Interfacial growth of the optimal BiVO4 nanoparticles onto self-assembled WO3 nanoplates for efficient photoelectrochemical water splitting

Journal of Colloid and Interface Science 557 (2019) 478–487

Contents lists available at ScienceDirect

Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis

Interfacial growth of the optimal BiVO4 nanoparticles onto self-assembled WO3 nanoplates for efficient photoelectrochemical water splitting Vijay S. Kumbhar, Hyeonkwon Lee, Jaewon Lee, Kiyoung Lee ⇑ School of Nano & Materials Science and Engineering, Kyungpook National University, 2559 Gyeongsang-daero, Sangju, Gyeongbuk, South Korea Research Institute of Environmental Science & Technology, Kyungpook National University, 80 Daehak-ro, Buk-gu, Daegu, South Korea

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 9 May 2019 Revised 8 September 2019 Accepted 11 September 2019 Available online 11 September 2019 Keywords: Core-shell WO3/BiVO4 heterojunction Photoelectrochemical water oxidation Green hydrogen Low cost

a b s t r a c t Photoelectrochemical water splitting is the most efficient green engineering approach to convert the sun light into hydrogen energy. The formation of high surface area core-shell heterojunction with enhanced light-harvesting efficiency, elevated charge separation, and transport are key parameters in achieving the ideal water splitting performance of the photoanode. Herein, we demonstrate a first green engineering interfacial growth of the BiVO4 nanoparticles onto self-assembled WO3 nanoplates forming WO3/BiVO4 core-shell heterojunction for efficient PEC water splitting performance. The three different WO3 nanostructures (nanoplates, nanobricks, and stacked nanosheets) were self-assembled on fluorine doped tin oxide glass substrates via hydrothermal route at various pH (0.8–1.2) of the solutions. In comparison to nanobricks and stacked nanosheets, WO3 nanoplates displayed considerably elevated photocurrent density. Moreover, a simple and low cost green approach of modified chemical bath deposition technique was established for the optimal decoration of a BiVO4 nanoparticles on vertically aligned WO3 nanoplates. The boosted photoelectrochemical current density of 1.7 mA cm 2 at 1.23 V vs. reversible hydrogen electrode (RHE) under AM 1.5 G illumination was achieved for the WO3/BiVO4 heterojunction which can be attributed to a suitable band alignment for the efficient charge transfer from BiVO4 to WO3, increased light harvesting capability of outer BiVO4 layer, and high charge transfer efficiency of WO3 nanoplates. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction

⇑ Corresponding author. E-mail address: [email protected] (K. Lee). https://doi.org/10.1016/j.jcis.2019.09.037 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

Global disparity between energy generation and usage is increasing continuously, whereas fossil fuel reserves worldwide are nearing their depletion. Furthermore, use of fossil fuels release

V.S. Kumbhar et al. / Journal of Colloid and Interface Science 557 (2019) 478–487

greenhouse gases, which are harmful to the environment and, consequently, to the human health [1–3]. Therefore, it is of great importance to generate electricity using renewable energy sources, and the worldwide scientific community is thus highly receptive to clean-burning hydrogen fuel from artificial photosynthesis, also known as photocatalytic water oxidation [4,5]. In photoelectrochemical (PEC) cells used for water splitting, a semiconductor material in contact with the liquid electrolyte absorb photons, thereby creating electron/hole pairs that can be separated to decompose water into H2 and O2. It is favorable if the semiconductor under consideration has a more negative conduction band (CB) than the redox potential of H2 (0 V vs. NHE) and a more positive valence band (VB) than the redox potential of O2 (1.23 V vs. NHE) [6–9]. Since the first investigation of n-type TiO2 semiconductors for PEC water splitting, different nanostructured photoanodes comprising TiO2, Fe2O3, ZnO, WO3, BiVO4, etc. have been widely studied for the same purpose [10–12]. In general, the water oxidation photocurrent (JPEC) is determined by the ability of a photoanode to harvest incident light and its charge separation-transfer efficiencies and PEC stability. As no single semiconducting oxide can ideally fit these properties, their PEC performances are limited. Therefore, different approaches such as doping, co-catalyst coating, formation of high surface area nanostructures, heterojunction design, band gap minimization, and graphene-based catalysis continue to be developed to improve the overall photoanode performance [13– 16]. Heterojunction formation has gained particular attention because it allows for better PEC performance enhancement via the combination of two or more semiconducting materials with almost complementary photoactive properties. Of the studied materials, n-type WO3 is decidedly appealing as it can absorb approximately 12% of solar radiation (band gap 2.5– 2.8 eV), has a moderate diffusion length (150 nm), and can transport electrons at up to 12 cm2 V 1 s 1. However, it suffers from several drawbacks such as sluggish hole kinetics and slow charge transfer rates at the semiconductor-electrolyte interface [17–19]. On the other hand, the recently investigated BiVO4 has excellent visible light absorption properties (band gap 2.4 eV) but displays fast electron/hole recombination rates. The CB and VB edges of BiVO4 are more negative than those of WO3 [20]. Therefore, a WO3/BiVO4 heterojunction can inject photogenerated electrons to the WO3 CB, which can greatly reduce the recombination of electron/hole pairs and thus promote H2 generation. Moreover, the holes can be transferred at the BiVO4 surface for oxygen evolution reaction. Several efforts have been made to fabricate WO3 and BiVO4 via sol–gel, hydrothermal, spin coating etc. techniques [21–22]. But, owing to PEC properties of the WO3 and BiVO4 semiconductors, it is unlikely that these photoanodes alone could achieve an efficient photocurrent density. Therefore, the formation of WO3/BiVO4 core-shell heterojunction is highly important for photocurrent enhancement. Several articles in the literature that reports the fabrication of WO3/BiVO4 core-shell heterojunction via spin-coating, polymer template route, electrospinning etc. for PEC water oxidation [23– 26]. But these methodologies are inadequate to form efficient core-shell heterostructures. In particular, the spin-coating technique that has been mostly used to grow BiVO4 layer exhibit several disadvantages with respect to both environmental and technical viewpoint. More explicitly, it gives a non-uniform, lack of material efficiency, wastage of coating materials, non-uniform microstructure for large area coating etc. Moreover, it is complex, expensive, and require different organic solvents such as IPA, ethanol, methanol, butane, etc. which are toxic to the human health and non-ecofriendly. Therefore, it is of high importance to find

479

an alternative eco-friendly, simple, and low method that could deposit the BiVO4 material uniformly on the surface of WO3 nanostructures for PEC water splitting application. In this article, we report the hydrothermal synthesis of various WO3 nanostructures (nanoplates, stacked nanosheets with puffedlike structures, and nanobricks) onto FTO substrates. The thin layers of BiVO4 nanoparticles at varying thicknesses were deposited on the WO3 nanoplates by using a green manufacturing approach called modified chemical bath deposition (M-CBD) method. The M-CBD method offers several advantages, including simplicity, cost-effectiveness, large area deposition, avoidance of loss of precursors, and thereby is highly suitable for practical applications [27,28]. The vertically aligned 2D WO3/BiVO4 heterojunction offered a large light scattering area for incident visible light and a high surface contact area to achieve a prominent PEC cell performance. 2. Material and methods 2.1. Synthesis of WO3 nanostructures All the chemicals as received from the Sigma Aldrich except NH4VO4 from Dae Jung were used without any purification. The purity of the chemicals used were as follows: Na2WO4 (99%), oxalic acid (98%), Na2SO4 (99%), Bi(NO3)2 (98%), NH4VO3 (99%) etc. The commercially available FTO substrates of a fixed size (3 cm  1 cm) were cleaned using ethanol, acetone, and water in an ultrasonicator and subsequently dried under N2 gas. The WO3 nanostructures were prepared using a hydrothermal method. Typically, 0.16 g of Na2WO4 was dissolved into 20 mL of deionized (DI) water and pH was adjusted to 1.2 using HCl. Then, 20 mL of a 35 mM oxalc acid solution was added, followed by 0.10 g of Na2SO4 as a morphological agent. The resultant mixture was transferred into a PPL-lined autoclave (50 mL), and a FTO substrate was immersed in it. The system was maintained at 160 °C for 2 h and then allowed to naturally cool to room temperature. Finally, the WO3-coated FTO was cleaned using DI water and annealed at 500 °C for 2 h. Similar experiments were performed with precursor solutions at pH 1.0 and 0.8. Fig. S1 shows a photograph of the WO3-nanoplate-containing film. 2.2. Synthesis of WO3/BiVO4 heterojunctions and BiVO4 nanoparticles Thin layers of BiVO4 nanoparticles were deposited onto the asprepared WO3 nanoplates by using the M-CBD method. Briefly, 0.36 g of Bi(NO)3 was dissolved into a 30 mL solvent mixture of acetic acid and DI water (1:9). As an anionic precursor, 0.17 g of NH4VO3 was dissolved into 30 mL DI water at temperature of 60 °C and cooled to room temperature. In the M-CBD process, the WO3 coated FTO substrate was dipped into the as-prepared Bi- precursor for 20 s to adsorb the Bi3+ ions onto the nanoplates. It was rinsed into the 30 mL of acetic acid-DI water (1:9) solvent mixture for 5 s. Then, the FTO substrate was dipped into the anionic Vprecursor to adsorb VO34 ions, followed by rinsing into DI water for 5 s to remove any loosely adsorbed nanoparticles and/or ions completing a single cycle. Such cycles were repeated 10, 20, and 30 times to form BiVO4 layers of varied thicknesses. Finally, the as-prepared WO3/BiVO4 heterojunctions were air annealed at 500 °C for 2 h. Fig. S2 shows the photographs of the WO3/BiVO4 heterojunctions formed after 10, 20, and 30 M-CBD cycles, and Fig. S3 is a schematic of the M-CBD method. For comparision, the BiVO4 nanoparticles were separately formed on the surface of the bare FTO using aforementioned procedure.

480

V.S. Kumbhar et al. / Journal of Colloid and Interface Science 557 (2019) 478–487

2.3. Characterization The phase purity and crystal structures of the WO3, BiVO4, and WO3/BiVO4 samples were analyzed with X-ray diffraction (XRD) technique using a PANalytical X’pert Pro MPD X-ray diffractometer. The sample surface microstructures were characterized with field emission scanning microscope (FE-SEM, JEOL JSM-7500 F). The nanoparticle size, elemental composition, and oxidation states were analyzed with the high-resolution transmission electron microscopy (HRTEM, JEM-2100 F, JEOL) and X-ray photoelectron spectroscope (XPS, ESCALAB-MKII). 2.4. PEC measurements The PEC measurements of all the samples were performed in 0.5 M Na2SO4 electrolyte with a potentiostat (Bio-logic, SP150) using a standard three-electrode system consisting of WO3 or WO3/BiVO4 as the working electrode, silver/silver chloride (Ag/ AgCl) as the counter electrode, and a platinum mesh as the reference electrode. Any dissolved O2 was removed by bubbling N2 at a constant rate prior to PEC and H2 production measurements to avoid unnecessary oxygen reduction. A solar simulator with an intensity of 1 sunlight was used as a light source for the PEC measurements. Linear sweep voltammetry (LSV) was performed with increasing potential at a scan rate of 5 mV s 1. The current–potential (I-V) variations of the photocurrents were conducted at in the dark, light, and chopped light with on/off cycles of 10 s. Potential scanning was recorded for 2 h and electrochemical impedance measurements were conducted within the frequency range 50 kHz-100 mHz at 0.6 V vs. Ag/AgCl. The measured potentials were converted into reversible hydrogen electrode (RHE) using Eq. S1. 3. Results and discussion 3.1. Formation process and structural analysis Fig. 1 shows a schematic of the formation process of the WO3/ BiVO4 core-shell heterojunction. The acidic pH (0.8) of the solution decomposed WO24 ions that initiated WO3 nucleating sites forming hydrated WO3. With time, the nucleation sites grew vertically via the morphological agent forming nanoplates (Eqs. S2–S4). The reduced VO34 ions were reacted with the predeposited Bi3+ ions constituting BiVO4 nanoparticles (Eqs. S5 and S6). Fig. 2 display the XRD patterns of all the samples. The most prominent diffraction peaks [Fig. 2(a)] observed at 2h = 23.1°, 23.6°, 24.4°, 28.9°, 34.1°, and 41.8°, 49.8°, and 54.7° belonged to the (0 0 2), (0 2 0), (2 0 0), (1 1 2), (2 0 2), (2 2 2), (4 0 0), and (2 4 0) planes of the WO3, respectively. The WO3 nanostructures formed at pH = 1.2 exhibited triclinic (JCPDS # 020-1323) whereas those formed at pH = 1.0, 0.8 exhibited monoclinic (JCPDS # 072-0677) crystal structures, respectively [29–31]. Moreover, as shown in Fig. 2(b), the diffracted peaks at 2h = 15.1°, 18.6°, 18.9°, 28.8°,

30.5°, 34.4°, 39.7°, 42.4°, 45.5°, 46.7°, 53°, 53.3°, 55.8°, and 58.5° were assigned to the (0 2 0), (1 1 0), (0 1 1), (1 2 1), (0 4 0), (2 0 0), (2 1 1), (0 5 1), (2 3 1), (2 4 0), (2 2 2), (1 6 1), (2 5 1), and (1 2 3) planes of the monoclinic structured (JCPDS # 014-0688) BiVO4 nanoparticles, respectively [32,33], respectively. Furtheremore, Fig. 2(c) shows the XRD patterns of the WO3/BiVO4-10, 20, and 30 samples indicating the presence of both WO3 and BiVO4 phases without any impurity confirming the formation of core/ shell heterostructures. The peaks marked by the ‘#’ correspond to the FTO substrate peaks. Fig. S4 shows the survey spectrum, indicating the presence of all the elements within the WO3/BiVO4-20 sample. Fig. 3(a–d) displays the high-resolution XPS spectra of W, Bi, V, and O, respectively. The W 4d spectrum binding energies at 259.8 and 247.3 eV for W 4d3/2 and W 4d5/2, respectively, confirmed the W6+ state within WO3 [34]. The Bi 4f spectrum was deconvoluted into two peaks at 164.3 and 159.1 eV, corresponding to the Bi 4f5/2 and Bi 4f7/2 states of the Bi3+. Moreover, the V 2p spectrum showed V 2p1/2 and V2p3/2 peaks centered at binding energies of 523.3 and 515.8 eV, respectively, which are typical values for V5+ in BiVO4 [35,36]. Finally, the observed O 1 s peak at 529.1 eV was related to the metal–oxygen bonds within WO3 and BiVO4, and the peak centered at 530.3 eV was attributed to surface AOH groups [37], 3.2. Surface microstructural analysis Fig. 4 shows the FE-SEM images of the WO3 and WO3/BiVO4 samples and the insets show corresponding high resolution images. Fig. 4(a–c) displays WO3 stacked nanosheets, nanobricks, and nanoplates synthesized at pH = 1.2, 1.0, and 0.8, respectively. The stacked nanosheets with puffy-like structure are nonuniformly distributed whereas the interconnected nanobricks are uniformly covered on FTO substrate. The nanoplates are uniform, interconnected, and grown normal to the FTO surface. Moreover, Fig. 4(d–f) indicates WO3/BiVO4-10, 20, 30 heterojunctions obtained with increasing thickness of outer BiVO4 layer, respectively. The WO3/BiVO4-10 contains fewer BiVO4 nanoparticles deposited discretely on the WO3 nanoplates while WO3/BiVO4-20 shows the continuous and uniform distribution of BiVO4 nanoparticles. The WO3/BiVO4-30 displays the overgrowth of the BiVO4 nanoparticles forming thick layer on WO3 nanoplates. Fig. 5(a) indicates the FE-SEM imgae of the BiVO4 nanoparticles coated separately on the surface of the bare FTO substrate. As shown in Fig. 5 (b), the nanoparticles of an average size about 200 nm are uniformly distributed on the surface of the FTO substrate. Fig. 6 show TEM and HRTEM results of the WO3/BiVO4-20 coreshell heterojunction, respectively. The BiVO4 nanoparticles of sizes about 50–100 nm were uniformly decorated on the WO3 nanoplate. The cross-section image clearly indicated a separation between WO3 and BiVO4, where the thickness of the BiVO4 nanoparticle layer was approximately 50 nm. Each BiVO4 nanoparticle was composed of a large number of 5–10 nm size smaller nanoparticles as agreement with the FE-SEM images. The crys-

Fig. 1. Schematic illustration of the synthesis process for WO3/BiVO4 core-shell heterojunction.

V.S. Kumbhar et al. / Journal of Colloid and Interface Science 557 (2019) 478–487

481

Fig. 2. XRD patterns of the (a) WO3 nanostructures, (b) FTO, BiVO4 nanoparticles, and (b) WO3/BiVO4 core-shell heterojunctions.

Fig. 3. XPS spectra of the WO3/BiVO4-20 core-shell heterojunction.

talline nature of the BiVO4 nanoparticles was again confirmed from the HR-TEM image [Fig. 6(c)]. Additionally, the SAED pattern shown in Fig. 6(d) shows both fringes and bright spots belonging to WO3 and BiVO4 in agreement with the XRD analysis [38]. Moreover, Fig. 7 indicates the HADDF-STEM and the EDS mapping images of the WO3-BiVO4-20 elctrode confirmed the uniform spatial distributions of W, Bi, V, and O elements within WO3/BiVO4 heterojunction. Fig. S5(a and b) shows low and high-resolution TEM images of the WO3 nanoplates, and Fig. S6(a–d) displays the HADDF-STEM image and elemental mappings of the W and O elements within the WO3 nanoplate. Moreover, the chemical composition of the WO3/BiVO4-20 heterojunction was studied and the results are indicated in Fig. 8. Inset depcits the atomic and weight percentage of the W, Bi, V, and O elements within the WO3/BiVO420 heterojunction in accordance with the elemental mapping results.

3.3. PEC performance evaluation Fig. 9 shows the LSV, Mott-Schottky (M-S), and incident photonto-electron conversion efficiency (IPCE) plots of the WO3, BiVO4, and WO3/BiVO4 core-shell heterojunctions, respectively. As shown in Fig. 9(a), the onset potential was 0.6 V vs. RHE for all the WO3 electrodes. However, the photocurrent of the WO3 stacked nanosheets increased linearly for bias potentials less than 1.1 V vs. RHE whereas it extended up to 1.6 V vs. RHE for the WO3 nanobricks and nanoplates. The lower saturation potential of the stacked nanosheets was owned due to its smaller active area. On the other hands, the nanobricks and nanoplates provided a relatively higher number of active sites and hence had efficient charge carrier production for PEC applications with an applied bias potential [39]. With a further increase in the bias potential, the increase in the photocurrent was small and non-linear, which

482

V.S. Kumbhar et al. / Journal of Colloid and Interface Science 557 (2019) 478–487

Fig. 4. FE-SEM images of the (a–c) stacked nanosheets, nanobricks, and nanoplates of the WO3 and (d–f) WO3/BiVO4-10, 20, 30 core-shell heterojunctions, respectively.

Fig. 5. (a) Low and (b) high resolution FE-SEM images of the BiVO4 nanoparticles.

was attributed to an almost steady state of charge-carrier production and/or recombination. In case of BiVO4 nanoparticles, the onset potential was lower than that of WO3 nanostructures. It could be attributed to the more negative CB energy level than that of the WO3. But the lower current density of 0.25 mA cm 2 of the BiVO4 nanoparticles could be attributed to the poor charge transfer efficiency due to high recomibation rates and thickness of the BiVO4 layer on the FTO substrate. On the other hand, the WO3 nanoplates exhibited a higher photocurrent density of 0.8 mA cm 2 at 1.23 V vs. RHE than those nanobricks or stacked nanosheets. This can be attributed efficient photo-induced charge carrier transport nature of nanoplates resulting from the vertical growth of the thinner WO3 nanoplates. Fig. 9(b) shows the LSV curves of the WO3/BiVO4 heterojunctions, exhibiting a lower onset potential of 0.15 V vs. RHE than all the WO3 nanostructures. This indicated the compatibility of the formed WO3/BiVO4 heterojunction, which allowed for easy electron transfer from the CB of BiVO4 to WO3 for PEC water split-

ting. The WO3/BiVO4-10 and WO3/BiVO4-30 heterojunctions exhibited a photocurrent densities of 1.4 and 0.82 mA cm 2 at 1.23 vs. RHE, respectively. The decrease in the photocurrent density of the WO3/BiVO4-30 was attributed to the thicker layer of BiVO4 nanoparticles, where most of the photogenerated charge carriers undergo recombination before reaching the CB of the WO3 nanoplates. On the other hand, the WO3/BiVO4-20 heterojunction showed the highest photocurrent density of 1.7 mA cm 2 at 1.23 V vs. RHE, which was 2.1 times higher than that of the WO3 nanoplates. The enhanced photocurrent density was attributed to the improved visible light harvesting capability of the optimum BiVO4 layer combined with the superior transportation behavior of the WO3 nanoplates. Figs. S7 and S8 show the LSV curves of the WO3 nanostructures, BiVO4 nanoparticles, and WO3/BiVO4 heterojunctions under continuous light and dark conditions, respectively. The energy band diagrams of the WO3/BiVO4 heterojunctions can illustrate the origin of their enhanced PEC performances.

V.S. Kumbhar et al. / Journal of Colloid and Interface Science 557 (2019) 478–487

483

Fig. 6. (a–d) TEM, HR-TEM, and SAED pattern images of the WO3/BiVO4-20 core-shell heterojunction.

Fig. 7. (a–h) HADDF-STEM and EDS elemental mapping images of the WO3/BiVO4-20 core-shell heterojunction, respectively.

Therefore, the flat band potentials (Vfb) and donor densities of the WO3 and WO3/BiVO4 samples were determined from the M-S relation shown in Eq. S7 [40]. The value of Vfb can be determined from the X-intercept of a linear fit of the M-S plot (1/C2s vs. Va). Fig. 9(c and d) shows the M-S plots of the different WO3 nanostructures, BiVO4, and WO3/BiVO4 heterojunctions, respectively. The inset of Fig. 9(c) is the M-S plots of the WO3 nanobricks and

nanoplates. The WO3 nanoplates and nanobricks had smaller Vfb values (0.36 and 0.38 vs. RHE, respectively) than BiVO4 nanoparticles (0.42 V vs. RHE) and the stacked nanosheets (0.51 V vs. RHE). This confirmed that the WO3 nanoplates can easily transport photogeneraterd electrons to the current collector (FTO) and therefore are compatible for the formation of a heterojunction with BiVO4 nanoparticles [41]. These results are in analogous to the LSV curves

484

V.S. Kumbhar et al. / Journal of Colloid and Interface Science 557 (2019) 478–487

Fig. 8. Elemental composition analysis of the WO3/BiVO4-20 heterojunction.

described earlier. The WO3/BiVO4 heterojunctions showed relatively more negative Vfb values (approximately 0 V vs. RHE). The formation n-type based WO3/BiVO4 heterojunction enhanced the space-charge region that lead to the electron-hole separation efficiency. Moreover, WO3/BiVO4 facilitated the transfer of photogenerated electrons from the CB of BiVO4 to the CB of WO3 and subsequently to the current collector. Similarly, the holes from the VB of the WO3 to the VB of BiVO4 for the oxygen evolution. It is confirmed from the LSV curves as shown in Fig. 9(a and b), indicating that the WO3/BiVO4 heterojunction can photo-transfer electrons from BiVO4 to WO3 and reverse-hole transfer for H2 and O2 production, respectively. The variation in photocurrent density as a function of wavelength was measured to calculate the IPCEs and band gaps of the photoanodes. The IPCE determines the photoconversion efficiency of a photoanode at each wavelength of incident monochromatic light. In this case, the IPCE values of the WO3 nanostructures and WO3/BiVO4-based heterojunctions were calculated from the observed photocurrent with a bias potential of 1.23 V vs. RHE under illumination with monochromatic light between 350 and 600 nm in steps of 10 nm. Fig. 9 (e and f) shows the variation in IPCE with respect to incident wavelength and the variation in (IPCE
hm)1/2 vs. hm of the WO3 nanostructures and WO3/BiVO4based heterojunctions, respectively. The input power intensity spectrum and the photocurrent density spectra of the samples are shown in Figs. S9 and S10, respectively. The WO3/BiVO4 heterojunctions showed a clear IPCE onset wavelength at approximately 500 nm, exhibiting a band gap of 2.45 eV. The WO3 nanostructures showed higher band gap of 2.69 eV owing to their larger onset wavelengths of approximately 450 nm, than BiVO4 nanoparticles, in agreement with the literature. Among the samples, the WO3/BiVO4-20 heterojunction displayed the highest IPCE values within the most visible region. In particular, at a violet light wavelength of 410 nm, the WO3/BiVO4-20 heterojunction had an IPCE of 33.8%, which was 4.7 times higher than BiVO4 nanoparticles, 1.9 times higher than WO3 nanoplates (17.7%), and 2 (17.3%) and 1.4 (23.4%) times higher than that of the WO3/BiVO4-10 and WO3/ BiVO4-30 heterojunctions, respectively. The enhanced IPCE of the WO3/BiVO4-20 heterojunction arose from its improved light harvesting behavior and the high charge transfer efficiency of the optimally thin layered BiVO4. EIS was used to understand the kinetics of the charge transfer process at the photoanode-electrolyte interface. Fig. 10(a and b) shows the Nyquist plots of the WO3 nanostructures, BiVO4 nanoparticles, and WO3/BiVO4 core-shell heterojunctions, respec-

tively, all of which exhibited the distorted semicircles. The inset of Fig. 10(a) shows a magnified view of the Nyquist plot for WO3 nanobricks and nanoplates. The semicircular shape signified electron transfer and recombination processes within the photoanode and at the photoanode-electrolyte interface. The first intercept is indicative of the equivalent series resistance (Rs) resulting from the combined effect of the intrinsic sheet resistance of WO3, FTO resistance, contact resistance, and ionic resistance of the Na2SO4. Another intercept is indicative of the charge-transfer resistance (Rct) [42]. The lowest Rs was found for the WO3 nanoplates (38 X) because of their 2D vertical alignment, which allowed faster electron transfer. Conversely, the WO3 nanobricks and stacked nanosheets and BiVO4 nanoparticles with higher mass loading and/or non-uniform distribution of nanostructures caused more electron collisions, thus enhancing their intrinsic resistance to 48, 55, and 52 X, respectively. Moreover, as the WO3 stacked nanosheets and BiVO4 nanoparticles exhibited the least number of PEC processes due to non-uniform surface microstructure, it yielded the highest Rct of about 7900 and 8400 X, respectively. On the contrary, in case of WO3 nanobricks and nanoplates it was estimated to be 320 and 130 X, respectively indicating favaurable activity due to the 2-dimensional vertical nanostructures toward the PEC water splitting. The detailed Nyquist plot fitting analysis was performed for the WO3 nanoplates and optimized WO3/BiVO4-20 heterojunction. The capacitance (C), Rd (diffusion resistance), and the Warburg element (W) resulted from the accumulation of photo-generated holes and SO24 ions forming a double layer in the mid-frequency region and negligibly diffused SO24 ions at low frequencies. Fig. 10(b) indicates the Nyquist plots of the WO3/BiVO4 core-shell heterojunctions and the inset shows its equivalent circuit consisting of Rs, Rct, and Rd, a capacitor (C), and W parameters. The WO3/BiVO4-10 heterojunction absorbed limited incident light because of the limited mass loading of the BiVO4 nanoparticles, whereas the optimally grown WO3/BiVO4-20 heterojunction harvested most of the incident light with a greater charge transfer efficiency. Although the WO3/BiVO4-30 heterojunction had the highest absorbance, the non-uniform and overgrown BiVO4 layer led to an increased recombination rate owing to the low charge transfer efficiency of BiVO4. The non-uniform distribution of the electrolyte sulfate ions at the WO3/BiVO4-20-electrolyte interface can form a double layer with the photogenerated holes (h+), affording capacitor-like behavior. The outer sulfate ion layer in turn acts as an h+ scavenger and promotes O2 production at the BiVO4 surface [43]. Table S1 lists the parameters of the fitted equivalent circuit of the WO3 nanoplates and WO3/BiVO4 heterojunctions. Fig. 10(c) shows the variation in PEC current and with time for all the photoanodes. The photocurrent of each photoanode decreased with time, which was related to the increased recombination rate of the photo-generated charge carriers [44]. The stacked puffy-like WO3 nanosheets had the large variations, which arose from their non-uniform distribution. The BiVO4 nanoparticles showed least photocurrent density of 0.25 mA cm 2 which might be associated with the high recomibation rates of the charge carriers due to their higher nanoparticle size. The WO3 nanobricks and nanoplates had photocurrent densities of 0.33 and 0.44 mA cm 2 and reached 52% and 54% of their initial values, respectively. Likewise, the BiVO4 nanoparticles retained a photocurrent density of 45% after 2 h of continuous operation indicating the degradation and/or dissolution of the nanoparticles after continuous PEC operation. On the other side, the WO3/BiVO4-10, WO3/BiVO4-20, and WO3/BiVO4-30 heterojunctions exhibited current densities of 0.9, 1.5, and 1.1 mA cm 2 and showed stabilities of 55%, 98%, and 90%, respectively, after 2 h. Since WO3 exhibited sluggish kinetics, accumulation of holes at the electrodeelectrolyte interface owing to slow charge transfer rates led to ano-

V.S. Kumbhar et al. / Journal of Colloid and Interface Science 557 (2019) 478–487

485

Fig. 9. (a and b) Chopped LSV curves, (c and d) Mott-Schottky plot at 500 Hz, (e) variation of IPCE vs. wavelength (k), and (f) (IPCE
hm)1/2 vs hm plot of the WO3 nanostructures, BiVO4 nanoparticles and WO3/BiVO4 core-shell heterojunctions measured at 1.23 V vs. RHE in 0.5 M Na2SO4.

dic photocorrosion of the WO3 nanostructures. On the other hand, for the WO3/BiVO4 heterojunctions, the compatible band alignments allowed for easy transfer of holes in the WO3 VB to the BiVO4 VB. There, they could readily oxidize hydroxyl groups through the SO4 hole scavenger for O2 production and therefore afford a relatively stable PEC performance. Overall, the decreased photocurrent could be due to the photocorrosion and/or the dissolution of the material under continuous illumination. The H2 evolution performance of the as-prepared WO3 nanoplates, BiVO4 nanoparticles, and all WO3/BiVO4 heterojunctions under 2 h of solar light irradiation was analyzed, and the results are shown in Fig. 10(d). All the electrodes exhibited a linear change in the volume of the produced H2 with time. The BiVO4 nanoparticles showed a lower H2 production capacity of about 88 lL cm 2 after about 2 h of continuous exposure which might be associated with

the high recombination rates, low charge transport ability of the nanoparticles due to their non-uniform distribution of nanostructures on the surface of the FTO. The WO3 nanoplates, WO3/BiVO410, WO3/BiVO4-20, and WO3/BiVO4-30 heterojunctions produced total H2 volumes of 203, 180, 382, and 232 lL cm 2, respectively. Fig. 11 shows a schematic of the electron-hole transfer process under light illumination.

4. Conclusions In summary, for the first time, a novel and low-cost modified chemical route was established to coat an optimal layer of BiVO4 nanoparticles on the surface of the WO3 nanoplates forming WO3/BiVO4 core/shell heterojunction for photoelectrochemical

486

V.S. Kumbhar et al. / Journal of Colloid and Interface Science 557 (2019) 478–487

Fig. 10. (a and b) Nyquist plots, (c) PEC stabilities of the WO3 nanostructures, BiVO4 nanoparticles, WO3/BiVO4 core-shell heterojunctions and (d) H2 production performance of the WO3 nanoplates and WO3/BiVO4-20 core-shell heterojunction at 1.23 V vs RHE in 0.5 M Na2SO4 under solar simulated sun light illumination.

WO3/BiVO4 core/shell heterojunction can be ascribed to favourable interfacial charge transportation due to optimial BiVO4 nanoparticles and feasible band alignment. Therefore, newly investigated fabrication methodology of WO3/BiVO4 core-shell heterojunction can provide insights into efficient and low-cost photoelectrochemical water oxidation systems for commercial applications. Moreover, fabrication of two heterojunctions with wide band gap energy outer layred material is planned to achive enhaced photoelectrochemical performance. Acknowledgement

Fig. 11. Schematic illustration for photogeneration, separation, and transport of charge carriers in the WO3/BiVO4-20 heterojunction under visible light illumination.

water splitting application. The efficient photoelectron transfer from the conduction band of BiVO4 to that of WO3 was achived by tunning the BiVO4 layer thickness. In particular, optimally decorated BiVO4 nanoparticles on the WO3 nanoplates displayed efficient photoelectrochemical performance with a photocurrent density of about 1.7 mA cm 2 at 1.23 V vs. reversible hydrogen electrode, incident photon-to-electron conversion efficiency of 33.8% at 410 nm wavelength, and photoelectrochemical stability of 98% as well the H2 production of about 382 lL cm 2 for the 2 h continuous operation. These achivemnets are higher than that of obtained by non-ecofriendly spin coated WO3/BiVO4 heterjunctions (photocurrent desnity of 1 and 1.6 mA cm 2), respectively. [23,45]. The superior photoelectrochemical performance of the

This research was supported by the National Research Foundation of Korea Grant funded by the Korean Government (NRF2019R111A3A01041454) and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2018R1A6A1A03024962). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.09.037. References [1] M.E. Munawer, Human health and environmental impacts of coal combustion and post-combustion wastes, J. Sust. Min. 17 (2018) 87–96. [2] N. Butt, H.L. Beyer, J.R. Bennett, D. Biggs, R. Maggini, M. Mills, A.R. Renwick, L. M. Seabrook, H.P. Possingham, Biodiversity risks from fossil fuel extraction, Science 342 (2013) 425–426. [3] I. Capellán-Pérez, I. Arto, J.M. Polanco-Martínez, M. González-Eguino, M.B. Neumann, Likelihood of climate change pathways under uncertainty on fossil fuel resource availability, Energy Environ. Sci. 9 (2016) 2482–2496.

V.S. Kumbhar et al. / Journal of Colloid and Interface Science 557 (2019) 478–487 [4] Y. Xu, L.-Z. Zeng, Z.-C. Fu, C. Li, Z. Yang, Y. Chen, W.-F. Fu, Photocatalytic oxidation of arylalcohols to aromatic aldehydes promoted by hydroxyl radicals over a CoP/CdS photocatalyst in water with hydrogen evolution, Catal. Sci. Technol. 8 (2018) 2540–2545. [5] H. Tian, S. Wang, C. Zhang, J.-P. Veder, J. Pan, M. Jaroniec, L. Wang, J. Liu, Design and synthesis of porous ZnTiO3/TiO2 nanocages with heterojunctions for enhanced photocatalytic H2 production, J. Mater. Chem. A 5 (2017) 11615– 11622. [6] K.L. Materna, R.H. Crabtree, G.W. Brudvig, Anchoring groups for photocatalytic water oxidation on metal oxide surfaces, Chem. Soc. Rev. 46 (2017) 6099– 6110. [7] M.R. Nellist, F.A.L. Laskowski, F. Lin, T.J. Mills, S.W. Boettcher, Semiconductorelectrocatalyst interfaces: theory, experiment, and applications in photoelectrochemical water splitting, Acc. Chem. Res. 49 (2016) 733–740. [8] M. Yamamoto, K. Tanaka, Artificial molecular photosynthetic systems: towards efficient photoelectrochemical water oxidation, ChemPlusChem 81 (2016) 1028–1044. [9] S. Bai, X. Yang, C. Liu, X. Xiang, R. Luo, J. He, A. Chen, An integrating photoanode of WO3/Fe2O3 heterojunction decorated with NiFe-LDH to improve PEC water splitting efficiency, ACS Sust. Chem. Eng. 6 (2018) 12906–12913. [10] S. Miranda, A. Vilanova, T. Lopes, A. Mendes, TiO2-coated window for facilitated gas evolution in PEC solar water splitting, RSC Adv. 7 (2017) 29665–29671. [11] P. Sharma, J.-W. Jang, J.S. Lee, Key strategies to advance the photoelectrochemical water splitting performance of a-Fe2O3 photoanode, ChemCatChem 11 (2019) 157–179. [12] B. Zhang, Z. Wang, B. Huang, X. Zhang, X. Qin, H. Li, Y. Dai, Y. Li, Anisotropic photoelectrochemical (PEC) performances of ZnO single-crystalline photoanode: effect of internal electrostatic fields on the separation of photogenerated charge carriers during PEC water splitting, Chem. Mater. 28 (2016) 6613–6620. [13] Y. Boyjoo, M. Wang, V.K. Pareek, J. Liu, M. Jaroniec, Synthesis and applications of porous non-silica metal oxide submicrospheres, Chem. Soc. Rev. 45 (2016) 6013–6047. [14] S. Wang, T. He, J.-H. Yun, Y. Hu, M. Xiao, A. Du, L. Wang, New iron-cobalt oxide catalysts promoting BiVO4 films for photoelectrochemical water splitting, Adv. Funct. Mater. 28 (2018) 1802685. [15] A.L. Sangle, S. Singh, J. Jian, S.R. Bajpe, H. Wang, N. Khare, J.L. MacManusDriscoll, Very high surface area mesoporous thin films of SrTiO3 grown by pulsed laser deposition and application to efficient photoelectrochemical water splitting, Nano Lett. 16 (2016) 7338–7345. [16] K. Afroz, M. Moniruddin, N. Bakranov, S. Kudaibergenov, N. Nuraje, A heterojunction strategy to improve the visible light sensitive water splitting performance of photocatalytic materials, J. Mater. Chem. A 6 (2018) 21696– 21718. [17] A. Tacca, L. Meda, G. Marra, A. Savoini, S. Caramori, V. Cristino, C.A. Bignozzi, V. G. Pedro, P.P. Boix, S. Gimenez, J. Bisquert, Photoanodes based on nanostructured WO3 for water splitting, ChemPhysChem 13 (2012) 3025– 3034. [18] D. Chandra, K. Saito, T. Yui, M. Yagi, Tunable mesoporous structure of crystalline WO3 photoanode toward efficient visible-light-driven water oxidation, ACS Sust. Chem. Eng. 6 (2018) 16838–16846. [19] J. Wiktor, F. Ambrosio, A. Pasquarello, Role of polarons in water splitting: the case of BiVO4, ACS Energy Lett. 3 (2018) 1693–1697. [20] S. Hilliard, G. Baldinozzi, D. Friedrich, S. Kressman, H. Strub, V. Artero, C. Laberty-Robert, Mesoporous thin film WO3 photoanode for photoelectrochemical water splitting: a sol–gel dip coating approach, Sust. Energy Fuels 1 (2017) 145–153. [21] T. Jin, D. Xu, P. Diao, W.-P. He, H.-W. Wang, S.-Z. Liao, Tailored preparation of WO3 nano-grassblades on FTO substrate for photoelectrochemical water splitting, CrystEngComm 18 (2016) 6798–6808. [22] W. Guo, D. Tang, O. Mabayoje, B.R. Wygant, P. Xiao, Y. Zhang, C.B. Mullins, A simplified successive ionic layer adsorption and reaction (s-SILAR) method for growth of porous BiVO4 thin films for photoelectrochemical water oxidation, J. Electrochem. Soc. 164 (2017) H119–H125. [23] I. Grigioni, K.G. Stamplecoskie, E. Selli, P.V. Kamat, dynamics of photogenerated charge carriers in WO3/BiVO4 heterojunction photoanodes, J. Phys. Chem. C 119 (2015) 20792–20800. [24] Y. Zhou, L. Zhang, L. Lin, B.R. Wygant, Y. Liu, Y. Zhu, Y. Zheng, C.B. Mullins, Y. Zhao, X. Zhang, G. Yu, Highly efficient photoelectrochemical water splitting

[25]

[26]

[27] [28]

[29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

487

from hierarchical WO3/BiVO4 nanoporous sphere arrays, Nano Lett. 17 (2017) 8012–8017. S.Y. Chae, C.S. Lee, H. Jung, O.-S. Joo, B.K. Min, J.H. Kim, Y.J. Hwang, Insight into charge separation in WO3/BiVO4 heterojunction for solar water splitting, ACS Appl. Mater. Inter. 9 (2017) 19780–19790. Z. Zhang, B. Chen, M. Baek, K. Yong, multichannel charge transport of a BiVO4/ (RGO/WO3)/W18O49 three-storey anode for greatly enhanced photoelectrochemical efficiency, ACS Appl. Mater. Inter. 10 (2018) 6218–6227. S. Lindroos, M. Leskelä, Successive ionic layer adsorption and reaction (SILAR) and related sequential solution-phase deposition, Techniques (2008) 239–282. V.S. Kumbhar, Y.R. Lee, C.S. Ra, D. Tuma, B.-K. Min, J.-J. Shim, Modified chemical synthesis of MnS nanoclusters on nickel foam for high performance all-solidstate asymmetric supercapacitors, RSC Adv. 7 (2017) 16348–16359. B. Ma, J. Kim, T. Wang, J. Li, K. Lin, W. Liu, S. Woo, Improvement of photocatalytic oxidation activity on a WO3/TiO2 heterojunction composite photocatalyst with broad spectral response, RSC Adv. 5 (2015) 79815–79819. H. Kim, D. Choi, K. Kim, W. Chu, D.-M. Chun, C.S. Lee, Effect of particle size and amorphous phase on the electrochromic properties of kinetically deposited WO3 films, Sol. Energy Mater. Sol. Cells 177 (2018) 44–50. Y. Liu, J. Li, W. Li, Y. Yang, Y. Li, Q. Chen, Enhancement of the photoelectrochemical performance of WO3 vertical arrays film for solar water splitting by gadolinium doping, J. Phys. Chem. C 119 (2015) 14834– 14842. M. Yang, H. He, A. Liao, J. Huang, Y. Tang, J. Wang, G. Ke, F. Dong, L. Yang, L. Bian, Y. Zhou, Boosted water oxidation activity and kinetics on BiVO4 photoanodes with multihigh-index crystal facets, Inorg. Chem. 57 (2018) 15280–15288. M. Lamers, W. Li, M. Favaro, D.E. Starr, D. Friedrich, S. Lardhi, L. Cavallo, M. Harb, R. van de Krol, L.H. Wong, F.F. Abdi, Enhanced carrier transport and bandgap reduction in sulfur-modified BiVO4 photoanodes, Chem. Mater. 30 (2018) 8630–8638. D. Nagy, D. Nagy, I.M. Szilágyi, X. Fan, Effect of the morphology and phases of WO3 nanocrystals on their photocatalytic efficiency, RSC Adv. 6 (2016) 33743–33754. C. Sengottaiyan, N.A. Kalam, R. Jayavel, R.G. Shrestha, T. Subramani, S. Sankar, J. P. Hill, L.K. Shrestha, K. Ariga, BiVO4/RGO hybrid nanostructure for high performance electrochemical supercapacitor, J. Solid State Chem. 269 (2019) 409–418. P. Guan, H. Bai, F. Wang, H. Yu, D. Xu, W. Fan, W. Shi, In-situ anchoring Ag through organic polymer for configuring efficient plasmonic BiVO4 photoanode, Chem. Eng. J. 358 (2019) 658–665. V.S. Kumbhar, D.-H. Kim, Hierarchical coating of MnO2 nanosheets on ZnCo2O4 nanoflakes for enhanced electrochemical performance of asymmetric supercapacitors, Electrochim. Acta 271 (2018) 284–296. J. Song, M.J. Seo, T.H. Lee, Y.-R. Jo, J. Lee, T.L. Kim, S.-Y. Kim, S.-M. Kim, S.Y. Jeong, H. An, S. Kim, B.H. Lee, D. Lee, H.W. Jang, B.-J. Kim, S. Lee, Tailoring crystallographic orientations to substantially enhance charge separation efficiency in anisotropic BiVO4 photoanodes, ACS Catal. 8 (2018) 5952–5962. M.G. Lee, D.H. Kim, W. Sohn, C.W. Moon, H. Park, S. Lee, H.W. Jang, Conformally coated BiVO4 nanodots on porosity-controlled WO3 nanorods as highly efficient type II heterojunction photoanodes for water oxidation, Nano Energy 28 (2016) 250–260. Z. Zhou, S. Wu, L. Qin, L. Li, L. Li, X. Li, Modulating oxygen vacancies in Sndoped hematite film grown on silicon microwires for photoelectrochemical water oxidation, J. Mater. Chem. A 6 (2018) 15593–15602. C. Liu, J. Su, L. Guo, Comparison of sandwich and fingers-crossing type WO3/ BiVO4 multilayer heterojunctions for photoelectrochemical water oxidation, RSC Adv. 6 (2016) 27557–27565. V.S. Kumbhar, A.C. Lokhande, N.S. Gaikwad, C.D. Lokhande, Facile synthesis of Sm2S3 diffused nanoflakes and their pseudocapactive behavior, Ceram. Int. 41 (2015) 5758–5764. B. Lamm, B.J. Trzes´niewski, H. Döscher, W.A. Smith, M. Stefik, Emerging postsynthetic improvements of BiVO4 photoanodes for solar water splitting, ACS Energy Lett. 3 (2018) 112–124. M. Huang, Y.-L. Zhao, W. Xiong, S.V. Kershaw, Y. Yu, W. Li, T. Dudka, R.-Q. Zhang, Collaborative enhancement of photon harvesting and charge carrier dynamics in carbon nitride photoelectrode, Appl. Catal. B 237 (2018) 783–790. T. Zhang, J. Su, L. Guo, Morphology engineering of WO3/BiVO4 heterojunctions for efficient photocatalytic water oxidation, CrystEngComm 18 (2016) 8961– 8970.