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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Enhanced Photoelectrochemical Water Oxidation Performance by Fluorine Incorporation in BiVO4 and Mo:BiVO4 Thin Film Photoanodes Martin Rohloff,†,‡,§,∥,+ Björn Anke,∥,+ Olga Kasian,⊥ Siyuan Zhang,⊥ Martin Lerch,∥ Christina Scheu,⊥ and Anna Fischer*,†,‡,§ ACS Appl. Mater. Interfaces Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 04/24/19. For personal use only.
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Freiburger Zentrum für interaktive Werkstoffe und bioinspirierte Technologien, Albert-Ludwigs-Universität Freiburg, Georges-Köhler-Allee 105, 79110 Freiburg, Germany ‡ Institut für Anorganische und Analytische Chemie, Albert-Ludwigs-Universität Freiburg, Albertstraße 21, 79104 Freiburg, Germany § Freiburger Materialforschungszentrum, Stefan-Meier-Straße 19, 79104 Freiburg, Germany ∥ Institut für Chemie, Technische Universität Berlin, Straße des 17. Juni 135, 10623 Berlin, Germany ⊥ Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Straße 1, 40237 Düsseldorf, Germany S Supporting Information *
ABSTRACT: Anion substitution is an emerging strategy to enhance the photoelectrochemical performance of metal oxide photoelectrodes. In the present work, we investigate the effect of fluorine incorporation on the photoelectrochemical water oxidation performance of BiVO4 and Mo:BiVO4 thin film photoanodes. The BiVO4 and Mo:BiVO4 thin film photoanodes were prepared by a straightforward organometallic solution route involving dip coating and subsequent calcination in air. Fluorine modification was realized by applying a soft and low-cost solid−vapor reaction route involving fluorine-containing polymers and an inert gas atmosphere leading to novel F:BiVO4 and F/Mo:BiVO4 thin film photoanodes with substantially increased photoelectrochemical water oxidation properties. Deposition of the cobalt phosphate (CoPi) water oxidation catalyst allowed further enhancement of the photoelectrochemical performance. While Mo doping mainly improves light-harvesting, charge transport, and charge separation efficiencies, F modification was demonstrated to primarily affect the charge transfer efficiency at the semiconductor−electrolyte interface, thereby leading to a photocurrent increase of 40 and 21% upon fluorination of the BiVO4 and Mo:BiVO4 photoanodes, respectively, and an applied bias photonto-current efficiency increase of 35 and 5%, respectively. We thereby could demonstrate that cation and anion co-doping in BiVO4 as demonstrated for Mo and F allows combining the photoelectrochemically relevant benefits associated with each type of dopant. KEYWORDS: bismuth vanadate, photoanode, fluorination, photoelectrochemical water oxidation
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INTRODUCTION
investigated oxide-based photoanode materials for photoelectrochemical water oxidation.1−13 However, one major drawback of BiVO4 is its poor electronic conductivity, which can be overcome by efficient doping strategies.14 In this context, molybdenum (Mo6+) and tungsten (W6+) were found to be compatible cationic n-type dopants to enhance the number of free charge carriers in BiVO4, increase the electronic conductivity, and improve the charge separation, thereby leading to enhanced PEC performance.15−20 Going beyond single-cation doping, cation co-doping has been shown to further increase the PEC performance of BiVO4
Photoelectrochemical (PEC) water splitting using sunlight as the energy source is considered to be the most elegant way for sustainable hydrogen production. A semiconductor material that absorbs a sufficient portion of solar light (Eg ≈ 2.1 eV), separates and transports the light-induced charge carriers efficiently, and favors the water oxidation half-reaction at its surface is crucial for the development of efficient watersplitting devices. In this context, ternary oxide bismuth vanadate (BiVO4) is a very promising photoanode material. Due to its suitable band gap (2.4 eV), its suitable valence band-edge positions matching the water oxidation redox potential, its stability in aqueous media at neutral and alkaline pH, its nontoxicity, and its low cost and abundance, BiVO4 has become one of the most © XXXX American Chemical Society
Received: September 23, 2018 Accepted: April 1, 2019
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DOI: 10.1021/acsami.8b16617 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
limited, reaching approximately 0.6 mA/cm2 for F:BiVO4 and 1.2 mA/cm2 for CoPi-modified F:BiVO4. In the present work, we apply for the first time fluorination as a strategy for the postsynthetic modification and performance optimization of BiVO4 thin film photoanodes for PEC water oxidation. Going beyond single-anion or single-cation doping/substitution, we also explore the effects of concomitant anion and cation substitution/doping on the PEC water oxidation performance of BiVO4 photoanodes, thereby revealing that F modification of both BiVO4 and Mo:BiVO4 photoanodes is beneficial for their PEC performance, achieving a performance increase of 40 and 21%, respectively, upon fluorination, mainly attributed to F-induced increased catalytic efficiency.
by combining the PEC-favorable benefits of each doping element. For example, cation co-doping with W (n-type doping) and Ti (p-type doping) has been shown to considerably improve (i) the charge separation efficiency (as a result of an increased electron and hole mobility due to W doping and Ti doping, respectively) and (ii) the charge transfer efficiency, due to the introduction of Ti-related surface sites with enhanced water splitting activity.21 As such, cation co-coping has been shown to allow the combination of the respective beneficial properties of each dopant to synergistically boost the PEC performance of BiVO4. In contrast to well-known cation doping, anion doping has been by far less explored. Starting with TiO2, anion doping of TiO2-based photocatalysts (e.g., with F−, N3−, and S2−) has been demonstrated to result in improved photocatalytic activity.22−27 As shown by Junqi et al. in 2011,26 among these non-metal dopants, fluorine incorporation into the TiO2 lattice reduced hole trapping at the TiO2/electrolyte interface presumably as a result of the strong electronegativity of surface-bound fluorine. As a result, the photocatalytic performance regarding the oxidative degradation of methyl orange was enhanced after fluorination. The concept of fluorine modification of oxide semiconductors has meanwhile been expanded to non-TiO2-based semiconductor materials. For example, fluorination of Bi2WO6 reported by the group of Zhu enhanced the photocatalytic activity of Bi2WO6 for Rhodamine B degradation, which has been ascribed to F ions serving as electron trapping sites.28,29 So far, little efforts have been made with respect to anion doping/substitution in BiVO4 and only a few reports on fluorinated BiVO4 can be found in the literature.30,31 For instance, Li et al. reported in 2013 a two-step hydrothermal synthesis method leading to F-doped BiVO4 (F:BiVO4) microspheres with improved optical absorption properties (red-shifted absorption edge) and higher photocatalytic activity than the pristine BiVO4 microspheres regarding the photocatalytic degradation of methylene blue under visible light irradiation.31 This improvement was ascribed to the generation of electron traps upon introduction of F− within the lattice, presumably restraining the recombination of photogenerated electron−hole pairs.31 In consideration of these first reported results, fluorine incorporation into BiVO4 represents a promising way to enhance its photocatalytic properties. Inspired by these first studies, we recently developed a solid−vapor phase reaction route allowing fluorine incorporation into BiVO4 powders.32 Photoanodes were prepared out of these powders by electrophoretic deposition and thermal post-treatments, allowing them to reach sufficient percolation and hence conductivity in the powder films. Investigations of the photoelectrochemical performance regarding water oxidation under visible light revealed increased photocurrents of the fluorine-containing BiVO4 photoanodes compared to the pristine counterparts, which we could ascribe to a higher number of free charge carriers, a cathodically shifted flat band potential, a slightly decreased optical band gap allowing absorption of a bigger portion of visible light, and improved surface kinetics. As such, we could demonstrate that F incorporation into BiVO4 is indeed beneficial for its PEC water oxidation performance. However, due to the particulate nature of the electrode film, the large particle size (larger nanometer to micrometer range), and the large thickness of the “powder” photoanodes, the photocurrents (@1.23 V vs RHE, 100 mW/cm2) of the fluorinated photoanodes were still
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EXPERIMENTAL SECTION
Synthesis. Reagents and Materials. Trichloromethane (CHCl3, 99,9%, anhydrous), vanadium(V) oxytriethoxide (VO(OEt)3, 95%), and molybdenum(VI) dioxydiacetylacetonate (MoO2(acac)2) were provided by Sigma Aldrich. Bismuth(III) 2-ethylhexanoate (92% in 2ethylhexanoic acid) was purchased from Alfa Aesar. Potassium dihydrogen phosphate (KH2PO4, ≥99%) and dipotassium hydrogen phosphate (K2HPO4, 99%) were provided by Carl-Roth. Poly(vinylidene difluoride) (PVDF) was purchased from Apollo Scientific. All chemicals were used as received without any further purification. Fluorine-doped tin oxide-coated glass slides (FTO, 30 × 30 cm, 8−12 Ω/square) were provided by Sigma-Aldrich and were cut in pieces of 3 × 1 cm2 and subsequently cleaned by ultrasonication in ethanol, isopropanol, and acetone before usage. Synthesis of BiVO4 Thin Films. The synthesis of the BiVO4 thin films was done according to our previous report.33 In brief, a solution of 123 μL (0.70 mmol) of VO(OEt)3 in 1.5 mL of CHCl3 was prepared. After stirring for 10 min, the deep-red solution was added to 445 mg (0.70 mmol) of Bi(2-ethylhexanoate)3 and stirred for 4 h. The solution was used for thin film deposition by dip coating FTOcoated glass slides under controlled conditions (
