Letter Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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Plasmon-Enhanced Layered Double Hydroxide Composite BiVO4 Photoanodes: Layering-Dependent Modulation of the WaterOxidation Reaction Ruirui Wang,† Lan Luo,† Xiaolin Zhu,‡ Yong Yan,*,‡ Bing Zhang,§ Xu Xiang,*,† and Jing He†
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†
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China ‡ Department of Chemistry and Environmental Science, New Jersey Institute of Technology, University Heights, Newark, New Jersey 07102, United States § School of Chemical Engineering, Zhengzhou University, Zhengzhou 450001, People’s Republic of China S Supporting Information *
ABSTRACT: Photo-electrochemical (PEC) generation of hydrogen from sunlight and water is a promising pathway to carbon-neutral energy. We describe a highly efficient three-layer photoanode, which consists of plasmonic Au-SiO2 core−shell nanoparticles (Au@SiO2 NPs), a layered double hydroxide (LDH), and semiconducting BiVO4. To understand the spatial dependence of water oxidation by the composite photoanode, we examined two configurations, LDH/Au@SiO2/BiVO4 (photoanode I) and Au@SiO2/ LDH/BiVO4 (photoanode II), which differed in the order in which the LDH nanosheets and Au@SiO2 layers were grown on BiVO4. In a PEC water-splitting cell under back-side illumination, at E0′(H2O/O2) = 1.23 V versus RHE, the photocurrent density of photoanode I reached 1.92 mA·cm−2, showing a 52% increase compared to that of photoanode II; the efficiency toward water oxidation of photoanode I was 69%, 1.3 times higher than that of photoanode II. This is because light absorption by Au@SiO2 in photoanode I excites a strong localized surface plasmon resonance that promotes the charge separation in BiVO4. However, under front-side illumination, photoanode II performed better than photoanode I because less light is transmitted to the Au@SiO2 layer in photoanode I. Our findings reveal that the BiVO4 layer contributed predominantly to charge separation, while the LDH layer served to catalyze water oxidation. The spatial dependence of the components of composite photoanodes provides a route to rational design of plasmonic PEC devices for solar energy conversion applications. KEYWORDS: BiVO4 photoanode, water-oxidation reaction, plasmon enhancement, charge separation, modulation
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INTRODUCTION Generation of hydrogen gas via photo-electrochemical (PEC) water splitting provides a route to convert and store solar energy as a chemical fuel and is a prominent topic in renewable energy research.1−3 Photoelectrodes are the core components of PEC water-splitting systems, and efficient systems require photoelectrodes that provide (1) sufficient absorption of light and efficient separation of the electron−hole pairs induced by light absorption, (2) suitable alignment between the energy bands of the photoelectrode and the potentials required for oxygen evolution and hydrogen evolution, (3) efficient transport of charge carriers across the interface with water, and (4) high chemical and photo-electrochemical stability.4,5 Several strategies have been developed to meet these requirements,6−12 such as constructing p−n heterojunctions to improve charge-carrier collection, combining a narrow band gap semiconductor with a wide band gap one to better match the absorption profile of the photoelectrode with the solar spectrum,13−15 or modifying photoanodes with water-oxidation catalysts (WOCs) to increase the rate of hole transport © XXXX American Chemical Society
across the interface between the photoanode and the electrolyte.16 Nanoparticles of some metals, such as Au and Ag, exhibit a specific and localized surface plasmon resonance (LSPR) that efficiently scatters and absorbs light at the resonance wavelength.17−19 Interactions between LSPR and light at the resonant wavelength result in an enhanced electromagnetic field near the nanoparticle and can enhance the generation of electron−hole pairs in a neighboring photocatalyst. These plasmonic interactions can be exploited to enhance the performance of PEC and photocatalytic water-splitting systems. For example, when plasmonic metal nanoparticles are coupled to an n-type semiconductor, hot electrons from the nanoparticles can be injected into the conduction band (CB) of the adjacent semiconductor, resulting in an increase in the Fermi level of the semiconductor.20−24 Ye et al. observed a Received: May 25, 2018 Accepted: July 18, 2018 Published: July 18, 2018 A
DOI: 10.1021/acsaem.8b00831 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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ACS Applied Energy Materials
the photoanode structures: LDH/Au@SiO2/BiVO4 (photoanode I) and Au@SiO2/LDH/BiVO4 (photoanode II). Photoanode I was prepared by first depositing Au@SiO2 NPs onto BiVO4 and then growing LDH on top of the Au@ SiO2/BiVO4 subassembly. Photoanode II was prepared by first growing LDH on BiVO4 and then depositing Au@SiO2 NPs onto the LDH/BiVO4 subassembly. Both Au@SiO2-modified photoanodes consisted of three layers: a layer of Au@SiO2, a layer of LDH, and a layer of BiVO4. XRD spectrometry was used to investigate the structures of the photoanodes (Figure 1). For as-grown BiVO4, reflections
more than 4-fold enhancement in photocatalytic activity due to the LSPR effect.25 In addition, LSPR is sensitive to the dielectric constant of the surrounding media and can be tuned to enhance the injection of hot electrons while reducing the rate of charge-carrier recombination.26−29 For instance, the LSPR of Au nanoparticles (NPs) increased remarkably when the NPs were covered by a thin layer of silica or alumina.30 Layered double hydroxides (LDHs) possess a two-dimensional (2D) structural framework and are efficient WOCs when transition metals such as Fe, Zn, Co, or Ni are incorporated into the layers of the LDH.31−37 Modification of BiVO4 photoanodes by LDHs has been shown to increase the efficiency of PEC water oxidation at the photoanodes.38,39 However, the effects of coupling a LDH and plasmonic metal NPs to a semiconductor have been little studied.40 Herein, we integrated a BiVO4 photoanode with a LDH and with plasmonic Au NPs coated with SiO2 (Au@SiO2). Two different configurations were constructed. The first configuration, photoanode I, was constructed by depositing the Au@ SiO2 NPs onto BiVO4 and then growing a LDH on top of the Au@SiO2/BiVO4 subassembly, yielding an LDH/Au@SiO2/ BiVO4 photoanode. The second configuration, photoanode II, was constructed by growing LDH on BiVO4 and then depositing Au@SiO2 onto the LDH/BiVO4 subassembly, yielding a Au@SiO2/LDH/BiVO4 photoanode. When illuminated from the back side, photoanode I achieved a 52% enhancement of photocurrent density relative to photoanode II, consistent with higher efficiencies for oxidation and charge separation. Under back-side illumination, the Au@SiO2 NPs in photoanode I can absorb enough light to excite an LSPR effect that increases charge separation in BiVO4 and thereby improves the water-oxidation performance of the device. However, under front-side illumination, photoanode II provides better light management than photoanode I. These findings indicate that the location of Au@SiO2 NPs in composite photoanode structures directly affects light absorption, which in turn impacts the oxidation performance of the composite photoanodes.
Figure 1. XRD patterns for BiVO4, Au@SiO2/BiVO4, LDH/BiVO4, Au@SiO2/LDH/BiVO4, and LDH/Au@SiO2/BiVO4 photoanodes.
were observed at 2θ = 18.8 and 28.9 degrees, consistent with the monoclinic scheelite phase of BiVO4 (JCPDS No. 751866), in addition to reflections attributable to the FTO substrate (solid square symbol). No other crystalline phases, e.g., V2O5 or Bi2O3, were detected in the BiVO4 sample. New reflections emerged after LDH was grown directly on BiVO4 (LDH/BiVO4). These new reflections were consistent with the characteristic (003), (006), (012), and (018) reflections of LDH and showed that the LDH phase was present.34,41 The decreasing intensity ratio of I(003)/I(012) suggests that LDH grew in a preferred orientation with its ab-plane perpendicular to the substrate. This growth mode has been observed in previous literature.38,42 No additional reflections were detected by the addition of Au@SiO2 to the composites, indicating that the loading of Au@SiO2 was insufficient to produce a diffraction peak. In order to confirm the successful incorporation of Au into Au@SiO2 NPs and of Au@SiO2 NPs into the composite photoanode structures, the morphologies of the samples were characterized by SEM and TEM. The SEM images in Supporting Information (Figure S1A,B) show a uniform dispersion of spherical Au@SiO2 NPs. The TEM results in Figure S1C,D show that the Au surface was coated by SiO2 in a core−shell structure, with Au cores approximately 32 nm in diameter and SiO2 shells of approximately 28 nm thickness. The photoanodes were characterized by SEM after each step in the pathway to the composite photoanode structures (Figure 2). Pristine BiVO4 presented a particulate morphology with an irregular and rough appearance, with particle sizes in the range of 200−300 nm (Figure 2A). After the Au@SiO2 NPs were loaded onto BiVO4, (Au@SiO2/BiVO4), some spherical particles (Au@SiO2) dispersed on the BiVO4 surface were observed (Figure 2B), evidencing the successful decoration of BiVO4 with Au@SiO2.
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RESULTS AND DISCUSSION Two composite structures were designed to explore the photoelectrochemical performance of BiVO4 photoanodes supporting plasmonic Au@SiO2 NPs and an LDH water-oxidation catalyst. Scheme 1 shows the two pathways used to assemble Scheme 1. Illustration of the Two Pathways Used to Fabricate Composite Photoanodes
B
DOI: 10.1021/acsaem.8b00831 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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ACS Applied Energy Materials
pathways. The absorption of visible light was greater for LDH/ BiVO4 than for BiVO4, due to absorption by the colored semiconductor CoAl-LDH.43,44 Au@SiO2/BiVO4 also showed improved absorption in the UV and visible spectral ranges relative to bare BiVO4. Figure S2 shows the UV−vis absorption spectra for Au and Au@SiO2. Au colloids in water showed a very intense surface plasmon absorption band at ca. 530 nm, while the absorption band for Au@SiO2 NPs in ethanol was observed at 540 nm. The weak shift in the peak from 530 to 540 nm most likely resulted from the change in the solvent because the refractive index for electronically inert silica is different in water than in ethanol.45,46 And the optical and electronic properties of the plasmon band are highly sensitive to the surrounding media.47 A comparison of the UV−vis absorption spectra for the two composite photoanode structures shows slightly greater absorption of shorter wavelengths (500 nm). The absorption spectra of LDH and Au@ SiO2 NPs overlap. Thus, the different spatial locations of the Au@SiO2 layer in the composite structures should directly affect light absorption, and thereby impact the oxidation performance. Under back-side illumination, the Au@SiO2 NPs in photoanode I can effectively capture photons, whereas LDH absorbs photons in photoanode II before they can reach the Au@SiO2 layer, hindering absorption by Au@SiO2 and thus G
DOI: 10.1021/acsaem.8b00831 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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ACS Applied Energy Materials The flat-band potentials (Efb) of the photoanodes were estimated from electrochemical Mott−Schottky measurements. The Mott−Schottky plots for all samples exhibited positive slopes, which are a characteristic feature of n-type semiconductors (Figure S9). The Efb values obtained from a fit of the data points extrapolated to the potential axis were 0.168 and −0.716 V versus RHE for BiVO4 and LDH, respectively. From the UV−vis absorption spectra of BiVO4 and LDH (Figure S10), we calculated the Tauc plot transformations for BiVO4 (Figure S11A) and LDH (Figure S11B) to yield band gaps (Eg) of 2.52 and 2.2 eV, respectively. In combination, Efb and Eg provide a picture of the energy levels in the photoanodes. These results allow a tentative mechanism for PEC water oxidation on the composite photoanodes to be proposed (Scheme 2). For n-type semiconductors in contact with an electrolyte containing the redox couple (O2/H2O), the electric field formed at equilibrium leads to energy-band bending that causes photogenerated holes to drift toward the surface with the semiconductor/electrolyte junction.63 For bare Au NPs, hot electrons can be injected into the conduction band of the semiconductor when they have sufficient energy and are in contact with an n-type semiconductor. However, as to SiO2encapsulated Au, i.e., Au@SiO2, the electron−hole recombination can be inhibited in an Au@SiO2-modified photoanode. Furthermore, the local-heat electric field near the plasmonic Au NPs is beneficial to catalytic reactions.30,64 Scheme 2 compares the energetics of the two photoanodes. Under back-side illumination, when Au@SiO2 was directly loaded on BiVO4, the Au@SiO2 layer can absorb ∼530 nm light and generate strong LSPR effect, showing local-heat electric field. BiVO4 contributes significantly to charge separation in the composite photoanodes. Consequently, the LSPR effect of Au@SiO2 may promote sufficient generation of electron−hole pairs from BiVO4. The photogenerated holes were transferred to the LDH and oxidized Co2+ to Co3+/Co4+, in which the Co3+/ Co4+ species oxidized water to release O2 and protons.35,38,62 For the back-side-illuminated composite with Au@SiO2 loaded on LDH, 530 nm light is absorbed by LDH, obstructing absorption by the Au@SiO2 layer and preventing LSPR in Au@SiO2 that promotes charge separation. When the direction of incident light is reversed, the order of absorption in the layers of the composite is reversed, affecting the results. For photoanode I under front-side illumination, visible light was absorbed first by LDH, hindering absorption by Au@SiO2. However, under front-side illumination, Au@ SiO2 can absorb enough photons to excite the LSPR effect in photoanode II, enhancing charge separation in BiVO4, and the water-oxidation capacity of photoanode II. Thus, the different spatial locations of the Au@SiO2 layer in the composite structures directly affect light absorption and impact the oxidation performance. Therefore, the composite structure of photoanode I can better utilize light energy, increase charge separation, and enhance the overall catalytic performance for water oxidation under back-side illumination.
(photoanode I). In the second configuration, LDH was grown on BiVO4 and then the Au@SiO2 NPs were deposited onto the LDH, yielding a Au@SiO2/LDH/BiVO4 structure (photoanode II). Under back-side illumination, photoanode I achieved a photocurrent density of 1.92 mA cm−2 at the thermodynamic potential for water oxidation (1.23 V versus RHE), a 52% enhancement relative to that of photoanode II (1.26 mA cm−2). The oxidation efficiency of photoanode I was 1.3 times greater than that of photoanode II. Under front-side illumination, photoanode II achieved better water-oxidation performance than photoanode I, although not as high as for photoanode II under back-side illumination. We therefore conclude that the different spatial location of the Au@SiO2 layer in the composite structure directly affects light absorption and impacts the oxidation performance. For photoanode I under back-side illumination, enough light can be absorbed by the Au@SiO2 NPs to excite the LSPR effect and improve charge separation, allowing photoanode I to yield the highest water-oxidation performance of the device explored in this work. These results also suggest that BiVO4 contributes significantly to charge separation in the composite photoanode, while LDH accumulates photogenerated holes and expedites the water-oxidation kinetics. This work opens up a new pathway to improve the PEC performance of photoanodes by functionally integrated structural design.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b00831. Experimental details, fluorescence decay, SEM, TEM, UV−vis, PL spectra, electrochemical and PEC measurements, Faradaic efficiency, and Mott−Schottky and Tauc plots (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*(X.X.) E-mail:
[email protected]. *(Y.Y.) E-mail:
[email protected]. ORCID
Xu Xiang: 0000-0003-1089-6210 Jing He: 0000-0002-2940-6675 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21576016, U1507202, and U1707603), the National Key R&D Program of China (Grant 2017YFA0206804), the Innovative Research Groups of National Natural Science Foundation of China (Grant 21521005), and the Key R&D Program of Qinghai Province (Grant 2017-GX-144).
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CONCLUSIONS In summary, we developed a three-layer Au@SiO2-mediated photoanode, consisting of Au@SiO2 NPs, a layered double hydroxide (LDH) water-oxidation catalyst, and BiVO4. Two configurations were constructed. In the first configuration, the Au@SiO2 NPs were deposited directly on BiVO4 and the LDH was then grown, yielding an LDH/Au@SiO2/BiVO4 structure
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DOI: 10.1021/acsaem.8b00831 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Letter
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DOI: 10.1021/acsaem.8b00831 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
