Enhanced Photocurrent of Transparent CuFeO2 Photocathodes by

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Enhanced Photocurrent of Transparent CuFeO2 Photocathodes by Self-Light-Harvesting Architecture Yunjung Oh, Wooseok Yang, Jimin Kim, Sunho Jeong, and Jooho Moon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01208 • Publication Date (Web): 07 Apr 2017 Downloaded from http://pubs.acs.org on April 8, 2017

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ACS Applied Materials & Interfaces

Enhanced Photocurrent of Transparent CuFeO2 Photocathodes by Self-Light-Harvesting Architecture Yunjung Oh,† Wooseok Yang,† Jimin Kim,† Sunho Jeong,§ and Jooho Moon*,†

†

Department of Materials Science and Engineering, Yonsei University

50 Yonsei-ro Seodaemun-gu, Seoul 120-749, Republic of Korea §

Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT)

141 Kajeong-ro, Yuseong, Daejeon 305-600, Korea

KEYWORDS: CuFeO2, front photoelectrode, transparent photocathode, two-dimensional photonic crystal, self-light-harvesting

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ABSTRACT Efficient sunlight-driven water splitting devices can be achieved by using an optically and energetically well-matched pair of photoelectrodes in a tandem configuration. The key for maximizing the photoelectrochemical efficiency is the use of a highly transparent front photoelectrode with a band gap below 2.0 eV. Herein, we propose two-dimensional (2D) photonic crystal (PC) structures consisting of a CuFeO2-decorated microsphere monolayer, which serves as self-light-harvesting architectures allowing for amplified light absorption and high transparency. The photocurrent densities are evaluated for three CuFeO2 2D PC-based photoelectrodes with microspheres of different sizes. The optical analysis confirmed the presence of a photonic stop band (PSB) that generates slow light and at the same time amplifies the absorption of light. The 410 nm-sized CuFeO2-decorated microsphere 2D PC photocathode shows an exceptionally high visible light transmittance of 76.4% and a relatively high photocurrent of 0.2 mA cm-2 at 0.6 V vs. reversible hydrogen electrode (RHE). The effect of the microsphere size on the carrier collection efficiency was analyzed by in situ conductive atomic force microscopy observation under illumination. Our novel synthetic method to produce self-light-harvesting nanostructures provides a promising approach for the effective use of solar energy by highly transparent photocathodes.


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■ INTRODUCTION Photoelectrochemical (PEC) water splitting on semiconducting materials has drawn significant attention as a potentially scalable and economically feasible technology for converting solar energy into hydrogen.1,2 Recently, intensive efforts have been devoted to developing multiple absorbers-based tandem devices with the combined use of front and back photoelectrode to achieve higher solar-to-hydrogen (STH) efficiency through the utilization of a broad range solar spectrum.2-4 The achievement of high STH efficiencies in tandem devices is strongly contingent on the combination of front and back photoelectrodes in a stacked configuration.5-7 Based on the recent calculation by Lewis et al., a maximum STH conversion efficiency of 29.7% can be achieved when a front photoelectrode with a band gap of 1.6–1.8 eV is paired with a back photoelectrode with a 0.95–1.1 eV band gap.3 In typically stacked tandem devices where the back photoelectrode mainly harvests the long-wavelength component of solar spectrum, the front photoelectrode should be semi-transparent, while utilizing the short-wavelength component of solar radiation. To date, studies on devices with tandem configuration have mostly focused on the use of wide band gap (>2.0 eV) semiconductors (such as Fe2O3, BiVO4, WO3, and Cu2O) as front photoelectrodes, predominantly due to a sufficient light transmission through front electrodes.8-11 This results in an unavoidable limited utilization of the solar spectrum. In this context, it is necessary to develop a front photoelectrode with both high transparency and photocurrent density to achieve efficient tandem devices, while having a narrow band gap of about 1.6 eV for high STH conversion efficiency. CuFeO2 is the most attractive candidate for front photoelectrode due to its suitable band gap (Eg = 1.6 eV). CuFeO2 is also known to possess appropriate absorption properties, structural stability in water, and relatively high conductivity.12-14 Despite desirable optical 3 ACS Paragon Plus Environment


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properties and favorable stability, however, it is challenging to adopt thin-film CuFeO2 as front photoelectrode. This is attributed to the fact that all previously developed CuFeO2 photocathodes have a dark brown colour with a thickness over 100 nm to absorb a considerable amount of light. 13,14 This leads to an unsatisfactory light absorption by back photoelectrode due to opaque front photoelectrode. Therefore, to achieve transparent CuFeO2 photoelectrode showing comparable photocurrent with thin-film based counterpart, the innovative architecture is necessary to effectively harvest photons from the sun, while being transparent enough to allow the back photoelectrode to absorb the transmitted light. Herein, we present a two-dimensional photonic crystal (2D PC) composed of silica microspheres decorated with a thin CuFeO2 layer. This microsphere@CuFeO2-based 2D PC shows unique optical properties with both optimal light transparency and light-harvesting ability.15-20 The assembled monodisperse microsphere monolayer provides a sufficient degree of transparency owing to the presence of empty spaces between the hexagonally assembled microspheres.19 In addition, the assembled microsphere monolayer behaves as a 2D PC, yielding a photonic stop band (PSB) originated from the periodic arrangement of different refractive index materials. In a PC, the light undergoes strong coherent multiple scattering and travels with very low group velocity near the energy of PSB edges, referred to as slow light; such slow photons can significantly increase the effective optical path length of light. If the PSB is located at a higher energy than the band gap of the absorber material, an amplified light utilization/absorption would be allowed near the PSB. Therefore, our 2D PC architecture can play a role as a self-light-harvester, which provides a strategy for enhancing light absorption and photocurrent at the same time.18 In view of these considerations, our CuFeO2-decorated 2D PC photocathode showed an unprecedented simultaneous high transparency and efficient photoelectrochemical activity. The well-engineered CuFeO2 photocathode is nearly invisible while demonstrating an exceptionally high current density of 4 ACS Paragon Plus Environment

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0.2 mA cm-2 at 0.6 V vs. reversible hydrogen electrode (RHE) in oxygen-saturated 1 M NaOH solution under 1 sun illumination; to the best of our knowledge, this is a record-high value for CuFeO2 thin-film photocathodes.12 Additionally, we propose a design rule to maximize both transparency and photocurrent density in 2D PC-based photoelectrodes, developed by analyzing the optical and photoelectrical properties of the CuFeO2 photocathode.

■ RESULTS AND DISCUSSION As mentioned above, the PC exhibits a specific PSB energy because the refractive index varies as a spatially periodic function.17,20 Thus, the PSB energies are dependent on the diameter of the microspheres, allowing for different, unique absorption ranges. For a microsphere@CuFeO2 monolayer photoelectrode, the diffusion length of the major photogenerated carriers is determined by the size of the microsphere support. Therefore, in order to understand the influence of the carrier travel length on the collected photocurrents, we prepared three 2D PCs composed of microspheres of different sizes (with diameters of 250, 410, and 630 nm). Firstly, monodisperse amorphous SiO2 particles of different sizes were synthesized by the Stöber method, which is based on the hydrolysis of tetraethyl orthosilicate (TEOS) in an ethanol solution in the presence of ammonia and water.21-23 The as-synthesized silica particles showed the monodisperse spherical shape and smooth surface, with a mean diameter of 410 ± 11 nm, as illustrated in Figure 1b (see Supporting Information Figure S1 for silica particles with mean diameters of 250 ± 9 nm and 630 ± 13 nm). To prepare CuFeO2-silica core-shell particles, the surfaces of the silica microspheres were coated with the photoactive material CuFeO2 by the Pechini process, as schematically depicted in Figure 1a.24,25 The as-synthesized silica particles were suspended in a polymeric precursor solution consisting of citric acid, polyethylene glycol (PEG), and metal ions. The 5 ACS Paragon Plus Environment


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surface of the silica particles obtained by the Stöber process contains a large amount of hydroxyl and silanol groups, which can easily adsorb metal ions through physical and chemical interactions. During the Pechini process, citric acid chelates metal ions to form stable complexes, while the other free carboxylic acids that are not participating in the complexation further react with PEG to form a polyester; this results in a characteristic morphological structure in which the metallic species are incorporated uniformly within the polymeric shell on the surface of the silica microspheres. Next, the polymeric shell-silica core particles were separated from the precursor solution by centrifugation, followed by annealing at 800 °C to produce CuFeO2-decorated silica particles through the thermal decomposition of the organic components inside the polymeric shell. Microspheres@CuFeO2 (CFSs) with support sizes of 250, 410, and 630 nm were denoted as 250-CFS, 410-CFS, and 630-CFS, respectively. Through a polymeric resin-derived chemical methodology, a uniform surficial layer of CuFeO2 was obtained on the surface of the 410 nm-sized silica particles; however, some aggregated CuFeO2 particles were also observed, as shown in Figure 1c. After the CuFeO2 coating process, the surfaces of the decorated particles were not as smooth as those of bare silica particles, because of the presence of CuFeO2 nanoparticles with a size of 100 nm. The optical analysis revealed the self-lightharvesting ability of the 2D PCs, which interact with light at a specific wavelength close to D. This leads to a strong light absorption near the PSB wavelength, thus amplifying the photocurrent. Next, we analyzed the effect of the sphere size on the carrier collection efficiency by in situ C-AFM under 0.2 sun illumination, which revealed that the travel length of the majority carriers is limited to 863 nm. Thus, we expected that the 2D PC containing microspheres@CuFeO2 with a diameter of 550 nm would generate a higher photocurrent because the location of PSB would be further utilized more incoming photons as well as fully collect photogenerated carriers. This novel architecture with self-light-harvesting ability will 16 ACS Paragon Plus Environment

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overcome the contradiction between light absorption and transmission of the front and back electrodes in a tandem device configuration. Our innovative structural design opens new possibilities for the development of bi-functional semiconductors suitable as PEC front electrodes regardless of the band gap of materials, i.e., semiconductors with high transparency and amplified light absorption.

■ EXPERIMENTAL SECTION Synthesis of monodisperse silica particles Monodisperse submicron silica spheres in the size range of 250 to 630 nm were synthesized by the Stöber process, in which TEOS (99%, Sigma-Aldrich, St. Louis, MO) is hydrolysed in an ethanol solution containing water and ammonia.19-21 This method yielded a colloidal solution of monodisperse silica particles with sizes depending on the relative concentration of the reactants. For the synthesis of monodisperse silica particles, a mixture containing ethanol (Duksan Pure Chemicals Co., South Korea), distilled water, 28% NH4OH (Duksan Pure Chemicals Co.), and TEOS was stirred at room temperature for 5 h, resulting in the formation of a white silica colloidal suspension. The experimental conditions for obtaining differently sized silica particles are listed in Table 2. The silica particles were centrifugally separated from the suspension and washed with ethanol and distilled water. Decoration of silica particles with CuFeO2 CuFeO2-decorated silica particles were prepared by using the Pechini sol-gel process. Stoichiometric amounts of Cu(NO3)3 3H2O (99%, Sigma-Aldrich) and Fe(NO)3 9H2O (98%, Sigma-Aldrich) were dissolved in ethanol containing citric acid (99.5%, Sigma-Aldrich), which acts as a chelating agent for metal cations. The molar ratio of metal ions to citric acid 17 ACS Paragon Plus Environment


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was 1:2. Next, PEG (molecular weight = 10,000; Alfa Aesar, Heysham, Lancashire, UK) was added to a final concentration of 0.5 g/mL. The precursor solution was stirred for 1 h to form a sol, and silica particles were added under vigorous stirring for 5 h. The silica particles coated with the precursor solution were separated by centrifugation and immediately dried at 100 °C. The dried samples were annealed at 500 °C in a box furnace with a heating rate of 1 °C/min and held for 2 h. The process was repeated several times to sufficiently cover the silica sphere. To obtain the CuFeO2 phase, the preheated samples were annealed in flowing nitrogen gas in a tube furnace at 800 °C for 12 h at a heating rate of 5 °C/min. Fabrication of 2D opal assembled monolayer on FTO substrates In order to fabricate PEI-coated FTO glass substrates, the FTO substrates were immersed into a Piranha solution (28% H2SO4:H2O4 = 3:1) for 20 min and rinsed several times with large amounts of distilled water and ethanol. The hydrophilic treated substrates were then dried by a stream of nitrogen gas. The FTO glasses were spin-coated with PEI (branched, molecular weight = 25,000; Sigma-Aldrich) at 600 rpm for 25 s and then 1,000 rpm for 75 s. The concentration of the PEI solution in distilled water for dry assembly of differently sized microsphere beads was 0.5% for 250 nm, and 1.0% for 410 nm and 630 nm. Some microsphere beads were placed on the PEI-coated FTO substrates and were repeatedly and uniformly rubbed with a polydimethylsiloxane (PDMS, Sylgard 184, Sigma-Aldrich) slab to form a 2D opal assembly of microspheres. The substrates were baked at 500 °C for 2 h in a box furnace with a heating rate of 5 °C/min and slowly cooled.23 Characterizations The phase evolution of the CFS powders was determined by XRD (MiniFlex 600, Rigaku, Tokyo, Japan) with Cu Kα radiation (λ = 0.15406 nm) and XPS (K-alpha, equipped with monochromated Al Kα, Thermo). Prior to XPS measurement, the sputtering in the area of 2 × 2 mm2 was performed with an Ar+ ion gun (0.2 kV) to clean the surfaces of the samples. The 18 ACS Paragon Plus Environment

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microstructure and elemental mapping of silica and CuFeO2-decorated spheres were analyzed using a field emission scanning electron microscope (JSM-7800, JEOL, Japan) equipped with an energy-dispersive X-ray spectroscopy system. Compositional analysis was conducted on the polished cross-section of the particles, prepared by a cross-section polisher (IB-19510CP, JEOL, Japan). Transmission electron microscopy (TEM) images were obtained using a Tecnai G2 F20 electron microscope at an acceleration voltage of 200 kV. The optical transmittance and absorbance spectra were recorded at room temperature using a UV-vis spectrophotometer (V-670, JASCO, Easton, MD, USA) equipped with an integrating sphere. The spectral density of absorbed photons is calculated from the absorbance weighted by the AM 1.5G solar spectrum according to the following equation:29 Spectral density of absorbed photons =

λ hc

IሺλሻA(λ);

700 nm λ

Integration of spectral density of absorbed photons = ‫׬‬300 nm

hc

IሺλሻA(λ) dλ ;

where A(λ) and I(λ) are the absorbance (%) and intensity of light at each wavelength, respectively. The enhancement factors were obtained by dividing the integral of absorbed photons in 410-CFS-AML or 610-CFS-AML over the integral of absorbed photons in 250CFS-AML. The photoelectrodes were fabricated by securing a copper wire on the exposed electric conductive parts of the FTO with a silver paste, and the unnecessary parts of the electrode and the wiring parts were then covered with epoxy resin. PEC measurements were conducted in a three-electrode configuration using a potentiostat (SI 1287, Solartron, Shildon, County Durham, UK). A coiled platinum counter electrode and a silver chloride electrode (E = +0.197 V vs. RHE) were used as counter and reference electrodes, respectively. The PEC properties were measured under simulated sunlight illumination (AM 1.5G, Newport Corp., Rochester, NY, USA) in a 1.0 M NaOH solution (pH 14). Linear sweep voltammetry was 19 ACS Paragon Plus Environment


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conducted using an Ag/AgCl reference electrode with a 5 mV s-1 sweep rateMott-Schottky analysis was performed in the dark at a frequency of 5 kHz in the Ar-purged 1 M NaOH solution using a potentiostat. IPCE spectra of CFS-AML photocathodes were measured under irradiation of monochromatic light at +0.6 V vs. RHE (CIMPS-QE/IPCE, Zahner, Germany). The photon flux was determined using a calibrated Si photodiode. The C-AFM (SPA 400, Seiko Instruments Inc., Chiba, Japan) measurements were performed using a gold-coated cantilever (SI-DF3-A) to obtain the topographic images and current maps of the photoelectrodes.

ASSOCIATED CONTENT Supporting Information. Figure S1-S12, Table S1, and Supporting Note : SEM images of 250 nm and 630 nm sized silica; SEM and TEM images of 250-CFS and 630-CFS; compositional mapping of cross sectioned 410-CFS by EDX analysis; extrapolation of CFS bandgap; photographic images and diffusive transmittance of photoelectrodes; Mott-Schottky plots of photoelectrodes; calculation of monolayer surface area; line profile from in-situ C-AFM. The following files are available free of charge. Additional figures (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID 20 ACS Paragon Plus Environment

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Jooho Moon: 0000-0002-6685-9999 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (Grant No. 2012R1A3A2026417).

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Figure 1. (a) Schematic illustration of the synthesis of silica microspheres decorated with CuFeO2 by the Pechini method. SEM images of (b) 410 nm-sized bare silica particles and (c) 410-CFS particles. (d) High-magnification image showing the surface structure of 410-CFS. Yellow arrows indicate aggregated CuFeO2 particles.


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Figure 2. (a) X-ray diffraction profile of the 410-CFS powder. (b-e) HRTEM images of 410CFS revealing the surface features of the decorated microspheres.



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Figure 3. (a) Schematic procedure for the fabrication of the assembled monolayer of the microspheres@CuFeO2 (i.e., CFS-AML)-based photocathode by a rubbing method. SEM images of CFS-AML photocathodes on FTO with different silica core sizes: (b) 250-CFSAML, (c) 410-CFS-AML, and (d) 630-CFS-AML.


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a Argon purged electrolyte

b Oxygen saturated electrolyte

-2

20 µA cm-2

0.6

0.7

0.8

0.9

Potential (E vs. RHE)

630-CFS-AML 410-CFS-AML 250-CFS-AML

Current density (µ A cm-2)


Current density (µ A cm )

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


1.0

50 µA cm-2

0.6

0.7

0.8

0.9

1.0

Potential (E vs. RHE)

Figure 4. Linear sweep voltammogram of CFS-AML photocathode in 1 M NaOH under 1 sun illumination. The photocathodes were immersed into different electrolytes: (a) Ar-purged electrolyte and (b) oxygen-saturated electrolyte. The sweep rate was 5 mV s-1 and the scan was in the cathodic direction.



a

b 100

c 80

30

Absorbance (%)

60

40


20

0 300

400

500

600

700

800

20

10

0 300

400

Wavelength (nm)

500

600

700

70

Spectral density of 16 -2 -1 absorbed photons (10 m s )


80

Angle resolved transmittance (%)

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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800

Wavelength (nm)

60


50 40 30 20 10 0 300

400

500

600

700

800

Wavelength (nm)

Figure 5. (a) Angle-resolved transmittance spectra of CFS-AML on FTO. (b) Absorbance spectra of the CFS-AML photocathodes. (c) Spectral density of absorbed photons per unit area and time, weighted by the AM 1.5G solar spectrum. A bare FTO substrate was used to obtain the baseline. The yellow and blue coloured regions represent the PSB positions of 410CFS-AML and 630-CFS-AML, respectively.


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Figure 6. Topographic images and corresponding in situ current maps in a 0.8 × 0.8 µm2 area for (a) 250-CFS-AML, (b) 410-CFS-AML, and (c) 630-CFS-AML photocathodes on FTO substrates. The left panel of each figure represents the schematic of the C-AFM mapping on each AML. Each C-AFM mapping was conducted under 0.2 sun illumination with an external bias of -0.1 V.



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Table

1.

Photocurrent

density

and

AVT

for

the

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assembled

monolayer

of

microspheres@CuFeO2-based photocathodes at each electrolyte condition

Sample name 250-CFS-AML 410-CFS-AML 630-CFS-AML a

Photocurrent density (µA cm-2)a Ar-purged O2-saturated electrolyte electrolyte 2.7 80.3 70.7 200.3 47.9 159.8

AVT (%)b 79.2 76.4 69.6

Photocurrent density was measured under 1 sun chopped illumination, 1M NaOH electrolyte, and 0.6 V vs. RHE.

b

AVT values are the arithmetic means of diffusive transmittance (Figure S9, Supporting Information) in the visible light range 400–700 nm.


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Table 2. Synthesis conditions of monodisperse silica particles with different size Size

TEOS

Ammonia

Water

Ethanol

250 nm

12 mL

4 mL

50 mL

140 mL

410 nm

5.5 mL

24 mL

13 mL

350 mL

630 nm

8 mL

37.5 mL

1.04 mL

140 mL



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Table of contents


Enhanced Photocurrent of Transparent CuFeO2 Photocathodes by

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