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Revealing the Beneficial Effects of FeVO Nano-shell Layer on the BiVO Inverse Opal Core Layer for Photoelectrochemical Water Oxidation Maheswari Balamurugan, Gun Yun, Kwang-Soon Ahn, and Soon Hyung Kang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12516 • Publication Date (Web): 14 Mar 2017 Downloaded from http://pubs.acs.org on March 16, 2017

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The Journal of Physical Chemistry

Revealing the Beneficial Effects of FeVO4 Nano-shell Layer on the BiVO4 Inverse Opal Core Layer for Photoelectrochemical Water Oxidation Maheswari Balamurugana, Gun Yunb, Kwang-Soon Ahnc, Soon Hyung Kangd,* a

Department of Chemistry, Chonnam National University, Gwangju 500-757, South of Korea

b

Department of Advanced Chemical Materials Engineering, Chonnam National University, Gwangju 500-757, South of Korea

c

School of Chemical Engineering, Yeungnam University, Gyeongsan 712-749, South of Korea

d

Department of Chemistry Education and Optoelectronic Convergence Research Center, Chonnam National University,

Gwangju 500-757, South of Korea

* E-mail: [email protected] (S.H. Kang) Tel: +82-62-530-2

In this paper we developed a template-assisted 3D-ordered BiVO4 inverse opal(IOs) film by sandwichtype infiltration through self-assembled colloidal polystyrene (PS) opal beads with a diameter of 410 nm (± 20 nm) for photoelectrochemical hydrogen production. Herein, the ordered BiVO4 inverse opal structure possessed a pore diameter of ~340 nm and wall thickness of ~20 nm, providing a large surface area. Their photoelectrochemical behavior were assessed under 1 sun illumination (100mW/cm2 with AM.1.5 filter) in 0.5 M Na2SO4 (pH = 7) which displayed a photocurrent density (Jsc) of 0.8 mA/cm2 at 1.23 V vs a normal hydrogen electrode (NHE). Low photocurrents of BiVO4 IO photoelectrode are due to their limited photoelectrochemical ability to split water under light irradiation and their intrinsically low electronic conductivities. To overcome these problems, BiVO4 IOs film was modified to deposit a nanolayer of n-type FeVO4 having a narrow band gap (Eg = 2.06 eV). The bilayered BiVO4/FeVO4 coreshell film has the efficient photoelectrochemical (PEC) properties compared to unmodified BiVO4, showing a photocurrent density of 2.5 mA/cm2 at 1.23V vs NHE, probably resulted from a favorable charge-transfer/transport phenomenon under beneficial band alignment as well as visible light absorption by the FeVO4 layer.

Introduction Current research interests toward a cost-effective and clean, alternative renewable energy source have intensified because of the need to resolve modern global energy issues intimately related to environmental and atmospheric matters. A renewable energy source, based on solar energy, will perhaps be a practical

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alternative to replace conventional fuels because of the unlimited, clean and endless potentiality. Furthermore, hydrogen is an energy carrier that can be used in hydrogen fuel cells to produce the electricity and heat. Therefore, the solar energy driven hydrogen production would be attractive as the eco-friendly sustainable way. Among these tools, hydrogen is produced from water using solar light to directly dissociate water molecules into hydrogen and oxygen without any external bias in photoelectrochemical (PEC) water splitting, corresponding to not emit any environmental pollution source.1 This concept was first reported by Fujishima and Honda who investigated the photocleavage of water under light using a TiO2 photoelectrode.2 This reaction led to significant advancement over later on pursuing the new materials and structure as well as the new functional design of the device to attain the high efficient PEC performance. Herein, the discovery of the new materials has been regarded as one of the crucial steps in overcoming the current efficiency (< 10 %). There has been a tremendous concentration on the synthesis and development of new materials and their proper combinations by enhancing their strong points and improving their weak points. Currently, the well-investigated metal oxides (e.g., TiO2, SnO2, ZnO, WO3, etc.) have not only showed quite high photo corrosion resistance in the aqueous electrolyte but have also become good electron conductors, whilst most of them shows a high energetic band gap (Eg) in the vicinity of ~ 3 eV, revealing the low visible light capturing yields in situations where the visible light occupies approximately ~ 45 % of solar light.3-5 Therefore, the theoretically achievable PEC performance is still low (< 5 %) depending on the chosen materials. For example, the theoretical limitation of the PEC performance in the unmodified TiO2 exists just in about ~ 1 %6,7. In addition, in the case of WO3 having an energetic band gap of approximately 2.8 eV, the theoretically achievable PEC performance is only about 3 %.8,9 Accordingly, various approaches exploring low band gap materials have been examined to improve the PEC performance. As a representative, the BiVO4 (Eg = 2.4 eV) has attracted enormous attention because of the suitable band gap to more absorb the visible light, the proper band position to evolve the hydrogen and oxygen from water photoelectrolysis and the excellent stability of the liquid-solid interface.10,11 However, it has also suffered from an inefficient charge separation between the interfaces and charge transportation through the photoelectrode, still displaying the low photocurrent density in PEC working condition. Thus, the heterojunction composed of the core and shell layer showed good band alignment and could potentially cope with the weak points of BiVO4 material. As a recent representative example, the controlled growth of BiVO4 layer on the inverse opal WO3 photoanodes was fabricated by a templateassisted route to form the heterojunction of the WO3/BiVO4 layer.12 The confined morphology of photoelectrodes exhibited a photocurrent density of ~3.3 mA/cm2 at 1.23 V vs NHE, which leads to an efficient transfer of electrons to WO3 and it can suppress the charge recombination in BiVO4. Also, the 1D Co-Pi modified BiVO4/ZnO junction showing the cascading band alignment exhibited a photocurrent


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of ~3 mA/cm2, at the highest photocurrent density. This probably resulted from enhanced visible light absorption and fast electron transport along 1D ZnO nanorods.13 The significant advancement of inverse opal 3D structure facilitates photonic crystal effect to the compound as well as it has mainly offered defect free well-ordered morphology. IOs with high surface area macroporous array beneficial to reducing bulk recombination through shortening the diffusion length for charge carriers. Also, photonic crystals having unique qualities towards the optical properties that enabling novel opportunities and strategies to improve ability in photocatalyst .moreover, optical absorption can be enhanced due to the longer photon lifetime in the photonic crystal. Hence, rise to impede the generation of the electron-hole pair and better photocatalytic performance, when used in PEC application.14-17 In the structural aspect, the 3-dimensional (3D) nanostructure enables to supply the large electroactive sites as well as beneficial charge transfer/transport events. In this study, the multilayered PS beads with a size of 410 nm (± 20 nm) were assembled in a hexagonal close packing array and upon this template, the 3D inverse opals (IOs) nanostructure was developed to apply for to the photoelectrode for PEC applications. The regular and well-ordered IO films exhibited superior photonic crystal effects to enhance the light absorption at a specific wavelength, thereby advancing the PEC performance. In the configuration of core-shell IO heterojunction, the FeVO4 material has still not yet investigated, despite its fascinating properties. Herein, we first employed the FeVO4 material as a shell layer on the BiVO4 core layer. In general, the FeVO4 contains a low Eg of about 2.06 eV to promote more efficient visible light absorption by producing the ultrafast photogenerated charges, even though this material also suffers from the poor electrical conductivity.18 Also, this material can be used to build a proper band alignment between the BiVO4 and FeVO4, positioning the conduction band above -0.4 eV compared to that of BiVO4. On the basis of these properties, the use of a very thin FeVO4 layer as a light-absorbing medium on the BiVO4 film was explored to determine the effects of the BiVO4/FeVO4 IO films.

Experimental Section Development of BiVO4, FeVO4, and BiVO4/FeVO4 IOs films: The established synthesis procedure for the development of the BiVO4, FeVO4 and BiVO4/FeVO4 IOs film was described in detail in Figure 1. At first, the fluorine-doped tin oxide (FTO) (Hartford glass corp.; sheet resistance: 15 Ω/sq) substrate with a size of 1.5 × 1.5 cm2 was dipped in a solution of H2SO4:H2O2:H2O (3:1:1 volume ratio) for 30 min to increase the surface hydrophilicity and rinsed with acetone and deionized water. These steps induced to the crack-free samples during the template preparation. Then, a PS opal sphere with an average diameter of 410 nm (± 20 nm) was self-assembled on



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a cleaned FTO substrate via spin coating at 1000 rpm for 15 sec. Afterwards, the drying at 70 °C for about 30 min was progressed to eliminate the organic impurities. The BiVO4 IO photoelectrode was developed by a sandwich-like infiltration method. At first, 0.2 M

Figure 1. Schematic illustration to describe methods used to fabricate the BiVO4, FeVO4 and BiVO4/FeVO4 IO films using a polystyrene opal template.

ammonium metavanadate (NH4VO3) in 5 mL of deionized (DI) water was continuously stirred for about 10 min. To completely dissolve the NH4VO3 solution, 5 mL of the 60 % HNO3 solution was added to the above-mentioned solution and we observed a change in the solution color from white to a transparent light yellow. Then, 0.2 M of bismuth(III) nitrate (Bi(NO3)2.5H2O) was added in the vanadium solution and the mixture was continuously sonicated for 30 min to ensure complete mixing of all the ingredients. To control the viscosity and surface tension of the precursor solutions, 10 mL of absolute ethanol was incorporated to induce an easy penetration of the metal precursor into the PS opal template without any hindrance. Finally, the prepared precursor solution was infiltrated to the PS opal template at room temperature via a sandwich-like infiltration method, as checked in Figure 1. This method enabled us to supply a well-ordered and hexagonally structured BiVO4 IO film without any overcoating and agglomeration after the sample was kept in the vacuum oven at 50 °C for about an hour. The vacuum drying process served as a good contact between the metal species, and most of the impurities were also eliminated in this step. To improve the crystallinity of the sample and burn-out of the PS bead template used to make the IOs structure, high-temperature annealing at 500°C for 2 hrs in the air was done. The spin coating assisted FeVO4 layer were also deposited in the same way upon this film, as shown in Figure


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1. After the 0.04 µL infiltration of the precursor solution consisting of 0.2 M iron nitrate nonahydrate (Fe(NO3)3.9H2O) and 0.2 M NH4VO3 to the BiVO4 IO film, the samples were kept under air atmosphere at 500 °C about 2 hrs. As a control sample, the FeVO4 IOs film was prepared by the same method using a precursor solution composed of 0.2M Fe(NO3)3.9H2O and NH4VO3 at an equal ratio. Then, the precursor solution consisted of solvents such as 5ml of 60% HNO3; 5ml of deionized water, respectively. Subsequently, the precursor mixer was turned homogeneous solution after diluted with 10 ml of methanol under constant stirring at room temperature for 30 min.The prepared solution was spin-coated onto the PS opal template in the same condition, followed by the annealing at 550 °C about 2 hrs to obtain the wellordered uniform FeVO4 IO photoelectrodes.

Characterization. The morphology and their corresponding cross-sectional views of BiVO4, FeVO4, and BiVO4/FeVO4 IOs were checked by field emission scanning electron microscopy (FE-SEM, S4800, HITACHI Inc.) operating at 10 kV and 20 mA. In order to identify the crystallinity of the films, X-ray diffraction was measured using PANalytical X’Pert PRO instrument operating at 40 kV and 30 mA. The bright and dark field images of BiVO4, FeVO4, and BiVO4/FeVO4 IOs were confirmed by high-resolution transmission electron microscopy (HR-TEM) using a JEOL-3010 instrument at an operating voltage of 300 kV. Moreover, the crystallinity and elemental composition of the films are also confirmed by scanning transmission electron microscopy (STEM, Tecnai G2 F30). The transmittance was measured by ultraviolet-visible (UV-Vis)/diffuses reflectance spectroscopy (DRS) using a PerkinElmer LAMBDA-900 UV/VIS/IR spectrometer to analyze the optical properties of the BiVO4, FeVO4, and BiVO4/FeVO4 IOs in the wavelength range of 350‒750 nm. The PEC measurement was conducted in a three-electrode configuration under light illumination using a potentiostat (CHI Instruments, USA). BiVO4, FeVO4 and BiVO4/FeVO4 IOs were used as working electrodes with an active area of 0.2 cm2. The Pt and saturated (sat.) Ag/AgCl electrodes were used as counter and reference electrodes, respectively. The potential of homemade sat. Ag/AgCl electrode (0.11 V vs normal hydrogen electrode, NHE) was converted to be presented in this manuscript. An aqueous electrolyte containing 0.5 M Na2SO4 (pH 7) was used for the PEC test after nitrogen bubbling to remove the dissolved oxygen in the solution. A Xe lamp was used as a light source at 150 W with a light intensity of 100 mW/cm2 with AM 1.5 filter. The current-voltage (J-V) curves and the chopped light on/off J-V curves were performed at a scan rate of 20 mV/s during the potential sweep. Incident photo-to-current conversion efficiency (IPCE) was measured as a function of wavelength from 400 nm to 650 nm using an applied potential bias of 0.61 VNHE using a specially designed IPCE system for PEC water splitting. Herein, a 150 W Xe lamp was utilized to generate the monochromatic beam. The



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calibration was done employing an NREL-certified Si photodiode. Electrochemical impedance spectroscopy (EIS) of BiVO4, FeVO4 and BiVO4/FeVO4 IOs was measured in order to assess the cell resistance of each component as well as the interfacial issues in the PEC. Working condition was explored using a standard potentiostat (AUTOLAB/PGSTAT, 128N) equipped with an impedance-spectra analyzer (Nova). The measured frequency ranged from 0.1 Hz to 10 kHz at amplitude of ±10 mV.

Results and Discussion

Figure 2. Surface FE-SEM images of (a, b) BiVO4 IO, (d, e) FeVO4 IO, (g, h) BiVO4/FeVO4 IO and (c, f, i) the cross-sectional images of corresponding IO films.

Figure 2 represents the low and high magnifications observed using FE-SEM of the hexagonally aligned BiVO4, FeVO4, and BiVO4/FeVO4 IOs films. Surface and cross-sectional images of Figure 2a-c are from BiVO4 IOs, revealing the smooth, dense coverage and crack-free IOs structure. Compared with the diameter (410 nm) of the PS opal beads, the pore diameter of IOs is reduced to become approximately 340 nm and the pore shrinkage of approximately 17 % was achieved, probably resulting from the complete removal of organic impurities via annealing process under an air atmosphere and the


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coalescence of Bi3+ and V5+ species to form BiVO4 nanoparticles during the annealing process.19 Furthermore, the wall thickness and length of the prepared BiVO4 IO film are in the range of ~ 20 nm and 3.1 µm, respectively. Figure 2d-f displayed the FeVO4 IOs film prepared by the same method, showing a similar IOs nanostructure having a pore diameter, wall thickness and film thickness of 320 nm (± 20 nm), ~ 26 nm and about 2.9 µm, respectively. The pore shrinkage of about 22 % was attained due to the elimination of organic contaminants corresponding to the thicker wall thickness. However, it was dominantly observed that the overall IO skeleton was broken with not connecting each other, maybe influencing the charge transport event. Figure 2g-i describe the BiVO4/FeVO4 IOs images, disclosing well-ordered and hexagonal closed packing IO arrays without any nanoparticle agglomeration showing an almost identical thickness of ~3.1 µm by the coating of the very thin and uniform FeVO4. This indicates that the intimate contact between FeVO4 and BiVO4 layer by the infiltration method can lead to the rapid charge separation and transfer between the BiVO4 IO and FeVO4 shell layer. However, the conventional infiltration techniques such as spin coating, dip coating and drop casting methods were also investigated in order to fabricate the BiVO4 IOs film. All these methods showed the overlayer formation on the top of IO structure, which was confirmed by FE-SEM images. (as seen in the Figure S1.)

Figure 3. XRD patterns of (a) BiVO4 IO, (b) FeVO4 IO and (c) BiVO4/FeVO4 IO films.

To confirm the crystallinity of the BiVO4, FeVO4, and BiVO4/FeVO4 IO films, Figure 3 shows the XRD patterns of BiVO4, FeVO4, and BiVO4/FeVO4 IOs films with an FTO substrate. The XRD pattern of



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the pristine BiVO4 IO film (Figure 3a) after calcined at 500 °C under air ambient shows the well crystallized monoclinic structure corresponding to the (013), (004), (024) and (116) planes with lattice parameters of a = 5.195 , b = 5.093 and c = 11.704 Å, which are in excellent accordance with the standard data (JCPDS NO.: 83-1699).20 No other peaks for impurity and other binary or ternary compounds were found, thereby showing the development of a high-quality BiVO4 IO film. In the case of FeVO4 layer, all the peaks were indexed to the triclinic structure, corresponding to (012), (112), (212) and (023) planes with lattice parameters of a = 6.719, b = 8.060 and c = 9.254 Å, which is in agreement with the reference data (JCPDS NO.:71-1592).21 In the case of the BiVO4/FeVO4 IO film, both monoclinic and triclinic structures together appeared, to match with JCPDS NO.: 83-1699 and 71-1592 for BiVO4 and FeVO4, respectively. Except for the FTO substrate peaks, any impurities, and other phases were not detected, thereby proving the high purity of the prepared bilayered film.

Figure 4. HR-TEM images of (a, b) BiVO4 IO, (c, d) FeVO4 IO, (e, f) BiVO4/FeVO4 IO, (g, h) SAED pattern of BiVO4/FeVO4 IO films.

To definitively identify the morphology and the crystalline properties of the BiVO4, FeVO4, and BiVO4/FeVO4 3D-ordered IO films, HR-TEM was used for the measurements and images shown in Figure 4. These images clearly show the perfect structural ordering with periodical arrangements corresponding to the hexagonal close-packed arrays. The pore diameter of all IOs is distributed from 340 nm to 320 nm, dependent on the wall thickness. That is to say, the average wall thickness (~ 20 nm) of


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BiVO4 IO film (Figure 4a and 4b) is thinner than that (~26 nm) of FeVO4 IO film (Figure 4c and 4d), as already confirmed by FE-SEM image of Figure 2. In particular, the incorporation of FeVO4 nanolayer reduces the wall thickness of overall IO structure in the core-shell BiVO4/FeVO4 IO film (Figure 4e and 4f). Also, the interfacial region between the BiVO4 IO and FeVO4 nanolayer was surely observed to form the uniform and intimate interface. In the high-magnification view focusing on the surface region of BiVO4/FeVO4 IOs (Figure 4g), the interplanar lattice spacing of 0.31 nm and 0.32 nm with clear lattice fringes corresponded to the monoclinic (013) plane and triclinic (012) plane of BiVO4 and FeVO4 crystal system, respectively. Also, Figure 4h shows the selected area electron diffraction(SAED) pattern of BiVO4/FeVO4 IO films, disclosing the crystalline properties of (121) plane from BiVO4 and (112) planes from FeVO4 materials to confirm the well-crystallized BiVO4 and FeVO4 layer.

Figure 5. Elemental mapping images of (a) Bi, (b) Fe, (c) V, (d) O, and (e) the core-shell BiVO4/FeVO4 film and EDX spectra of (f) BiVO4, (g) FeVO4 and (h) the core-shell BiVO4/FeVO4 film.

To examine the element distribution in the core-shell BiVO4/FeVO4 IOs films, Figure 5 shows the elemental mapping images including the EDX spectra of BiVO4, FeVO4, and BiVO4/FeVO4 IOs films. It can be seen that all the elements are uniformly distributed over the entire area of IO films. In particular, the Bi elements (Figure 5a) exist in the central region of the IO skeleton, whilst the Fe element (Figure 5b) exists in focusing on the shell region, judged from the extent of colour intensity. The V and O elements (Figure 5c and d) were evenly distributed through the entire IO area. All elements from BiVO4/FeVO4



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IOs films (Figure 5e) was displayed together, showing a variety of the coloured IO films, indicative of the uniform position of all elements. Moreover, in order to survey the quantitative atomic composition of each component in all samples, the EDX spectra of BiVO4, FeVO4 and BiVO4/FeVO4 IOs films was measured and compared in Figure 5f-h and Table 1. Figure 5f-h shows the strong signals from Bi/V, Fe/V, and Bi/Fe/V elements, respectively, but it was noticeably observed that the Fe signal in the BiVO4/FeVO4 IOs is relatively weak due to the low loading of FeVO4 layer. Quantitative elemental compositions in Table 1 indicate the similar atomic ratios of Bi/V, Fe/V, and Bi/V elements from BiVO4, FeVO4, and BiVO4/FeVO4 IOs films containing the relatively low Fe content in the bilayered film. Thus, it can be seen that the FeVO4 layer on BiVO4 IO skeleton was uniformly coated with a shell thickness of about 5 nm. Table 1. Elemental composition of Bi, Fe, V and O obtained from EDX analysis of core-shell BiVO4/FeVO4 IO film. Samples

Bi(at.%)

Fe(at.%)

V(at.%)

O(at.%)

BiVO4 IO

30

0

21.9

47.4

FeVO4 IO

0

22.3

19.2

58.6

BiVO4/FeVO4 IO

35.5

23.5

31.9

10.1

Figure 6 stands for the optical transmittance spectra of BiVO4, FeVO4 and BiVO4/FeVO4 IO films fabricated on the FTO substrate. The BiVO4 IO film starts to absorb the light at the wavelength of 525 nm, while FeVO4 IO films start to absorb the meaningful light in the vicinity of 600 nm wavelength. Taking into account the optical properties of intrinsic BiVO4 and FeVO4 materials, the core-shell BiVO4/FeVO4


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Figure 6. Optical Transmittance spectra of BiVO4, FeVO4 and BiVO4/FeVO4 IO photoelectrodes.

IO films show the improved light absorption from the wavelength of 600 nm, steadily continuing the light absorption toward the shorter wavelength, and certainly influencing the photocurrent in the PEC cell. In addition the corresponding bilayer planar film absorbs the light wavelength at 550 nm. The exact direct optical bandgap(Eg) of all samples was evolved by extrapolating the linear portion of the (αhν)2 vs hν plot, as shown in the inset of Figure 5, where α is the absorption coefficient and hν is the incident photon energy. The estimated Eg of BiVO4, FeVO4, BiVO4/FeVO4 Planar and BiVO4/FeVO4 IO films are 2.35 eV, 2.05 eV,2.24 eV and 2.12 eV respectively. Herein, the Eg of core-shell BiVO4/FeVO4 IO films was shifted from 2.35 to 2.12 eV due to the hybridization of valence and conduction band orbitals of the BiVO4 film by FeVO4 layer, resulting in the extension of visible light absorption to the wavelength at 580 nm.22-25



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Figure 7. Core-level XPS spectra of (a) Bi, (b) Fe and (c) V and (d) O elements

To find the surface composition and oxidation state of the elements existing in the sample, core-level X-ray photoelectron spectroscopy (XPS) spectra of BiVO4, FeVO4, and BiVO4/FeVO4 IO films were measured and compared, as presented in Figure 7. The binding energy of all samples containing Bi, V and Fe was calibrated to the C 1s peak (280.60 eV). The wide scan spectrum confirms the presence of Bi, V and O atom in BiVO4 IO film, Fe, V and O atom in FeVO4 IO film and Bi, Fe, V and O atom in core-shell BiVO4/FeVO4 IO film (not shown here). The core-level XPS spectra of Bi in the BiVO4 and FeVO4 layer coated onto a BiVO4 layer are shown in Figure 7a. The Bi core level spectra exhibited the Bi 4f7/2 peaks located at 158.01 and 158.11 eV respectively, which is good agreement with the typical reported values for Bi2O3 nanoparticles, revealed the oxidation state of Bi to be 3+.26 In addition, the core-level peak of the Bi 4f7/2 in core-shell IO film slightly shifted towards higher binding energy value of about 0.10 eV compared to that of BiVO4 IO film. This shift ascribed to the strong interaction between BiVO4 and FeVO4 nanoparticles, which causes modified bonding state of Bi atom in the core-shell surface, resulted from the decreased oxygen vacancies due to the incorporation of FeVO4 into the BiVO4 lattice. Therefore, the binding energy of the Bi 4f7/2 is higher in core-shell IO film than that in BiVO4 film.27 As shown in Figure 7b, the position of core level of Fe 2p3/2 spectra appears at 709.93 eV, indicating the oxidation state


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of Fe is trivalence (3+).28 Compared to FeVO4 IO film, the Fe 2p core level peak of the core-shell IO film shift to the higher binding energy at 710.03 eV. Generally, when the FeVO4 and BiVO4 nanocrystals are tanged and attach continuously, their corresponding Fermi energy levels would be adjusted to be the equilibrium state. The electrons transferred from FeVO4 to BiVO4, while the hole transferred from BiVO4 to FeVO4 due to the carrier diffusion caused by the concentration gradient near the junction. The diffusion process leads to the higher binding energy of Fe 2p core level peak in core-shell BiVO4/FeVO4 IO films. Figure 7c shows the XPS core-level peaks of V 2p3/2 spectra of the prepared samples. In general, vanadium exists in various oxidation states from V3+ (515.15-515.85 eV) and V4+ (515.65-516.2 eV) to V5+ (516.9-517.2 eV), and usually, the broad asymmetrical V spectra can be deconvoluted into two components, assigned to V4+ and V5+, respectively. In the case of BiVO4 IO film, the V 2p1/2 core level peak also agrees well with 523.80 eV from V5+ and 516 eV from partially reduced V4+, respectively. An unambiguous assignment for the vanadium oxidation state is reconfirmed by the binding energy (BE) separation between O 1s and V 2p3/2. On the basis of the reported value of the BE differences showing 14.84 eV for V3+, 14.35 eV for V4+ and 12.8 eV for V5+, respectively, the measured BE separations of 14.23 eV and 12.6 eV are coming from the reduced phase (4+) and the oxidized phase (5+) in vanadium element, respectively. Moreover, V4+ species contains unpaired electrons(d1). When the electron vacancies at a core level are created by photoionization, there can be the coupling between the unpaired electrons in the core level and the unpaired electrons in the outer shell. The resultant spin-orbital splitting of V4+ species induces to the additional peak at 516 eV.29 In the case of V5+, there are no unpaired electrons(d0), thus showing no multiplet splitting. From the XPS spectra, it was clearly concluded that vanadium cations exist with two oxidation states on the oxide surface and the mixed valence of V is formed in the deficient environment of oxygen. That is, the loss of one oxygen from the lattice created one oxygen vacancy

••

( Vo

), leaving two electrons. This may be represented by the following reaction;

•• O o → ½ O 2 + Vo + 2e -

(3)

The reduction in vanadium caused from one oxygen deficiency may be explained as, V5+ (3p6 3do) + 2e- → 2V4+ (3p6 3d1) However, not all the V

5+

(4)

ions are affected by the process and the amount of reduced vanadium ions

depends on the extent of oxygen deficiencies, closely associated with the electrical conductivity of the IO films. Also, the XPS analysis also reveals that the surface atomic ratio of Bi, V, and Fe is 30%, 35%, and 25%, respectively, manifesting that this result is well correlated with the EDX results. Surface oxygen vacancies and oxygen bonded to the samples play an important role in the materials performance. Therefo re, the corresponding O1s core level peaks of all samples are enlarged in Figure 7(d). The high-resolution spectrum of O 1s is a convolution of two peaks for BiVO4 IO film. In order to increase the binding



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energies, BiVO4 O1s peaks are marked as 528.97 and 531.98 eV, respectively. The broad lowest binding energy peaks of BiVO4 and FeVO4 located at 528.97 eV correspond to the O2- ion in Bi2O3 and Fe2O3, res pectively. The higher binding energy peak of BiVO4 at 531.98 eV is related to the surface oxygen and OH- species which absorbed on the surface.30 Meanwhile, the careful observation shows that the O 1s cor e-level peak in the core-shell IO film shift toward low binding energy at 528.80 eV which is thanked to su rface absorbed oxygen and oxygen vacancies. Hence, it can be concluded that both surface oxygen and ox ygen vacancies enable to influence the PEC performance of the core-shell IO film. The photoelectrochemical ability of the BiVO4, FeVO4 and BiVO4/FeVO4 IO films were evaluated in the 0.5 Na2 SO4 solutions (pH = 7), as shown in Figure 8. Figure 8a illustrates the linear sweep voltammograms (LSVs) curves of all samples measured under AM 1.5 illumination condition. In the case of BiVO4 IO film, the onset potential near 0 VNHE was observed, similar to the preceding literature value.17 This indicates that the BiVO4 material itself well takes part in the PEC reaction. After the onset potential, the photocurrent is slowly increasing in the potential range of 0.25 VNHE – 1.0 VNHE, implying the highly resistive charge transfer/transport phenomenon, maybe attributed from the intrinsic BiVO4 property with low electrical conductivity. The maximum photocurrent of 0.8 mA/cm2 at 1.23 VNHE was achieved, revealing the pretty low value. On the other hand, FeVO4 IO film exhibits no meaningful photocurrent density through the scanned potential range, despite the great light absorption ability. Original FeVO4 may suffer from either the quite low electrical conductivity or suppressed hole transfer in the interface region of FeVO4 and electrolyte. On the contrary, the coating of FeVO4 nanolayer on the BiVO4 IO skeleton to make core-shell structure displayed the outstanding PEC activity with an enhanced photocurrent of 2.5 mA/cm2 at 1.23 VNHE. In particular, the onset potential is shifted from 0 VNHE to -0.5 VNHE, demonstrating that the thin FeVO4 layer well works in the PEC cell. Also, the photocurrent in the potential of 0.25 V – 1.0 VNHE is more sharply increased, exhibiting the improved photocurrent density. At the 1.23 VNHE, the photocurrent density of ~ 2.5 mA/cm2 was achieved, revealing three times higher than that of BiVO4 IO film. Herein, to more clearly assess the reason why the PEC activity is dramatically improved in terms of structure effect and the generation of oxygen vacancies will result in more photogenerated electrons, thereby has enhanced the PEC performance in the formation of the functional coreshell IOs structure31,32. The planar bilayered BiVO4/FeVO4 IO films were prepared in the same condition showing a photocurrent of 1.35


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Figure 8. (a) LSV curve, (b) chopped LSV curve, (c) IPCE spectra of BiVO4, FeVO4, and BiVO4/FeVO4 IO films. ( d) OCV decay curves as a function of time as soon as light shut off.

mA/cm2 at 1.23 VNHE. Accordingly, it was concluded that the 30 % enhancement is from the proper material combination and remnant 40 % improvement is from the structural effect. Therefore, the IOs structure composed of core-shell BiVO4/FeVO4 IO films leads to the synergetic effects in terms of the materials combination as well as the structure effect. To examine the prompt photoresponse of the IO films, the LSVs under the chopped light illumination were collected and are displayed in Figure 8b. The recorded photocurrent densities are agreeable with the values from the LSVs (Figure 8a), which dedicates that the photocurrents are generally stable without photoinduced charging effects through the scanned potential range.33 In addition, to figure out the overall enhancement of photocurrent density as a function of wavelength, Figure 8c shows the IPCE spectra of BiVO4, FeVO4, BiVO4/FeVO4 Planar and BiVO4/FeVO4 IO photoele ctrodes in the potential of 0.61 VNHE. In general, the IPCE value is described by the following equation to enable to compare with the quantitatively J-V curve.34



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IPCE = (1240 x J)/(λ x Plight) Where J is the measured photocurrent density(mA/cm2) at a particular wavelength; λ is the wavelength of incident light (nm), and Plight is an incident light (mW/cm2). Considering the pristine BiVO4 IO film as a skeleton, the detectable photoresponse starts at about 525 nm, giving an Eg of 2.35 eV. The maximum IPCE of BiVO4 IO film is about 8 % at the wavelength of 400 nm, identifying the photocurrent value achieved from the J-V curve. On the other hand, FeVO4 IO film exhibits no meaningful IPCE value through the scanned wavelength, consistent with the J-V curve. Hence, planar BiVO4/FeVO4 films, the exist onset wavelength at around 600 nm which promotes 25 % of IPCE at the wavelength of 450 nm with their corresponding J-V curve.In the case of BiVO4/FeVO4 IO films, the onset wavelength shifted to 650 nm contributed from the FeVO4 layer, and the maximum IPCE is reached to be approximately 75 % at the wavelength of 450 nm. In particular, the markedly enhanced IPCE values of about 30 % were attained at the visible wavelength from 450 nm to 625 nm. This definitely confirms that the overall enhanced photocurrent is mainly contributed from the thin FeVO4 shell layer and oxygen deficiencies are further enhanced by capturing the visible light. Moreover, the photovoltage decay measurement has been performed to judge the degree of trap or defect states in the photoelectrodes. Figure 8d showed the photovoltage-time (V-t) spectra of BiVO4, FeVO4, BiVO4/FeVO4 planar and BiVO4/FeVO4 IO films and the decay lifetimes of each V-t profiles by fitting to a biexponential function with two-time constants were calculated.35 y(t) = A0 + A1e-t/τ1 + A2e-t/τ2 τm = (τ1τ2)/ (τ1 + τ2)

(1) (2)

where τm is the harmonic mean of the lifetime, and the total half-life is log (2 x τm). The total half-life of BiVO4, FeVO4, BiVO4/FeVO4 planar and BiVO4/FeVO4 IO films were estimated to be 0.86s, 0.91s, 0.77s and, 0.69s respectively. This result indicates that the core-shell BiVO4/FeVO4 IO film shows shorter decay lifetime than that of BiVO4 BiVO4/FeVO4 planar and FeVO4 IO films. This was resulted from the modification of BiVO4 IO to form the core-shell structure, which favoured good electron transporting paths and suppressed recombination of photo-induced carriers through the well-ordered close-packed arrangement of IO films. Furthermore, in order to investigate the interfacial working condition between the FeVO4 shell layer and BiVO4 IO films, the EIS measurements under light illumination were carried out, as presented in Figure 9. Figure 9 compares the Nyquist plots for BiVO4, FeVO4 and BiVO4/FeVO4 IO films including the fitting data using the equivalent circuit in which RS dedicates the series resistance, including the FTO substrate and photoelectrode, the resistance associated with the ionic conductivity of the electrolyte and the external contact resistance and RCT is associated to the semiconductor/electrolyte charge transfer resistance at the low-frequency arc and RSC is related to the charge-transfer process in the semiconductor


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depletion layer at the high-frequency range.36 In our system, the similar RS with about 348 Ω was achieved in the similar working condition, while RCT of BiVO4/FeVO4 IO film at the high-frequency range is a quite low (2113 Ω), following the BiVO4 IOs (3048 Ω) and FeVO4 IOs (5525 Ω), revealing that the beneficial material combination to make the core-shell structure, as well as the surface passivation, can boost the favorable hole transfer event in the interfacial region between the photoelectrode and Table 2. Quantitative values obtained from the electrical equivalent circuit for BiVO4, FeVO4, and BiVO4 /FeVO4 IO films.

RS (Ω)

RCT (Ω)

RSC (Ω)

BiVO4 IO

348

3048

20957

FeVO4 IO

348

5525

66887

BiVO4/FeVO4 IO

340

2113

48

electrolyte. Furthermore, RSC of BiVO4/FeVO4 IO film at the low-frequency range is a greatly low (48 Ω), following BiVO4 IOs (20957 Ω) and FeVO4 IOs (66887 Ω), presenting the beneficial charge transfer in the interfacial region between the FeVO4 shell layer and BiVO4 core layer on account of the similar materials’ properties and the surface passivation of BiVO4 IO film by thin FeVO4 layer. On the contrary, the high charge transfer resistance in the depletion region was shown in the BiVO4 and FeVO4 IO films, resulted from a great deal of surface states (e.g., defect or trap sites) and original material properties are showing low electrical conductivity. In particular, it was noticed that RSC of FeVO4 IO film is above three times higher than that of BiVO4 IO film, representing the quite low electrical conductivity of FeVO4 IO film in the PEC working cell. However, the core-shell BiVO4/FeVO4 IO films show a pretty low RSC, probably resulted from the favorable material modification (e.g., enough oxygen vacancies or surface passivation) from FeVO4 layer. In summary, the core-shell BiVO 4 /FeVO 4 showed the dramatically enhanced PEC performance as compared with the B i V O 4 and FeVO4 IO films, attributed for the following reasons. (1) FeVO4 nanolayer can sufficiently absorb the visible light and the photogenerated charges quickly transfer to the BiVO4 core skeleton in the suitable band alignment between both materials. (2) Thin coating of FeVO4 nanolayer on the BiVO4 IO film can passivate the surface trap/defect states to suppress the charge recombination reaction, inducing to the favorable hole transfer into the electrolyte. (3) The deposition of FeVO4 Nano layer results in the improved electrical conductivity closely correlated to the increased oxygen vacancies of the core-shell IO film to deduce the dramatically decreased RSC.



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Figure 9. EIS spectra of BiVO4, FeVO4 and BiVO4/FeVO4 IO films with the fitted data using the electrical equivalent circuit shown in the inset.

Conclusion In this work, a BiVO4 IO film showing a regular close-packed 3D arrangement was prepared by a sandwich-like infiltration technique, followed by addition of a FeVO4 Nano layer to construct the coreshell BiVO4/FeVO4 IO films. The high-crystalline IO films with monoclinic BiVO4 IO and triclinic FeVO4 layers were identified by their XRD patterns. In addition their optical properties from the UV-Vis measurement were compared, revealing that the core-shell BiVO4/FeVO4 IO films exhibited band gap energies of about 2.12 eV. These values were lower than that of the BiVO4 IO film (2.35 eV), apparently indicating the significant visible light absorption from FeVO4 Nano layer. The PEC behavior of the coreshell BiVO4/FeVO4 IO films was tested in the electrolyte of 0.5 M Na2SO4 (pH = 7), showing the photocurrent density of 2.5 mA/cm2 at 1.23 VNHE under 1 sun illumination. This value is three times higher than that of the BiVO4 IO film (0.8 mA/cm2 at 1.23 VNHE), and no photo response was detected in the FeVO4 IO film throughout the scanned potential region. This result is mainly due to the beneficial visible light absorption from the FeVO4 layer, the fast charge separation, the increased conductive electron pathways from the BiVO4 IO skeleton and retarded charge recombination due to the surface passivation effect from thin FeVO4 layer, signifying the critical points that need to be considered in order to enhance the PEC performance in terms of the core-shell functional material combination and the 3D IO structure. Based on these results, we propose that the advanced material modification and combination will lead to improvement in the PEC performance.


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ASSOCIATED CONTENT Supporting information FE-SEM images of BiVO4 film prepared by the conventional infiltration technique. (PDF)

Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and future Planning (grant number 2014R1A2A2A04004950).

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TOC Graphic


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Revealing the Beneficial Effects of FeVO - ACS Publications

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