suwon 16419, Republic of Korea

of Mechanical Engineering, Stanford University, 440 Escondido Mall, Stanford,

of Advanced Materials Science and Engineering, Sungkyunkwan University, 2066

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c Advanced

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California 94720, United States

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d SLAC

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United States

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e Department

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University, 206 World cup-ro, Suwon 16499, Republic of Korea

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‡These authors contributed equally to this work.

Light Source, Lawrence Berkeley National Laboratory, 6 Cyclotron Rd, Berkeley,

National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025,

of Materials Science & Engineering and Energy Systems Research, Ajou

ACS Paragon Plus Environment

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ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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[email protected] | Fax: +82-31-290-7410 | Phone: +82-31-290-7403

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[email protected] | Fax: +1-650-723-1748 | Phone: +1-650-736-8953

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KEYWORDS: CuFe2O4, Flame synthesis, Photoelectrochemical water splitting, Hydrogen

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production

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ABSTRACT

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Copper ferrite (CuFe2O4) possesses an indirect bandgap in the range of 1.54–1.95 eV. It is used as

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an attractive p-type photocathode in photoelectrochemical (PEC) water splitting, and theoretically

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it can yield a maximum photocurrent density of ~27 mA/cm2 and a maximum solar-to-hydrogen

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conversion efficiency of ~33%. To date, only a few reports have been published on CuFe2O4

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photocathodes with very low-photocurrent densities, with a maximum value of 0.4 mA/cm2 at 0.4

13

V vs. RHE. Herein, we prepared a CuFe2O4 photocathode on FTO glass with the sol–gel method

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followed by either high-temperature flame annealing or furnace annealing. We found that the

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flame-annealed CuFe2O4 photocathode generated a photocurrent density of 1.82 mA/cm2 at 0.4 V

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vs. RHE that is approximately 3.5 times higher than the furnace-annealed CuFe2O4 (0.52 mA/cm2).

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This photocurrent density is also higher than all the reported CuFe2O4 photocathodes, and any Cu

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containing ternary oxide (Cu–M–O, M: Fe, Bi, V, and Nb) photocathode (0.1–1.3 mA/cm2 at 0.4

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V vs. RHE). An improved PEC performance of the flame-annealed CuFe2O4 photocathode is

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elicited owing to the beneficial effects of flame annealing on the physical, optical, and electrical

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properties of CuFe2O4. Flame annealing enhances the light absorption property of the CuFe2O4


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photocathode by slightly reducing the bandgap, and by forming a thicker film with increased

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porosity. Flame annealing also reduces the oxygen vacancy concentration in CuFe2O4, thus

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facilitating charge transport and interfacial charge transfer processes. Moreover, flame annealing

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requires only 16 min, which is much shorter than the time required for furnace annealing (~9 h).

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These results demonstrate that flame annealing is a rapid and effective means for fabricating metal

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oxide photoelectrodes with an enhanced PEC water splitting performance.

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INTRODUCTION

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Photoelectrochemical (PEC) water splitting has been extensively studied as a possible

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hydrogen production technique that utilizes solar energy to split water.1-3 A potential bias-free PEC

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configuration could be represented by the tandem cell that consists of p-type photocathodes and n-

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type photoanodes.1,

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photoanode materials, such as BiVO4,8, 9 WO3,10, 11 Fe2O3,12, 13 and TiO2.14, 15 In comparison, there

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is lack of suitable p-type photocathode candidates.16, 17 Good candidates for photocathodes are

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copper-based metal oxides because they are mostly nontoxic and cost-effective. For example,

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CuO18, 19 and Cu2O20-22 photocathodes have been used for PEC water reduction and have elicited

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good performance but poor stability. In addition, CuFeO2,23-25 CuBi2O4,26,

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CuNb2O629 photocathodes have been shown to have modest performance and stability.

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There is a general consensus for inexpensive, active, and stable n-type

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Cu3VO4,28 and

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Copper ferrite (CuFe2O4) is another p-type semiconductor in the family of copper-based

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metal oxides. CuFe2O4 has several attractive properties that makes it a possible candidate for

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photocathodes. CuFe2O4 has a narrow indirect bandgap in the range of 1.54–1.95 eV, thus yielding

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a theoretical maximum photocurrent density of ~27 mA/cm,2 and a maximum solar-to-hydrogen


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conversion efficiency of ~33%.30-34 CuFe2O4 has a conduction band minimum that is suitable for

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hydrogen evolution reactions.3, 26 In fact, several studies have tested CuFe2O4 in photocathodes for

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PEC hydrogen evolution, but its elicited performance has been poor.31, 32, 35-37 Diez-Garcia et al.

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coated commercial, cubic CuFe2O4 nanoparticles with 30 nm which possessed a high-degree of

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crystallinity on fluorine-doped tin oxide (FTO) glass substrate, followed by annealing at 450 °C

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for 1 h and electrochemical pretreatment. This CuFe2O4 photocathode finally attained a film

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thickness of 14 μm and achieved a current density of -40 μA/cm2 at 0.75 V vs. RHE.32 Li et al.

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grew cubic spinel structure of CuFe2O4 nanoparticle with average diameters in the range of 50–60

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nm using the hydrothermal method followed by the coating of the particles on FTO and annealing

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at 400 °C for 2 h. This CuFe2O4 photocathode has a PEC performance of -0.4 mA/cm2 at 0.4 V vs.

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RHE.36 Among the various factors that account for the poor performances of these CuFe2O4

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photocathodes was the poor contact between CuFe2O4 and FTO, as CuFe2O4 was coated on FTO

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with moderate post sintering temperature in the range of 400–450 °C. Better interface can be

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achieved by directly growing CuFe2O4 on the FTO substrate. The challenge is attributed to the fact

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that the crystallization temperature of CuFe2O4 (800–1000 °C) is much higher than the glass

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transition temperature of FTO (~564 °C).

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Herein, we apply our rapid and high-temperature flame annealing method (> 980 °C)12, 14,

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15, 38-40

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with high crystallinity and good contact with FTO glass. For comparison, we also prepared a

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control sample of a CuFe2O4 film which was coated on FTO glass was after it was annealed in a

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conventional box furnace at 750 °C. The flame-annealed CuFe2O4 photocathode elicited a

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photocurrent density which was ~3.5 times higher (-1.82 mA/cm2 E) than that of furnace-annealed

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CuFe2O4 (-0.52 mA/cm2) at 0.4 V vs. RHE under 1 sun illumination in Ar purged 1 M NaOH

to anneal CuFe2O4 precursor films coated on FTO glass to prepare CuFe2O4 photocathode


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electrolyte. The enhanced PEC performance of the flame-annealed CuFe2O4 photocathode was

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attributed to several reasons, as described next. First, flame annealing makes the CuFe2O4 film

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more porous and thicker that increases light absorption in the visible light region (400–700 nm),

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and increases the surface area.41-43 Second, flame-annealed CuFe2O4 has fewer oxygen vacancies,

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thus leading to more efficient charge transport and transfer processes. Third, flame-annealed

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CuFe2O4 has a slightly smaller optical bandgap that extends the range of light absorption.

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Additionally, the flame annealing method shortens the annealing time from ~9 h to 16 min. To the

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best of our knowledge, the photocurrent density of our flame-annealed CuFe2O4 is higher than all

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the reported Cu-based ternary oxide photocathodes (Table 1).

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EXPERIMENTAL SECTION

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Synthesis of CuFe2O4 film on FTO substrate by flame and furnace. All samples were prepared

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on fluorine-doped tin oxide (FTO) coated-glass substrates (TEC 7, 7~8 Ω/sq, MSE Supplies). The

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FTO substrates were cleaned in acetone, IPA, and DI water, with sonication for 15 min, and were

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dried by an air gun. The CuFe2O4 precursor was prepared by the sol–gel method and was spin-

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coated on FTO glass for both annealing methods (see Supporting Information) at 3000 rpm for 60

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s. The samples were then dried on a hotplate at 100 °C for 10 min. For furnace annealing (Figure

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1a), the samples were annealed in a box furnace at 450 °C in air for 1 h after each spin coating,

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and the optimized thickness was achieved after five spin coatings. The samples are finally annealed

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at 750 °C for 20 min. For flame annealing (Figure 1b), the samples were annealed in an oxygen-

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rich postflame region at 980 °C for 2 min after each spin coating until all the five layers were

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optimized, followed by the final annealing at 980 °C for 8 min. Flame annealing was employed


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using a co-flow premixed flat flame burner (McKenna Burner).12, 14, 15, 38-40 The flow rates of CH4

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and air were set to 2.05 and 31.3 SLPM so that the fuel to oxygen equivalence ratio (Φ) was 0.63.

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Photoelectrochemical characterization. The PEC performances were measured in a standard

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three-electrode PEC cell using a Gamry 1000 potentiostat44. The CuFe2O4 samples were used as

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the working electrodes with a glassy carbon as the counter electrode and Ag/AgCl reference

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electrode. Argon purged 1 M sodium hydroxide (NaOH) electrolyte (pH~13.5) was used in all

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PEC measurements if not specified otherwise. All the PEC measurements were performed by

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employing a solar simulator (Model LCS–100, Newport), and the intensity was calibrated for a

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standard 1 sun condition (AM 1.5 G, 100 mW/cm2) using a photovoltaic reference (Model 91150V,

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Newport). Electrochemical impedance spectroscopy (EIS) measurements were performed in the

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same, standard three-electrode conﬁguration, using a potentiostat (SP–200, Biologic) using 1 sun

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illumination. The incident photon-to-current conversion efficiency (IPCE) was measured at 0.4 V

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vs. RHE with the use of a 150 W Xenon arc lamp source (ABET Technologies) and a

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monochromator (Mmac 200, 600 gr/mm blazed at 500 nm) with a standard silicon photodiode cell.

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RESULTS AND DISCUSSION

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Methodological differences between furnace and flame annealing. As shown in Figure 1a and

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1b, the main differences between furnace and flame annealing methods are the annealing

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temperature and duration. There is an upper annealing temperature limit for FTO glass as its glass

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transition temperature is approximately 564 °C,38 and its conductivity decreases when annealed at

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a temperature above 800 °C.45,

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environment, and our test shows that furnace annealing at 750 °C for 20 min is the upper

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Furnace annealing provides a homogeneous temperature


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temperature limit before severe FTO sheet resistance effects are documented. In contrast, many of

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our previous studies have demonstrated that flame annealing provides a heterogeneous temperature

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field. The coated materials on FTO (e.g., CuFe2O4 precursors) are exposed to high temperature but

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most of FTO glass is kept at a lower temperature due to large spatial temperature gradient, thus

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protecting its integrity and conductivity14, 15, 38-40. The sheet resistances of FTO before and after

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flame annealing are close to the pristine value provided by vendor (Figure S1), confirming little

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impact on FTO from flame annealing. In addition, flame annealing not only enables the use of

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higher temperatures, but also allows negligible ramping/cooling rates compared to the furnace.

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Therefore, flame annealing dramatically reduces the annealing time from ~9 h (furnace) to 16 min

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(flame).

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Morphological comparison of CuFe2O4 photocathodes annealed by furnace and flame. We

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first investigated how the furnace and flame annealing methods affect the morphology of CuFe2O4

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thin films coated on FTO glass. SEM images show the morphology of the CuFe2O4 film annealed

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by furnace (Figure 1c and 1d) and flame (Figure 1e and 1f). The CuFe2O4 (furnace) film is very

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dense (~450 nm) with connected grains, while the CuFe2O4 (flame) is porous and thicker (~530

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nm). The greater porosity and thickness of the CuFe2O4 (flame) enhances its light absorption

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response and increases its contact area with the electrolyte, yet both factors improve the PEC

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performance, as discussed below.

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X-ray characterizations of CuFe2O4 annealed by furnace and flame. The crystallinity

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of CuFe2O4 (flame and furnace) film was examined by X-ray diffraction (XRD) as shown

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in Figure 2a. The XRD data of both CuFe2O4 films match the standard CuFe2O4 spectra


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(JCPDS, #34–0425), thus confirming the successful crystallization of the tetragonal copper

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ferrite phase that is a extensively used as the structural phase for high-performance

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photocatalysts.32 Figure S2 shows that the CuFe2O4 phase, which annealed at 450 °C for 1

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h, is amorphous, thus confirming the need for high-temperature annealing for its

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crystallization.

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To better understand the chemical states of CuFe2O4 (flame and furnace), we

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performed Cu and Fe K-edge X-ray absorption near-edge structure (XANES) experiments.

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The Cu K-edge XANES spectra of CuFe2O4 synthesized by furnace and flame annealing

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are shown along with the reference sample spectra of Cu foil and CuO in Figure 2b. The

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spectra of both CuFe2O4 samples show a peak at 8,997 eV (orange arrow) and have an edge

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rising position of ~8,980 eV (gray arrow), thus indicating the absence of Cu1+ cations and

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the occupancy of the octahedral sites of Cu2+ cations.25, 47 The Fe K-edge XANES spectra

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of both CuFe2O4 samples (Figure 2c) yielded a pre-edge peak at 7,114 eV (purple arrow),

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which is the characteristic feature of Fe3+ cations in the tetrahedral sites.47 The pre-edge

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peak is attributed to the transition from 1s to 3d, which is dipole-forbidden for the absorbers

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in a centrosymmetric environment.48 The enhancement of the pre-edge peak in the Fe K-

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edge XANES spectra is owing to the breaking of the symmetry owing to the

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noncentrosymmetric tetrahedral crystal field of Fe3+ cations in CuFe2O4.

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The XANES spectra analysis revealed a minor difference between flame- and

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furnace-annealed CuFe2O4 samples. We then performed extended X-ray absorption fine

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structure (EXAFS) measurements to compare the local atomic structure of CuFe2O4 (flame

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and furnace). Figure 2d and 2e show the Fourier transformed (FT) curves of Cu and Fe K-

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edge EXAFS spectra, respectively. For the Cu K-edge FT curves, the first peak at 1.45 Å


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originates from the Cu–O bond,47, 49 and the second peak at 2.57 Å is ascribed to the bonds

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of Cu–Fe and Cu–Cu with both cations occupying the octahedral sites.47, 49 The CuFe2O4

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(flame) elicits a higher intensity for the first peak (Cu–O bond), while the CuFe2O4

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(furnace) has a higher intensity for the second peak (Cu–Fe and Cu–Cu bonds). This

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difference suggests that the CuFe2O4 (flame) has fewer oxygen vacancies.50 The Fe K-edge

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results yielded a similar behaviour in that CuFe2O4 (flame) shows a higher relative intensity

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for the first peak (Fe-O bond) than the second peak (Fe-Cu and Fe-Fe) compared with those

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of CuFe2O4 (furnace).47, 49 The difference in the oxygen vacancy between CuFe2O4 films

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by flame and furnace is expected to affect their electronic properties and performance for

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PEC hydrogen production (Figure S3).

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Electronic band structure comparison of CuFe2O4 annealed by furnace and flame. The

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electronic structures of the CuFe2O4 (flame and furnace) films were investigated by UV–vis

14

spectroscopy and ultraviolet photoelectron spectroscopy (UPS). The UV–vis spectroscopy

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determined the band gap from the Tauc plot method (Figure 3a). The Tauc plots show that both

16

CuFe2O4 (flame and furnace) films have an indirect bandgap, as shown in the inset of Figure 3a.

17

The CuFe2O4 (flame) has a slightly smaller optical bandgap (1.84 eV) than that of CuFe2O4

18

(furnace, 1.95 eV). The CuFe2O4 (flame) and CuFe2O4 (furnace) have theoretical photocurrent

19

density limits of ~17.5 mA/cm2 and ~15.3 mA/cm2, respectively, which were calculated based on

20

reference solar spectral irradiances (AM1.5, ASTM G173, NREL). The bandgap values are close

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to the previously reported bandgap range of CuFe2O4 from 1.54 to 1.80 eV.30-33 Moreover, the

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CuFe2O4 (flame) had a higher light absorption efficiency than CuFe2O4 (furnace) over the


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wavelength range from 400 nm to 700 nm (Figure S5), which is a consequence of its higher

2

porosity and thickness (Figure 1c–1f).

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The UPS measurement (Figure S6) was used to determine the work function and the

4

valence band maximum (VBM) of CuFe2O4. The band positions and band edge values for CuFe2O4

5

(flame and furnace) were estimated by UPS measurements, as shown in Figure 3b. According to

6

the UPS and UV–vis data, the calculated conduction band minimum values of CuFe2O4 (flame and

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furnace) films are -0.69 and -0.92 V vs. NHE, and the maximum values of the valence band are

8

1.15 and 1.03 V vs. NHE, respectively (see the experimental section for details). The obtained

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band structures confirmed that the CuFe2O4 photocathodes synthesized by flame and furnace

10

annealing were thermodynamically favorable to drive the hydrogen evolution reaction.26

11 12

Photoelectrochemical characterizations of CuFe2O4 annealed by furnace and flame. Figure

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4a shows the linear sweep voltammetry of CuFe2O4 (flame and furnace) in Ar purged 1 M NaOH

14

electrolyte in the dark (dashed lines), and under 1 sun illumination (solid lines). These polarization

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curves were constructed with a scan rate of 10 mV/s. Correspondingly, the current density values

16

were calculated based on the projected area of the FTO glass substrate. The thickness of the

17

CuFe2O4 film had been optimized based on the photocurrent density values under 1 sun

18

illumination (Figure S7). The optimal thickness is shown in Figure 1d and 1f.

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The CuFe2O4 (flame) clearly elicited a better PEC performance than CuFe2O4 (furnace)

20

with an earlier onset and a higher photocurrent density in the range of the voltage sweep. The

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CuFe2O4 (flame) photocathode exhibited a positively shifted onset potential of ~1.05 V vs. RHE,

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in comparison to that of the CuFe2O4 (furnace) of ~0.85 V vs. RHE. This characteristic establishes


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the CuFe2O4 (flame) as a better photocathode for coupling with the photoanode in a PEC tandem

2

cell.3, 26 The photocurrent density of CuFe2O4 (flame, -1.82 mA/cm2) is ~3.5 times higher than that

3

of CuFe2O4 (furnace, -0.52 mA/cm2) at 0.4 V vs. RHE under 1 sun illumination. We noted that the

4

photocurrent density for CuFe2O4 (flame) has a step change at ~ 0.65 V vs. RHE, and this step

5

change is reduced by increasing the scan rate (> 10 mV/s, Figure S8). These results indicate the

6

presence of surface states for CuFe2O4 (flame) and they contribute to surface recombination.

7

Figure 4b shows the incident photon-to-current conversion efficiency (IPCE) data of CuFe2O4

8

(flame and furnace) photocathodes at 0.4 V vs. RHE. The integrated photocurrent densities from

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IPCE are 1.40 mA/cm2 and 0.36 mA/cm2 for CuFe2O4 (flame) and CuFe2O4 (furnace) respectively,

10

and are lower than the light Jph but consistent with the values of (Jph - Jdark) at 0.4 V vs. RHE in the

11

J–V plot (Figure 4a). This suggests that side reactions occur, even in the dark, which could be

12

attributed to the exposure of the FTO substrate and the instability of copper-based oxides. Similar

13

dark currents are also commonly observed for copper-containing photocathodes, as listed in Table

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1. Importantly, the photocatalytic activity of CuFe2O4 (flame) is the best among the recently

15

reported Cu- and Fe-based ternary oxides for the photocathode, as characterized by the

16

photocurrent density at 0.4 V vs. RHE (Table 1).

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The improved PEC performance of CuFe2O4 (flame) photocathode is only partially caused

18

by its enhanced light absorption property in the visible light region (Figure S5) owing to its thicker

19

and more porous film morphology. In addition, we further conducted electrochemical impedance

20

spectroscopic (EIS) analyses to compare the effect of furnace and flame annealing on the charge

21

transport and transfer efficiency. The EIS data were measured at 0.4 V vs. RHE under 1 sun

22

illumination were fitted to a circuit model, as specified in an inlet of Figure 4c. In this model, the

23

resistances and capacitances are composed of a series resistance (R1) that describes the charge


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transport resistance at the interface between the CuFe2O4 film and the FTO substrate, a charge

2

transport resistance (R2) and a capacitance of the space charge region (C2) in the bulk of material,

3

and a charge transfer resistance (R3) and a Helmholtz capacitance (C3) at the interface between

4

the CuFe2O4 film and the liquid electrolyte. The fitted resistance values are listed in Table 2.51

5

First, the flame-annealed film has a better electrical contact between the CuFe2O4 film and the

6

FTO substrate since the values of R1 of flame-annealed CuFe2O4 (19.46 Ω) is much smaller than

7

that of furnace-annealed CuFe2O4 (156.8 Ω). In addition, the charge transport resistance of

8

CuFe2O4 (flame) is lower (~11 kΩ) than that of CuFe2O4 (furnace, ~20 kΩ). This is attributed to

9

the reduced oxygen vacancies from the EXAFS analysis in Figure 2d and 2e.52 Moreover, the

10

charge injection resistance was dramatically reduced from ~667 Ω for CuFe2O4 (furnace) to ~6 Ω

11

for CuFe2O4 (flame). The EIS results indicate that flame annealing improves both the charge

12

transfer and transport efficiencies, thus leading to the enhanced PEC performance of CuFe2O4

13

(flame).

14

Additionally, we measured the electrochemical active surface area (ECSA) to understand

15

the charge transfer process. The capacitive currents were measured at 1.13 V vs. RHE because

16

there was no noticeable Faradic current in the potential range from 0.78 to 1.18 V vs. RHE. Figure

17

4d plots the capacitive current as a function of scan rate for CuFe2O4 (flame and furnace) in which

18

the slope is proportional to the ECSA ratio.53 The ECSA ratio of the CuFe2O4 based on flame and

19

furnace is 2.78. The higher ECSA value of CuFe2O4 (flame) indicates a better interfacial charge

20

transfer efficiency, which is consistent with the EIS results (Figure 4c).

21

Finally, the stability of the CuFe2O4 photocathode was tested using chronoamperometry

22

using chopped illumination at 0.15 V vs. RHE for 10 min in Ar purged 1 M Na2SO4 electrolyte in

23

agreement with the J–V curves (Figure S9). The stability test was not conducted in 1 M NaOH but


12

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1

in 1 M Na2SO4 to reduce the chemical influence from the strong alkaline electrolytes. Both

2

CuFe2O4 films by flame (red) and furnace (black) show acceptable stabilities, as compared to many

3

reported Cu- and Fe-based ternary oxides (Table 1)23,

4

protective layers and electrocatalyst.21, 54, 55 The stability of the CuFe2O4 photocathode still needs

5

to be further improved to be a practical photocathode. The main reason for the instability is the

6

chemical reduction of Cu2+ under the PEC bias range needed for hydrogen evolution reaction

7

(Figure S10-S12). Potential methods for improving the stability of the CuFe2O4 include elemental

8

doping, deposition of protection layers, addition of electrocatalysts, and a combination of them.

25

and Cu2O20 without the use of any

9 10

CONCLUSIONS

11

In summary, we investigated the potential of flame-annealed CuFe2O4 as a photocathode for PEC

12

hydrogen production. The flame-annealed CuFe2O4 photocathode achieved a photocurrent density

13

of -1.82 mA/cm2 at 0.4 V vs. RHE that was higher than a) the density of the furnace-annealed

14

CuFe2O4, and b) the corresponding density values of all the reported Cu-based ternary oxide

15

photocathodes. The enhanced PEC performance of the flame-annealed CuFe2O4 photocathode was

16

attributed to several factors. First, the flame-annealed CuFe2O4 film was more porous and thicker,

17

and led to a better light absorption and a higher surface area. Second, the flame-annealed CuFe2O4

18

had fewer oxygen vacancies, and hence achieved more efficient charge transport and transfer

19

processes. Third, flame-annealed CuFe2O4 had a slightly smaller optical bandgap that extended

20

the range of light absorption. On top of that, the flame annealing method shortened the annealing

21

time from ~9 h to 16 min. These results showed that the CuFe2O4 photocathode could elicit a good

22

performance for PEC hydrogen production. Nevertheless, the stability of the CuFe2O4


13


Page 14 of 30

1

photocathode is still not adequate and it has to be further improved for practical PEC water

2

splitting.

3 4

Supporting Information

5

The Supporting Information is available free of charge on the ACS Publications website at DOI:

6

XXX

7

Sheet resistance of FTO, SEM images and XRD of Cu and Fe mixture before high temperature

8

annealing, J-V curves of CuFe2O4 with different equivalence ratio, Tauc plots of CuFe2O4, optical

9

properties of CuFe2O4 annealed by flame and furnace, UPS analysis for investigation to

10

each band position, film optimization according to the coating layers, J-V curves of CuFe2O4

11

with different scan rate, J-V curves and stability test (at 0.15 V vs. RHE) of CuFe2O4 (Flame)

12

and CuFe2O4 (Furnace) photocathodes in Ar purged 1 M Na2SO4 (pH ~5.5) electrolyte,

13

Cyclic voltammogram of CuFe2O4 in Ar purged 1M NaOH, SEM images of CuFe2O4 after

14

stability test, and XPS spectra of CuFe2O4 before and after stability test (PDF)

15 16

Author Contributions

17

The manuscript was written through contributions of all authors. All authors have given approval

18

to the final version of the manuscript. ‡These authors contributed equally.

19 20

ACKNOWLEDGMENTS


14

Page 15 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

X.L.Z acknowledges generous financial support from the Stanford Woods Institute for the

2

Environment and the Stanford Natural Gas Initiative for their generous support. This work was

3

also supported by Basic Science Research Program through the National Research Foundation of

4

Korea (No. 2017R1A2B3010927), Global Frontier R&D Program of the Center for Multiscale

5

Energy

6

(2016M3D1A1027664).

System

(2012M3A6A7054855),

and

Creative

Materials

Discovery

Program

7 8

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1 2

Figure 1. Schematic of CuFe2O4 on FTO annealed by furnace and flame. a Illustration

3

of the furnace annealing conditions and the dense film structure of the CuFe2O4 (Furnace)

4

on FTO. b Illustration of the flame annealing conditions and the porous film structure of

5

the flame-annealed CuFe2O4 on FTO. c, d SEM images of top (c) and cross (d) view of

6

CuFe2O4 on FTO (Furnace). e, f SEM images of top (e) and cross (f) view of CuFe2O4 on

7

FTO (Flame).

8


24

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1

2 3

Figure 2. X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS)

4

characterization on CuFe2O4 annealed by furnace and flame. a, XRD spectra of

5

CuFe2O4 film on FTO annealed by flame (top) and furnace (middle). Corresponding peaks

6

signals of CuFe2O4 (red) and FTO (black) from the JCPDS database are shown in the

7

bottom figure. b, c Cu K-edge (b) and Fe K-edge (c) X-ray absorption near-edge structure

8

(XANES) spectra of CuFe2O4 by furnace (black solid) and flame (red solid). d, e. Cu K-

9

edge (d) and Fe K-edge (e) extended X-ray absorption fine structure (EXAFS) functions of

10

CuFe2O4 by furnace (black solid) and flame (red solid).


25


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1

2 3

Figure 3. UV-vis spectra and energy band positions of CuFe2O4 annealed by furnace

4

and flame. a UV-vis spectra and the corresponding Tauc plots for indirect bandgaps shown

5

in insets. The corresponding Tauc plots for direct bandgaps are shown in Figure S4. b

6

Comparison of the energy band positions for CuFe2O4 annealed by furnace and flame in

7

contact with aqueous electrolyte at pH 0.

8


27


Page 28 of 30

1 2

Figure 4. Photoelectrochemical (PEC) characterizations on CuFe2O4 photocathode by

3

furnace and flame. a J-V curves of CuFe2O4 photocathode measured in Ar purged 1M

4

NaOH under simulated 1 sun illumination (100 mW/cm2) for hydrogen production. The

5

scan rate was 10 mV/s and the scan direction was in cathodic sweep. b Incident photon to

6

current efficiency (IPCE) of CuFe2O4 photocathode. c Electrochemical impedance spectra

7

(EIS) of CuFe2O4 photocathode at 0.4 V vs. RHE under simulated 1 sun illumination. d

8

Capacitive currents measured at 1.13 V vs. RHE as a function of scan rate for the relative

9

electrochemical active surface area (ECSA) measurements. In all figures, CuFe2O4

10

(Furnace) is black and CuFe2O4 (Flame) is red.

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Table 1. Summary of various Cu-based ternary oxide photocathodes. Jph @ 0.4VRHE (mA/cm2)

Jdark @ 0.4VRHE (mA/cm2)

Vonset (VRHE)

Electrolyte

Thickness

Stability****

Ref

CuFe2O4 (Flame)

-1.82

-0.35

1.05

Ar purged 1M NaOH

530nm

600s @ 0.15VRHE in Ar purged 1M Na2SO4

This work

CuFe2O4 (Furnace)

-0.52

-0.15

0.85

Ar purged 1M NaOH

450nm

450s @ 0.15VRHE in Ar purged 1M Na2SO4

This work

CuFe2O4

-0.04*

~0

1.1

N2 purged 0.1M NaOH

14μm

-

32

CuFe2O4

-0.4

-0.25

0.5

0.2M Na2SO4

60nm NPs

Over 240s @ -0.2VSCE

36

CuFeO2

-1.25

-0.25

0.9

O2 purged 1M NaOH

290nm

Over 600s*****

23

CuFeO2

-0.3

-0.1

0.81

Ar purged 1M NaOH

130nm

70s @ 0.6VRHE

24

CuFeO2

-1.3

-0.2

0.8

Ar purged 1M NaOH

6 times spin coating

20min w/ NiFe/RGO in Ar purged 1M NaOH

25

CuBi2O4

-0.8**

~0

1.25

1M NaOH

Over 1μm

Over 1h @ 0.6VRHE

26

Cu3VO4

-0.77***

-0.55

0.2VSCE

0.5M Na2SO4

1μm

250s w/ CuO NPs

28

CuNb2O6

-0.25

-0.1

0.78

CO2 purged 0.1M NaHCO3

20nm NPs

-

29

* at -0.2 V vs. Ag/AgCl ** at 0.6 V vs. RHE. *** at -0.18 V vs. SCE. **** time for Jph to be halved from its initial value. ***** with AZO/TiO2/Pt @ 0VRHE in Ar purged 0.5M Na2SO4.

Table 2. Resistances of each material measured by EIS shown in Figure 4c. Unit [Ω] R1 R2 R3 CuFe2O4 (Furnace)

156.8

19989

666.9

CuFe2O4 (Flame)

19.46

10962

6.037

18


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“For Table of Contents Use Only”

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TOC Image: Flame annealing process improves CuFe2O4 photocathode for better hydrogen production from sunlight and water through photoelectrochemical water splitting process

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Rapid Flame-annealed CuFe2O4 as Efficient Photocathode for

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