Article pubs.acs.org/JPCC
Role of WO3 Layers Electrodeposited on SnO2 Inverse Opal Skeletons in Photoelectrochemical Water Splitting Gun Yun,† Maheswari Balamurugan,‡ Hyun-Sik Kim,§ Kwang-Soon Ahn,∥ and Soon Hyung Kang*,⊥ †
School of Applied Chemical Engineering and ‡Department of Chemistry, Chonnam National University, Gwangju 500-757, S. Korea § Testing Analysis Center, Korea Institute of Ceramic Engineering and Technology, Jinju 660-031, S. Korea ∥ School of Chemical Engineering, Yeungnam University, Gyeongsan 712-749, S. Korea ⊥ Department of Chemistry Education and Optoelectronics Convergence Research Center, Chonnam National University, Gwangju 500-757, Republic of Korea ABSTRACT: Tin dioxide (SnO2) inverse opals (IOs) were developed by a spin-coating-assisted sol−gel process. The SnO2 IOs exhibited a pore size of ∼260 nm in the 370 nm sized polystyrene bead (PS) templates. Electrodes having a WO3 layer with a band gap (Eg) of 2.6 eV were electrodeposited on the SnO2 IOs under a constant potential (−0.47 VAg/AgCl), where the thickness of the WO3 layer depended on the applied charge amount for WO3 electrodeposition (200− 800 mC/cm2). As a control sample, a pure WO3 IO film with the same thickness of ∼3.1 μm was also prepared by electrodeposition. The pore diameter of the SnO2 IO structure declined noticeably as the deposited charge amount of the WO3 layer increased from 200 to 800 mC/cm2, leading to eventual coverage of the SnO2 IO structure with the WO3 (800 mC/cm2) layer. Moreover, X-ray diffraction analysis indicated a steady increase of the signal intensity of the monoclinic WO3 planes as the deposited charge amount of the WO3 layer increased, indirectly indicating an increased loading amount of the WO3 layer. However, the optimum photoelectrochemical (PEC) response was achieved with the SnO2/WO3 (600 mC/cm2) IO electrode, which exhibited the highest photocurrent density (Jsc) of 2.8 mA/cm2 under full-sun conditions and 0.91 mA/cm2 under visible light, indicating that the enhancement of the Jsc under visible light contributed significantly to the improvement of the total Jsc, compared with the values for the pure SnO2, SnO2/WO3 (200, 400, and 800 mC/cm2), and WO3 IO electrodes. Furthermore, the favorable cascading band alignment between the SnO2 and WO3 layers promoted rapid charge separation and charge transport through the conductive SnO2 IO skeleton. Therefore, the heterojunction, formed from the highly conductive SnO2 core layer and visible-light-absorbing WO3 shell layer, can boost the PEC activity by complementary combination of the unique advantages of the materials.
1. INTRODUCTION Increasing international awareness concerning carbon dioxide emissions and the dependence on declining fossil fuel resources that currently account for ∼85% of the global energy supply is a prime motivation for pursuing research in the field of ecofriendly and renewable energy systems.1 Solar energy has received significant attention as a clean and unlimited energy resource that is also abundant with a wide distribution. However, the technology to store solar energy in related devices or equipment (e.g., photovoltaic cells, solar heating panels, thermoelements, etc.) is still too limited to be practicable.2−4 Photoelectrochemical (PEC) water splitting to produce H2 and O2 from water under sunlight is the most promising approach to address this energy issue. Since Fujishima and Honda’s report on PEC water splitting using semiconducting TiO2 materials adapted for the photoelectrode,5 other semiconducting materials having strong resistance to photocorrosion, a proper band gap (Eg) of around 2.0 eV for efficient absorption of ultraviolet light, and a © 2016 American Chemical Society
band position adequate to straddle the energy position related to H2/O2 evolution have been intensively surveyed.6,7 However, an ideal combination of materials has not yet been discovered. Therefore, different approaches have been suggested to overcome the inherent disadvantages of the existing materials. The first involves modification of the band structure by homogeneous or inhomogeneous doping to induce narrowing of the band gap to absorb visible light.8,9 The second involves the introduction of multi-band-gap photoactive materials employing organic dyes or quantum dots.10,11 This can also improve visible-light absorption by semiconducting metal oxide (TiO2, SnO2, ZnO, WO3, etc.) photoelectrodes possessing large band gaps (>2.8 eV). However, these approaches have also revealed critical stability issues that hamper practical application. Furthermore, the large-scale Received: January 4, 2016 Revised: February 28, 2016 Published: March 2, 2016 5906
DOI: 10.1021/acs.jpcc.6b00044 J. Phys. Chem. C 2016, 120, 5906−5915
Article
The Journal of Physical Chemistry C
Scheme 1. Schematic Diagram Showing Fabrication of SnO2, SnO2/WO3, and WO3 3D IO Electrodes Using PS Bead Templates
on large-area conductive substrates, simple and cheap manufacturing, and easy controllability of the film thickness by monitoring the charge consumption during deposition. Previous literature19,20 surveys have indicated the possibility of preparation of WO3 by facile electrodeposition for application to PEC electrodes. Baeck et al. prepared nanoparticulate WO3 thin films by pulse electrodeposition, where the enhanced photocurrent was attributed to a decrease in the particle size, associated with an increase of the surface area.20 Kang and coworkers also reported the direct electrodeposition of WO3 on various metal substrates under a constant potential, where the thickness and morphology varied with the applied potential.21 The extremely thin WO3 films exhibited the best PEC performance due to the low electric conductivity and highly porous structure, which favored better electrolyte penetration. Moreover, the grain size of 40−80 nm obtained in this process is somewhat larger than that (5−20 nm) obtained with the sol− gel process, which possibly induced rapid charge transport. Three-dimensional (3D) IO structures can provide large surface areas, beneficial charge transport, as well as photonic crystal effects, where the strong light absorption in specific wavelength regions relies on the diameter of the IOs and the refractive index of the materials. For example, the enhanced PEC performance has been achieved with the hematite catalystcoated 3D nanocrystalline antimony-doped tin oxide IO nanoarchitecture, which is primarily due to more effective charge collection, decreased charge transfer resistance, and an increase in the number of available active surface sites at the semiconductor−liquid junction.22 Moreover, the pore diameter of the IOs can be easily controlled based on the diameter of the polystyrene beads to give various nanoarchitectural 3D porous structures. The formation of a heterojunction (i.e., core−shell structure) is proposed as a highly promising method to increase the photoabsorption and promote charge transport in WO3 films. The conducting core medium would act as a skeleton given that the thick WO3 film exhibited high charge transport resistance in the 3D nanoporous IO structure. Hence, a core/shell SnO2/ WO3 IO photoelectrode (i.e., heterojunction system) was developed herein by a spin-coating-assisted sol−gel process, followed by direct WO3 electrodeposition to cover the SnO2 IO film. The deposited charge amount was varied in the range of 200−800 mC/cm2 to adjust the thickness of the WO3 layer. As a control sample, pure SnO2 and WO3 IO photoelectrodes (i.e., single-junction system) with an identical thickness were also prepared to compare their PEC properties under comparable conditions.
implementation of such processes is difficult for most semiconductor materials. In alternative approaches, plasmonic assistance or photonic crystal effects have been applied to the design of photoelectrodes to impart the unique ability to strongly absorb light at specific wavelengths to the photoelectrodes.12 Moreover, from the charge transfer/transport point-of-view, cocatalysts have frequently been incorporated into the surface of photoelectrodes to enhance the charge transfer rate.13 This induces a significant shift of the photocurrent onset potential toward the flat band potential, which is ascribed to a decrease in the electron−hole recombination rate. Furthermore, core−shell-based photoelectrodes such as one-dimensional (1D) nanorods/wires or frameworks of conducting channels have been suggested to promote rapid electron−hole separation and charge transport. For example, a combination of the core n-type TiSi2 nanonet with shell hematite (α-Fe2O3) acts as an effective charge collection framework for photogenerated carriers in the TiSi2 core layer with improved visible-light absorption by the αFe2O3 shell layer.14 Further, SnO2/TiO2 core−shell inverse opal (IO) structures have been shown to enhance the photoabsorption of the photoactive TiO2 shell layer and promote charge transport in the conducting SnO2 core layer.15 These factors led to an improvement in the PEC performance. That is, SnO2 exhibits superior electron mobility (240 cm2/V· s), which is 2−3 orders of magnitude higher than that of TiO2, and the conduction band of SnO2 (−4.66 eV) is approximately 300−500 mV lower than that of TiO2.16 The type-II alignment between SnO2 and TiO2 employed herein may promote fast charge separation/transfer via the formation of core−shell SnO2−TiO2 materials. Furthermore, SnO2 has a small lattice mismatch with TiO2, leading to good structural compatibility. Nevertheless, the photoactive TiO2 has a large band gap of 3.2 eV in the anatase phase and 3.0 eV in the rutile phase and, hence, is not able to absorb visible light. Therefore, in order to increase the light absorption in the visible wavelength region, WO3 can be used as a proper photoelectrode in core−shell systems. WO3 possesses an optical band gap of 2.6−2.8 eV and is able to absorb a wider region of the visible spectrum than TiO2. The conduction band of WO3 is also appropriately positioned to transfer the photogenerated electrons to the substrate with a stable photoresponse in strongly acidic solutions with high resistance against corrosion and photocorrosion.17 Further, WO3 can be easily synthesized by various methods such as sol−gel synthesis, the hydrothermal method, electrochemical anodization, hot-wire chemical vapor deposition, arc discharge deposition, etc. Herein, electrodeposition was adopted because this methodology offers a number of attractive properties in comparison with other deposition techniques.18,19 These include the possibility of direct coating 5907
DOI: 10.1021/acs.jpcc.6b00044 J. Phys. Chem. C 2016, 120, 5906−5915
Article
The Journal of Physical Chemistry C
Figure 1. FE-SEM images of (a) 370 nm sized PS bead template, (b) SnO2 IO, (c) WO3 IO, (d) SnO2/WO3 (200 mC/cm2) IO, (e) SnO2/WO3 (400 mC/cm2) IO, (f) SnO2/WO3 (600 mC/cm2) IO, (g) SnO2/WO3 (800 mC/cm2) IO, and cross-sectional views of (h) SnO2/WO3 (600 mC/ cm2) IO and (i) WO3 IO films.
2. EXPERIMENTAL SECTION 2.1. Preparation of 3D SnO2, SnO2/WO3, and WO3 IO Film. Scheme 1 presents a simple representation of the fabrication process used for the SnO2/WO3 IO films. Typical SnO2 IOs were prepared by using modified methods from previous literature.15 Briefly, a cleaned fluorine-doped tin dioxide (FTO, Hartford Glass Corp.; sheet resistance: ∼ 15 Ω/sq.) substrate with dimensions of 1.25 × 1.25 cm2 was treated with a H2SO4:H2O2:H2O (3:1:1, volume ratio) solution for 30 min to improve the hydrophilicity. This is a crucial step influencing the uniformity and covering density of the multilayered polystyrene (PS) bead template on the FTO substrate. An aqueous solution containing 10 wt % PS beads with an average diameter of 370 (±10) nm was spread on the surface of the FTO substrate and subsequently spin-coated at 1000 rpm for 10 s. The substrate was then dried at 70 °C for a short period (20 min) on a hot plate to remove the remnant solvents or impurities and to enhance the structural stability of the film. This drying process promotes the attachment of neighboring PS beads. Subsequently, the precursor solution consisting of tin tetrachloride (SnCl4·5H2O, 30 μL, 0.75 M) and absolute ethanol as a solvent was dropped on the multilayered PS bead template on the spin-coating boat, followed by spinning the sample at 1000 rpm for 10 s. After drying the sample for a short time, high-temperature annealing was performed at 500 °C for 3 h under ambient air (ramping rate of 0.98 °C/min) to slowly remove the PS bead templates. In order to cover the SnO2 IO film with a WO3 layer, direct electrodeposition was carried out by following the preceding literature,21 where a three-electrode cell with saturated (sat.) Ag/AgCl (0.11 V vs NHE), platinum and FTO, or the SnO2 IO film as reference, counter, and working electrode, respectively, was used. The bath solution was prepared by dissolving the sodium tungstate dihydrate powder (Na2WO4·2H2O, 0.025 M)
and hydrogen peroxide (0.03 M) in distilled (DI) water. The pH value of the resulting solution was then adjusted to 0.8 by adding concentrated HNO3. The related reactions are as follows23 2WO4 2 − + 4H 2O2 → W2O112 − + 2OH− + 3H 2O
(1)
W2O112 − + 6H+ + 4e− → 2WO3 + O2 + 3H 2O
(2)
In strongly acidic solution, the dominant product of the bath solution is a peroxytungstate species (W2O112−), which is subsequently transformed to a WO3 phase by the reduction reaction due to the applied potential. Herein, the stirring rate of the bath solution was maintained at 500 rpm, and a constant applied potential (−0.47 VAg/AgCl) was used during the electrodeposition process (Autolab Potentiostat model AUTOLAB (PGSTAT302N.FRA2)). The amount of applied charge (200−800 mC/cm2) is a determining factor in controlling the thickness of the WO3 layer on the SnO2 IO film. WO3 deposition using a charge of 800 mC/cm2 required ∼200 s. The measured current density of most samples was around 3 mA/cm2 during the electrodeposition at room temperature (25 °C). As a control sample, a pristine WO3 IO film was also prepared using the same process, except for the deposition time (which was ∼30 min) to achieve a thickness of approximately 3 μm. After finishing the reaction, the as-deposited samples were rinsed with DI water and subsequently annealed at 350 °C for 30 min in air atmosphere to enhance the crystallinity. 2.2. Characterization. The morphological changes and crystalline properties of the SnO2, SnO2/WO3, and WO3 IO films were evaluated using a field-emission scanning electron microscope (FE-SEM, S4800, HITACHI Inc.) operating at 10 kV and 20 mA and a high power X-ray diffraction (HP-XRD, PANalytical, X′Pert PRO) instrument operating at 40 kV and 30 mA. Bright-field (BF) and high-resolution (HR) trans5908
DOI: 10.1021/acs.jpcc.6b00044 J. Phys. Chem. C 2016, 120, 5906−5915
Article
The Journal of Physical Chemistry C
Figure 2. Low-magnification TEM images of (a and a′) SnO2 IO, (b and b′) SnO2/WO3 (600 mC/cm2), and (c and c′) WO3 IO film. Highmagnification TEM images of (a″) SnO2 IO, (b″) SnO2/WO3 (600 mC/cm2), and (c″) WO3 IO film.
3. RESULTS AND DISCUSSION Figure 1 presents the FE-SEM images of the polystyrene bead template, SnO2, WO3, and SnO2/WO3 IOs with the crosssectional images. The multilayered PS template (Figure 1(a)) with a diameter of 370 (±5) nm facilitated the deposition of the photoactive materials (e.g., SnO2 and WO3) in a regular array of close-packed layered colloids, corresponding to a facecentered cubic structure (ABCABC···). The stacking of the PS microspheres in the FCC arrangement is the densest and most stable alignment. Figure 1(b) shows the well-defined microporous 3D SnO2 IOs with a pore diameter and wall thickness of 260 (±18) and 30 (±2) nm, respectively, revealing the honeycomb-shaped hexagonal close-packed array. Compared with the initial diameter of the PS beads, ∼30% pore shrinkage occurred in the IO structure. This could be due to the loss of liquid volume from the precursor and the densification of SnO2 during the phase transformation from the amorphous to the rutile phase. This shrinkage is similar to the previous case (25− 30%) recorded for the sol−gel-based structure.24 The wall thickness between each micropore was ∼20 (±20) nm with thicker walls at the contact area. In the case of the pure WO3 IO films (Figure 1(c)), although partial disruption of the IO structure occurred during the electrodeposition or post-thermal treatment, the IO structure was observed to be completely maintained, as confirmed by the cross-sectional image of the WO3 IO film (Figure 1(i)). The thin WO3 layer was coated on the SnO2 IO film by electrodeposition at a constant potential (−0.47 VAg/AgCl) by adjusting the charge from 200 (Figure 1(d)) to 800 mC/cm2 (Figure 1(g)). In the case of WO3 (200 mC/cm2, Figure 1(d)) on the SnO2 IO film (abbreviated as SnO2/WO3 (200 mC/cm2)), the IO structure was observed with a diameter and wall thickness of ∼220 (±12) and 56 (±8) nm, respectively, with maintaining the well-ordered hexagonal-
mission electron microscopy (TEM) were used to confirm the crystallinity and thickness of the SnO2, SnO2/WO3, and WO3 IO films using a JEOL-3010 instrument at an operating voltage of 300 kV. Elemental mapping images and energy-dispersive Xray spectra (EDS) were acquired using a Tecnai G2 F30 scanning transmission electron microscopy (STEM) instrument equipped with a high-angle annular dark-field (HAADF) unit. The transmittance (%) of the SnO2, SnO2/WO3, and WO3 IO films was measured using UV−vis spectroscopy (PerkinElmer LAMBDA-900 UV/VB/IR spectrometer) at room temperature. To assess the photoelectrochemical performance in the 3electrode configuration comprised of SnO2, SnO2/WO3, and WO3 IO films, Pt wire, and a sat. Ag/AgCl electrode as the working, counter, and reference electrodes, respectively, linear sweep voltammograms (LSVs) were acquired in a solution of Na2SO4 (0.5 M) under one-sun conditions (Xe lamp, 100 mW/ cm2, AM 1.5). The data were automatically recorded by a computer. Furthermore, chopped illumination voltammograms were acquired at a scan rate of 20 mV/s to analyze the dynamics of the photogenerated charges. The incident photon-to-current conversion efficiency (IPCE) was measured under a potential bias of 0.61 VNHE using a tungsten lamp source with an illumination intensity of ∼1 mW/ cm2. Electrochemical impedance spectroscopy (EIS, 10 mV perturbation in the frequency range of 105−0.1 Hz) was performed on an Autolab PGSTAT equipped with an FRA2 frequency response analyzer with Nova 1.7 controlled data acquisition under open-circuit voltage. The impedance data were fitted using the suggested equivalent circuit (shown in the inset of Figure 7(b)) by using the Zview program with relative errors below 5%. 5909
DOI: 10.1021/acs.jpcc.6b00044 J. Phys. Chem. C 2016, 120, 5906−5915
Article
The Journal of Physical Chemistry C
Figure 3. HAADF-STEM and elemental mapping images of (a, a′, a″, and a‴) SnO2 IO, (b, b′, b″ and b‴) SnO2/WO3 (600 mC/cm2), and (c, c′, c″, and c‴) WO3 IO film.
small crystallites of SnO2 with an average size of ∼5 nm. The neighboring pores were connected to each other by thin walls (Figure 2(a and a′)). To confirm the crystallinity of the SnO2 IO film (Figure 2(a″)), the TEM measurement was repeated in HR mode. Herein, the interplanar lattice spacing was determined to be 0.343 nm with clear lattice fringes that conform to the rutile (110) planes of the SnO2 crystal system. In the case of SnO2/WO3 (600 mC/cm2), a less porous IO structure was observed due to the thick WO3 coating layer composed of larger grains with a size of ∼73 (±26) nm, ascribed to the electrodeposition at −0.47 VAg/AgCl, in which the growth rate of the grains is quite fast and the porosity of the WO3 layer is relatively low. Additionally, the inner and outer surfaces of the SnO2 layer were perfectly covered with small WO3 grains (Figure 2(b and b′)). This coverage reflects the possibility of uniform coating of the WO3 layer throughout the macroporous 3D IO structure by direct electrodeposition. The crystalline properties of the SnO2/WO3 IO film were also investigated by measuring the interplanar lattice spacing by focusing on the surface region of the IO film. Here, two lattice spacings of 0.337 nm from the (110) plane of the rutile SnO2 phase and 0.262 nm from the (220) plane of the monoclinic WO3 phase were observed with clear lattice fringes (Figure 2(b″)). Fortunately, the SnO2 as well as the WO3 phases were concurrently detected, confirming the crystallinity of the core/ shell SnO2/WO3 IO films. Finally, the WO3 film also exhibited the IO structure, with more rectangular-shaped IO pores derived from the growth of larger grains during the electrodeposition process (Figure 2(c and c′)), and the interplanar lattice spacing of 0.263 nm could be indexed to the (220) plane of the monoclinic WO3 phase (Figure 2(c″)). Furthermore, to check the composition and elemental distribution of the SnO2, WO3/SnO2, and WO3 IO films, EDS mapping was performed by accumulation of the integrated intensity of the oxygen, tin, and tungsten signals as a function of the beam position by operating the TEM in scanning mode (STEM). The HAADF-STEM images are displayed in Figure 3.
close packing arrangement. On further deposition of the WO3 layer on the SnO2 IOs, the pore diameter rapidly decreased from 180 (±10) nm (400 mC/cm2, Figure 1(e)) with a wall thickness of 100 (±10) nm to 49 (±7) nm (600 mC/cm2, Figure 1(f)) with a wall thickness of 184 (±13) nm with maintaining the nanoporous IO structure, accompanied by complete coverage of the entire area of the SnO2 IO surface (800 mC/cm2, Figure 1(g)). Therefore, it can be inferred that the increase in the charge amount is not directly proportional to the increase in the wall thickness. After a certain level (>600 mC/cm2), the pore diameter of the IO structure decreased sharply with an increase of the applied charge amount. This is closely related to the current efficiency during the electrodeposition process; the rapid rise in the WO3 thickness in the initial stages is due to the highly conducting SnO2 medium. Subsequently, the thickness remains constant due to the formation of an amorphous WOx layer upon increasing the deposited charge amount.25 At the end of the process, the SnO2 IO film was completely covered via WO3 electrodeposition using an applied charge of 800 mC/cm2. To evaluate the uniformity of the electrodeposited WO3 layer and the thickness in the SnO2 IOs film, the cross-sectional images of the SnO2/ WO3 (600 mC/cm2) and WO3 IOs films were acquired, as presented in Figure 1(h) and 1(i). Both samples exhibited a similar thickness of 3.1 μm, illustrating uniform coating of the WO3 layer into the interior of the SnO2 IO films, where the pure WO3 IO films also exhibited the normal inverse opal skeleton throughout the entire thickness of the film. That is, the prepared samples with identical thickness could facilitate comparison between samples with no corrections required for variations in sample preparation. The morphology of the IO films was evaluated by TEM analysis, as presented in Figure 2. Herein, SnO2, SnO2/WO3, and WO3 IO films were prepared for comparison. In the case of the SnO2 IO film prepared by the sol−gel process along with the spin-coating method, the rough walls of the 3D ordered macroporous network consisted of an assembly of extremely 5910
DOI: 10.1021/acs.jpcc.6b00044 J. Phys. Chem. C 2016, 120, 5906−5915
Article
The Journal of Physical Chemistry C
m2) and approaches a value of ∼2.6 eV (800 mC/cm2), and the Bragg diffraction peaks are red-shifted based on charge loading from 200 to 800 mC/cm2. That is, relative to the PBG of SnO2/WO3 (200 mC/cm2) centered at 2.4 eV, the PBG is reduced to 2.2 eV (400 mC/cm2), then to 2.1 eV (600 mC/ cm2), and finally to 1.9 eV (800 mC/cm2). These results indicate the recognition of the combined properties of the SnO2 and WO3 layer by the incident light, rather than the recognition of each single component (i.e., SnO2 or WO3). As the thickness of the WO3 layer increases, the overall optical properties are governed by the WO3 layer. Accordingly, the total Eg of the SnO2/WO3 IO films approaches the Eg of WO3. Meanwhile, the intensity of the absorption band around the PBG steadily decreases with an increase of the applied charge for WO3, reflecting the weakness of the photonic crystal effects. These effects would result from the irregular ordering of the IO structure as well as the rapid decrease in the pore diameter. Further, upon increasing the charge amount of the WO3 material, the transmittance declined steadily, indicating a direct increase of the light absorbed by the WO3 layer. Therefore, the total light absorbed by the SnO2 skeleton as well as the WO3 layer increased with increasing deposited charge amount of the WO3 layer. To confirm the crystallinity of the SnO2, SnO2/WO3, and WO3 IO films, XRD measurements were performed as shown in Figure 5. All of the diffraction peaks in the patterns of the
These images demonstrate that all the elements were distributed very homogeneously on the IO skeleton with no apparent element separation or aggregation. However, the signal intensity was notably strong due to the high loading of each element at the central area which connected with the IO mesh. Moreover, in the case of the SnO2 IO film (Figure 3(a, a′, a″, a‴)), no color index from the W element was apparent, and conversely, no color index from the Sn element was found for the WO3 IO film due to the absence of each element in the respective samples. In contrast, the SnO2/WO3 IO film exhibited strong signals from all elements throughout the IO skeleton (Figure 3(b, b′, b″, b‴)). In particular, the W signal was stronger than the Sn signal, probably due to the thick WO3 coating layer. The STEM EDS spectra (not presented) show that all elements were detected in each sample, and the SnO2/ WO3 IO films exhibited strong Sn and W peaks. The average Sn:O:W atomic ratio determined from EDS analysis was 34.04:37.54:28.42. On the other hand, the pure WO3 IO film (Figure 3(c, c′, c″, c‴)) exhibits the strong signal from O and W elements, corresponding to the very weak Sn signal. To verify the enhancement of light harvesting by this IO structure, the percentage transmittance of the SnO2, SnO2/ WO3, and WO3 IO films was evaluated as shown in Figure 4.
Figure 4. UV−vis transmittance spectra of SnO2, SnO2/WO3, and WO3 IO films.
The peak positions and the width of the photonic band gap (PBG) are highly sensitive to the dielectric medium surrounding the materials. The optical transmittance of the SnO2 IO film decreased sharply at wavelengths of incident light shorter than 345 nm due to the wide band gap energy (Eg, 3.6 eV). Moreover, in the case of the pristine SnO2 IO film, a transmittance of >80% was achieved in the wavelength range of 600−800 nm, accompanied by a PBG of 2.56 eV, resulting from Bragg diffraction and slow light propagation at wavelengths close to 484.4 nm. The asymmetric absorption curve reflects the partial covering of the disordered IO film.26 Furthermore, the WO3 IO film exhibited low transmittance ( 800 mC/cm2 (2.2 mA/cm2) > 400 mC/cm2 (1.84 mA/cm2) > 200 mC/cm2 (1.27 mA/cm2) at 1.23 VNHE. In particular, the photocurrent density of the SnO2/WO3 (600 mC/cm2) IO electrode was ∼6-fold higher than that of the pure SnO2 or WO3 IO electrodes. This result emphasizes the positive role of the core−shell SnO2− WO3 heterojunction in enhancing the photoactivity of the SnO2 or WO3 IO electrode under illumination. Furthermore, the modification of the macroporous SnO2 IO structure with the WO3 overlayer resulted in an anodic shift of the onset potential by ca. 200 mV in comparison to that (0.134 V) observed for the bare SnO2 IO electrode. However, the photocurrent magnitude was extremely low and possibly lost in the background signal due to the high overpotential in this region. To investigate the prompt photoresponse of the electrodes, the LSVs under chopped photoillumination were collected and are displayed in Figure 6(b). The recorded photocurrent densities are consistent with the values obtained from the LSVs (Figure 6(a)), which indicates that the photocurrents are generally stable without photoinduced charging effects.29 To closely evaluate the photoresponse of the WO3 layer with an Eg of 2.6 eV under visible light, the LSVs were again measured using a 400 nm cutoff filter, as presented in Figure 6(c). Unexpectedly, a significant photocurrent (∼1 mA/cm2) from the WO3 layer was achieved under visible light, coinciding with the sequence obtained from the LSVs under full-sun illumination; the quantitative values are summarized in Table 1. It can be inferred that approximately one-third of the total Jsc was attributed to the Jsc obtained under visible light. In particular, the SnO2/WO3 IO electrodes exhibited predominantly visible-light-enhanced Jsc, compared with that of the pure SnO2 or WO3 IO electrode, showing that approximately
Figure 6. (a) LSV and (b) chopped LSV under full-sun on/off cycles and (c) LSV under visible light of SnO2, SnO2/WO3, and WO3 IO films in 0.5 M Na2SO4 electrolyte.
one-third of the total Jsc could be attributed to the Jsc obtained under visible light. This indicates that the combination of the core−shell structures in the SnO2 and WO3 materials can improve the photoresponse under full-sun light as well as visible light. In order to thoroughly examine the enhancement of the photocurrent densities as a function of the wavelength and probe the effect of the WO3 layer, the IPCE spectra of the SnO2, SnO2/WO3, and WO3 IO films were acquired at 0.61 VNHE, as shown in Figure 7(a). Generally, the IPCE can be described using the following equation30 IPCE = (1240 × J )/(λ × Plight)
where J is the measured photocurrent density (mA/cm2) at specific wavelengths; λ is the wavelength of incident light (nm); 5912
DOI: 10.1021/acs.jpcc.6b00044 J. Phys. Chem. C 2016, 120, 5906−5915
Article
The Journal of Physical Chemistry C
Table 1. Summary of Photocurrent Densities (mA/cm2) at 1.23 VNHE, with and without a Cutoff Filter (420 nm), and IPCE Values of SnO2, SnO2/WO3, and WO3 IO Films at the Wavelength of 377 nm in 0.1 M KOH Solution SnO2 IO SnO2/WO3 SnO2/WO3 SnO2/WO3 SnO2/WO3 WO3 IO
(200 (400 (600 (800
mC/cm2) mC/cm2) mC/cm2) mC/cm2)
Jsc (mA/cm2) @ 1.23 VNHE
Jsc (mA/cm2) @ 1.23 VNHE with a cutoff filter
IPCE (%) @ 377 nm
0.47 1.26 1.83 2.8 2.2 0.47
0.02 0.54 0.76 0.91 0.71 0.18
0.5 1.7 7.2 9.6 18 14
declined relative to the maximum IPCE achieved with the SnO2/WO3 (600 mC/cm2) IO film. To determine the source of this decline, EIS analyses were conducted in the frequency range from 10 kHz to 0.1 Hz using 0.5 M Na2SO4 electrolyte. The experimental data were analyzed using the suggested equivalent electrical circuit.31 As presented in Figure 7(b), Rs indicates the series resistance including the resistance of the FTO substrate, the resistance related to the ionic conductivity in the electrolyte, and the external contact resistance, and RSC at high frequency indicates the interfacial charge-transfer resistance in the semiconductor depletion layer; RCT is correlated to the semiconductor/electrolyte charge transfer resistance in the low-frequency region. Here, the two constant phase elements, CPESC and CPECT, represent the nonideal capacitance of the space charge in the nanoporous semiconductor and the nonideal capacitance of the Helmholtz layer in the nanoporous semiconductor/electrolyte interface, respectively. The fitted value of the different resistances is summarized in Table 2. Table 2. Quantitative Value of Ohmic Resistance (Rs) and Charge-Transfer Resistance (RSC and RCT) of SnO2, SnO2/ WO3, and WO3 IO Films, Fitted by Using the Equivalent Electrical Circuit SnO2 IO SnO2/WO3 (400 mC/cm2) SnO2/WO3 (600 mC/cm2) SnO2/WO3 (800 mC/cm2) WO3 IO
Figure 7. (a) IPCE spectra and (b) EIS spectra with the fitted curve using the equivalent electric circuit (shown in inset) for SnO2, SnO2/ WO3, and WO3 IO films in 0.5 M Na2SO4 electrolyte.
Rs (Ω)
RSC (Ω)
RCT (Ω)
87.46 87.83 85.62 93.28 86.57
13709 11140 8876 7300 25685
593.6 658.5 750.2 838.2 928.7
Figure 7(b) shows the Nyquist plots for the SnO2, SnO2/WO3, and WO3 IO films, which reveal similar Rs values of ∼86 Ω under the same working conditions. Comparison of the RSC and RCT of the samples shows that the pure WO3 IO film exhibits a substantially high RSC (25685 Ω) and RCT (929 Ω), indicating an unfavorable environment for the charge transfer process at the semiconductor depletion region as well as the semiconductor/electrolyte interface. Moreover, the RSC value steadily decreased, possibly due to fast charge separation and transport via the favorable band alignment between the SnO2 and WO3 materials. Conversely, the RCT value steadily increased on moving from SnO2 to the SnO2/WO3 (800 mC/cm2) IOs, which is ascribed to the surface trap/defect states or high charge transfer resistance for the favorable hole transfer process of the WO3 material itself, compared to SnO2. This confirms that the core−shell SnO2−WO3 IO electrodes have lower charge transfer resistance and better intrinsic conductivity than the pristine SnO2 or WO3 IO electrode. On the basis of the PEC data, the optimum performance could be achieved with the SnO2/WO3 (600 mC/cm2) IO films with tuning of the RSC and RCT values.
and Plight is the measured intensity of the incident light (mW/ cm2). No meaningful IPCE value was obtained for the pure SnO2 or WO3 IO films over the scanned wavelengths; only a minor photoresponse was observed in the range of 370−380 nm. On the other hand, the SnO2/WO3 IO electrodes exhibited significantly enhanced IPCE values from ∼460 nm, where the IPCE (18% at 377 nm) of the SnO2/WO3 (600 mC/cm2) IO film was the highest, followed by that of the SnO2/WO3 (800 mC/cm2, 14%), SnO2/WO3 (400 mC/cm2, 7.2%), and SnO2/ WO3 (200 mC/cm2, 1.7%) films in the stated sequence, which is consistent with the J−V curves. In particular, the IPCE signal derived from visible light in the vicinity of 400−450 nm was prominently enhanced due to the visible light-absorbing WO3 medium with an onset wavelength of 460 nm. On the basis of the transmittance spectra of all the samples, the WO3 material itself works as the light-absorbing medium, and upon increasing the loading amount of the WO3 layer, the quantitative light absorption also increases, along with efficient charge collection. However, in the case of the SnO2/WO3 (800 mC/cm2) IO film possessing the highest optical light absorption, the IPCE 5913
DOI: 10.1021/acs.jpcc.6b00044 J. Phys. Chem. C 2016, 120, 5906−5915
Article
The Journal of Physical Chemistry C
surface of the SnO2 IO film. The SnO2/WO3 (600 mC/cm2) IO film exhibited the best PEC performance in terms of the Jsc (2.8 and 0.91 mA/cm2) at 1.23 VNHE with and without a cutoff filter, indicating that the photoresponse from the WO3 layer influences the total PEC performance. The combined core− shell structure of the SnO2 and WO3 layers functions well in cascading band alignment. However, the SnO2/WO3 (800 mC/ cm2) IO film exhibited reduced PEC performance, which could be attributed to the thicker and more resistive WO3 layer, as well as the unfavorable electrolyte penetration into the SnO2 IO skeleton. Overall, most single component candidates for the PEC photoelectrode are inefficient with respect to the three requirements for highly efficient PEC water splitting: (1) a suitable band gap energy (
