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Three-dimensional FTO/TiO2/BiVO4 Composite Inverse Opals Photoanode with Excellent Photoelectrochemical Performance Haifeng Zhang, and Chuanwei Cheng ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00060 • Publication Date (Web): 13 Mar 2017 Downloaded from http://pubs.acs.org on March 14, 2017
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ACS Energy Letters
Three-dimensional FTO/TiO2/BiVO4 Composite Inverse Opals Photoanode with Excellent Photoelectrochemical Performance Haifeng Zhanga nd Chuanwei Chengab* a
Shanghai Key Laboratory of Special Artificial Microstructure Materials and
Technology, School of Physics Science and Engineering, Tongji University, Shanghai 200092, P.R. China b
The institute of Dongguan-Tongji University, Dongguan, Guangdong, 523808, P. R.
China
The poor electrons transport property, short charge carriers diffusion lengths and slow water oxidation
kinetics severely
limited
the
photoelectrochemical (PEC)
performance of the BiVO4 photoelectrodes. To address these problems, we report the design and fabrication of three dimensional FTO/TiO2/BiVO4 core-shell inverse opals photoanode for photoelectrochemical hydrogen production by combining
atomic
layer deposition and electro-deposition routes for TiO2 and BiVO4 layer deposition on F: SnO2 (FTO)
inverse opal skeletons, respectively. Benefiting from the highly
conductive transparent FTO invese opal networks providing fast electron pathways and TiO2/BiVO4 heterojunctions，the as-fabricated 3D FTO/TiO2/BiVO4 inverse opals photoanode delivers excellent photoelectrochemical performance with a maximum photocurrent density of 4.11 mA cm-2 at 1.23 V vs reversible hydrogen electrode in the presence of holes scavenger in contrast to that of the counterparts FTO/TiO2 and FTO/BiVO4 inverse opals electrodes, respectively, which could be attributed to the significantly improved charge transport and separation efficiency.
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ACS Energy Letters
The PEC water splitting that convert the solar energy into hydrogen fuels has been considered to be a promising way to address the environmental pollution issues and energy crisis issues simultaneously.1, 2In a typical PEC cells, the semiconductor photoanodes with good light absorption could facilitate to generate excited charge carriers. Therefore, more research has focused on the development of materials with narrower band gaps for visible light harvesting.3,4 Among various semiconductors, BiVO4 (Eg = 2.4 eV) has attracted great attention for its suitable conduction band (0 V vs RHE) and valence band edges, chemical stability and excellent light absorption ability.5 However, the poor electron transport property and slow water oxidation kinetic greatly limit the performance of BiVO4.6 To address these issues, typical strategies are through nanostructured electrode design and metal elements doping to enhance the charge transport property.7 Another effective approach is to use two or three dissimilar materials taking the tasks of light absorption and charge transport, respectively. In this integrated heterojunction system, a high specific surface area and conductive support material is usually used as “host”, which is coated with a visible light absorber as “guest”. 8-11 To maximize the PEC performance, the selection of host material plays a significant role. For instances, BiVO4 has been combined with various metal oxides to form highly efficient type ІІ heterojunction photoanodes for water splitting, such as WO3/BiVO4, heterojunctions.
12, 13
ZnO/BiVO4,
14, 15
and TiO2/BiVO416-18
Among them, WO3 and ZnO are attractive for their high electron
mobility. However, WO3 has a relatively positive flat band (FB) potential (0.4 V vs RHE), leading to potential energy losses during the electrons transport process.19 ZnO is corroded easily out of the neutral solution.
For TiO2, it’s very stable, and more
importantly, it has a relatively negative FB potential (0.2 V vs RHE), which does not significantly limit the photovoltage from BiVO4.20 Nevertheless, TiO2 suffering from its intrinsically low mobility still greatly hinders the overall performance of TiO2/BiVO4 heterojunctions photoanode. To realize high light absorption, charge transport and separation efficiency simultaneously in TiO2/BiVO4 system is still a great challenge.
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Herein, we report the design and fabrication of a novel three-dimensional (3D) FTO/TiO2/BiVO4 inverse opals electrode for PEC hydrogen production. In this electrode design, the highly conductive transparent FTO inverse opals serve as 3D skeletons to support TiO2 and BiVO4 layers, providing fast electron transport pathways to improve the electron transport and collect efficiency, while TiO2 acts as a hole-blocking layer to form a type II heterojunction with BiVO4 to enhance the charge separation efficiency, and BiVO4 works as a visible absorber. Furthermore, the 3D inverse opals with large surface area allow for loading guest material, and the periodical and continuous void structure are advantageous for enhancing light scattering and electrolyte infiltration, which is also beneficial to improve the PEC performance.21-23 As a result, the as-fabricated FTO/TiO2/BiVO4 photoanode presents excellent PEC performance with a maximum photocurrent density of 4.11 mA cm-2 at 1.23 V vs RHE with hole scavenger, which is much higher than that of the FTO/TiO2 and FTO/BiVO4 inverse opals electrodes, respectively. The fabrication processes of the 3D FTO/TiO2/BiVO4 inverse opals are schematically illustrated in Figure 1. First, multi-layers of polystyrene (PS) opals were assembled on the FTO glass substrate acting as sacrificial templates. After that the FTO precursors were infiltrated into the voids of the PS templates. Then, the PS spheres were burned out and left the FTO inverse opals structure as skeletons. In a subsequent step, a thin TiO2 layer of ~ 6 nm was deposited on the surface of the FTO inverse opals by an atomic layer deposition (ALD) technique. Finally, the BiVO4 layer was electro-deposited on the FTO/TiO2 skeletons to form a core-shell type ІІ heterojunctions structure. The morphology of the as-fabricated samples was investigated by scanning electron microscope (SEM). Figure S1a (Supporting Information) shows the typical SEM image of the PS spheres opals, the compactly arranged PS spheres with diameters of
∼408 nm are clearly observed. Figure 2a displays the top-view SEM image of the FTO inverse opals, from which 3D ordered macroporous structure with periodical hexagonal close-packed porous can be observed, and the geometrical characteristics
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ACS Energy Letters
are composed of two kinds of pores: one is the macropores with a diameter of ∼290 nm, which shows 29% shrinkage in contrast with that of the PS spheres template; the other is the small pores with a diameter of 86-124 nm. The pores connect to the neighboring pores through the necks. The cross-section image of the FTO inverse opal (inset of the Figure 2a) further confirms the periodically ordered porous structures. When 63 cycles ALD TiO2 was deposited on the FTO inverse opal, the morphology of the porous structure was not significantly changed (Figure S1b Supporting Information). As shown in Figure 2b, after the BiVO4 layers deposited on the surface of 3D porous FTO/TiO2 skeletons, the wall thickness is distinctly increased, and the pore diameter decreased accordingly.
As shown in Figure S2，
the thickness of the TiO2 layer and BiVO4 layer are ~5.9 nm and 27.9 nm, respectively, which is thinner to the carrier diffusion lengths of BiVO4 (
