Letter pubs.acs.org/JPCL
Preparation of Bi-Based Ternary Oxide Photoanodes BiVO4, Bi2WO6, and Bi2Mo3O12 Using Dendritic Bi Metal Electrodes Donghyeon Kang,†,§ Yiseul Park,†,§ James C. Hill,‡ and Kyoung-Shin Choi*,† †
Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, Wisconsin 53706, United States Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States
‡
S Supporting Information *
ABSTRACT: The major limitation to investigating a variety of ternary oxides for use in solar energy conversion is the lack of synthesis methods to prepare them as high-quality electrodes. In this study, we demonstrate that Bi-based n-type ternary oxides, BiVO4, Bi2WO6, and Bi2Mo3O12, can be prepared as high-quality polycrystalline electrodes by mild chemical and thermal treatments of electrodeposited dendritic Bi films. The resulting oxide films have good coverage, adhesion, and electrical continuity, allowing for facile and accurate evaluation of these compounds for use in solar water oxidation. In particular, the BiVO4 electrode retained the porosity and nanocrystallinity of the original dendritic Bi film. This feature increased the electron−hole separation yield, making this compound more favorable for use as a photoanode in a photoelectrochemical cell.
SECTION: Energy Conversion and Storage; Energy and Charge Transport
R
millivolts, even when hole acceptors with fast oxidation kinetics were used for photocurrent generation.22 This suggested that these electrodes possessed considerable surface states in the bandgap region that can cause rapid surface recombination. Because photoelectrochemical properties of polycrystalline electrodes can vary significantly by synthesis methods or conditions, which affect the intrinsic doping/defect levels as well as surface states, developing new synthesis conditions that can produce Bi2WO6 electrodes with morphologies and surface conditions to suppress surface recombination would be desirable. In this study, we report an electrochemical synthesis procedure to prepare high-surface-area dendritic Bi metal electrodes, which can be converted to BiVO4 and Bi2WO6 using simple chemical and thermal treatments (Scheme 1). We also attempted to produce n-type Bi2MoO6, which is isostructural to Bi2WO6,25 but we obtained n-type Bi2Mo3O12 instead. Because not much is known about Bi2Mo3O12 for use in solar energy conversion and no methods to directly grow Bi2Mo3O12 as a film-type electrode have been available,26−29 we characterized and investigated the resulting Bi2Mo3O12 electrode as a photoanode along with the BiVO4 and Bi2WO6 electrodes in this study. The major challenge for preparing high-quality Bi electrodes by electrodeposition using aqueous plating solutions is that the Bi(III)-containing salts are not soluble unless the solution is
ecently, ternary metal oxides containing Bi(III) have been identified as promising semiconductor electrodes for use in solar energy conversion, including BiVO4 and Bi2WO6. BiVO4 is an n-type semiconductor with a bandgap of 2.4 to 2.5 eV, which is identified as one of the most promising photoanodes for use in a water-splitting photoelectrochemical cell (PEC).1−11 Our latest study showed that the electron−hole separation yield of BiVO4 can be significantly enhanced when BiVO4 electrodes are composed of particles with sizes smaller than its hole diffusion length.4 However, synthesis methods to prepare nanoporous BiVO4 electrodes have been quite limited. Therefore, developing facile synthesis methods to produce nanoporous BiVO4 electrodes would be beneficial. Bi2WO6 is another n-type semiconductor that has been mainly tested for photodegradation of organic dyes and pollutants.12−18 Its bandgap (2.7 to 2.8 eV) is wider than the ideal bandgap for solar energy conversion, but its conduction band minimum is estimated to be at a more negative potential than that of water reduction to H2.19−22 Therefore, it could conceivably photoelectrochemically split water with little or no applied bias. Also, it may be combined with a smaller bandgap p-type oxide to form a p−n junction solar cell that could potentially achieve a high photovoltage. However, previous studies mostly focused on the synthesis and properties of Bi2WO6 nanocrystals as photocatalysts, and only a few synthesis methods to grow Bi2WO6 as thin-film type electrodes have been reported to date.12,18,21−24 When Bi2WO6 electrodes were prepared by the conversion of nanoporous WO3 electrodes, the observed photocurrent onset potential was shifted in the positive direction from its flatband potential by several hundred © 2014 American Chemical Society
Received: July 22, 2014 Accepted: August 19, 2014 Published: August 21, 2014 2994
dx.doi.org/10.1021/jz501544k | J. Phys. Chem. Lett. 2014, 5, 2994−2999
The Journal of Physical Chemistry Letters
Letter
Scheme 1. Schematic Representation for the Conversion of Dendritic Bi Metal Electrodes to Bi-Based Ternary Metal Oxide Electrodes
strongly acidic. Therefore, a strongly acidic Bi(III) solution should be used as the plating solution.30−33 However, the acidity of the plating solution is detrimental to Bi metal deposits because Bi metal dissolves in acid. When the deposition ends and Bi metal deposits are no longer under cathodic protection in acidic media, the dissolution of Bi occurs immediately, weakening the Bi substrate junction and resulting in poor adhesion and coverage. 31,32 This problem is pronounced when Bi is deposited on a conducting oxide substrate such as fluorine-doped tin oxide (FTO) because the Bi metal-oxide substrate interaction is not strong. For example, Bi metal deposited on an FTO substrate using a 1 M HNO3 aqueous solution containing 20 mM of Bi(NO3)3 is shown in the Supporting Information (Figure S1A), which shows poor coverage. To overcome this problem, we used a nonaqueous ethylene glycol solution containing Bi(III) ions, as the plating solution. This not only increased the solubility but also eliminated the dissolution issue of the Bi deposits, resulting in the formation of Bi films with good coverage and adhesion. SEM images showed that the resulting Bi electrodes were composed of compact Bi dendrites, creating high surface areas (Figure 1A). Highmagnification SEM images showed that the diameters of the trunks and branches of Bi dendrites are in the range of 50−80 and 10−30 nm, respectively (Figure 1B). The side-view SEM image confirmed the uniform and dense coverage of the Bi dendrites on the substrate (Figure 1C). The thickness of the Bi film was estimated to be 1.3 μm. The deposits were confirmed to be pure, crystalline Bi metal by X-ray diffraction (Figure 2A). To convert Bi to BiVO4 electrodes, 100 μL of 150 mM VO(acac)2 dimethyl sulfoxide (DMSO) solution was placed onto the as-deposited Bi electrode (geometric area = 1.8 cm2) to fully cover its surface. The film was then heated to 450 °C for 2 h in air (ramping rate = 1.8 °C/min). During the heating procedure, Bi metal and VO2+ were oxidized and reacted with each other to form BiVO4. Excess VO2+ ions were used to ensure the complete conversion of Bi to BiVO4 because any residual V2O5 formed can be easily removed by soaking the electrode in a 1 M NaOH solution for 30 min while stirring. The conversion of Bi to Bi2WO6 was achieved by adding 50 μL of 20 mM (NH4)2WS4 DMSO solution onto the Bi electrode and annealing it at 600 °C for 2 h in air (ramping rate = 2.4 °C/min). After annealing, excess WO3 was removed by soaking the electrode in a 1 M NaOH solution for 3 min at 80 °C while stirring.
Figure 1. (A,B) Top-view and (C) side-view SEM images of Bi metal electrodes deposited from an ethylene glycol solution and top-view SEM images of (D) BiVO4, (E) Bi2WO6, and (F) Bi2Mo3O12 electrodes.
To prepare Bi2Mo3O12 electrodes, 100 μL of 50 mM (NH4)2MoS4 DMSO solution was placed onto the Bi electrode and the electrode was annealed at 550 °C for 4 h in air (ramping rate = 2.2 °C/min). After annealing, excess MoO3 was removed by soaking the electrode in a 1 M NaOH solution for 5 min while stirring. The purity and crystallinity of resulting BiVO4, Bi2WO6, and Bi2Mo3O12 electrodes were confirmed by X-ray diffraction (Figure 2B−D). The atomic Bi:V, Bi:W, and Bi:Mo ratios in the BiVO4, Bi2WO6, and Bi2Mo3O12 electrodes, respectively, were also analyzed by energy-dispersive X-ray spectroscopy (EDS) to confirm the absence of amorphous impurities (e.g., Bi2O3). All electrodes showed expected atomic metal ratios (Supporting Information, Table S1). The SEM images showed that the BiVO4 electrodes retained the porosity and nanocrystallinity of the original dendritic Bi electrodes, resulting in high surface areas, although the morphology of the nanoparticles was slightly altered (Figure 1D). The average BiVO4 2995
dx.doi.org/10.1021/jz501544k | J. Phys. Chem. Lett. 2014, 5, 2994−2999
The Journal of Physical Chemistry Letters
Letter
Figure 2. XRD patterns of (A) Bi, (B) BiVO4, (C) Bi2WO6, and (D) Bi2Mo3O12 electrodes. The peaks generated from the FTO substrate are denoted by asterisks.
particle size was estimated to be ∼150 nm based on highmagnification SEM images. The Bi2WO6 electrodes also retained some porosity (Figure 1E), but the porosity of Bi2Mo3O12 was significantly reduced because the conversion of Bi metal to Bi2Mo3O12 requires considerable volume and mass increases due to the incorporation of 1.5 Mo atoms and 6 oxygen atoms per Bi atom (Figure 1F). The side-view SEM images show that the average film thicknesses are ∼1, ∼0.8, and ∼1.2 μm for BiVO4, Bi2WO6, and Bi2Mo3O12, respectively (Supporting Information, Figure S1B−D). The UV−vis absorption spectra of BiVO4, Bi2WO6, and Bi2Mo3O12 electrodes are shown in Figure 3. The bandgaps of
Figure 4. J−V plots for sulfite (blue) and water oxidation (red) for (A) BiVO4, (B) Bi2WO6, and (C) Bi2Mo3O12 electrodes under AM 1.5G (100 mW/cm2) illumination in 0.1 M phosphate buffer (pH 7.6) with and without 1 M sodium sulfite (scan rate = 10 mV/s). Dashed lines represent dark currents for sulfite (blue) and water (red) oxidation. IPCE (red) and APCE (black) for (D) BiVO4 and (E) Bi2WO6 electrodes measured for sulfite oxidation at 0.6 V versus RHE.
be used for sulfite oxidation without being lost to surface recombination.3 Therefore, measuring photocurrent for sulfite oxidation allowed us to assess the intrinsic performance of the photoanode (i.e., generation and separation of electron−hole pairs), not limited by the poor catalytic nature of BiVO4 for water oxidation.1−4,10 As previously mentioned, most of the previously reported BiVO4 electrodes were nonporous and their performances were mainly limited by their low electron−hole separation yields (
