Article pubs.acs.org/JPCC
Synthesis and Characterization of CuV2O6 and Cu2V2O7: Two Photoanode Candidates for Photoelectrochemical Water Oxidation Wenlong Guo,†,∥ William D. Chemelewski,‡ Oluwaniyi Mabayoje,§ Peng Xiao,⊥ Yunhuai Zhang,*,∥ and C. Buddie Mullins*,†,‡,§ †
McKetta Department of Chemical Engineering, ‡Texas Materials Institute, and §Department of Chemistry and Biochemistry, Center for Electrochemistry, and Center for Nano- and Molecular Science, University of Texas at Austin, 1 University Station C0400, Austin, Texas 78712-0231, United States ∥ College of Chemistry and Chemical Engineering and ⊥College of Physics, Chongqing University, Chongqing 400030, China S Supporting Information *
ABSTRACT: Thin films of two copper-based metal vanadates (CuV2O6 and Cu2V2O7) were synthesized by a facile dropcasting method. The primary photoelectrochemical (PEC) and physical properties of these two materials including photocurrent response, band gap, flat band potential, incident photon to current conversion efficiency, chemical stability, and oxygen evolution faradaic efficiency were researched. The photocurrent density of CuV2O6 and Cu2V2O7 films at 1.23 V vs RHE in 0.1 M sodium borate buffer solution was about 25 and 35 μA/cm2, respectively. At 1.58 V vs RHE, however, the photocurrent density reached approximately 220 and 120 μA/cm2, respectively. Although the photocurrents observed for these two materials at 1.23 V vs RHE were relatively low, the photocurrents were much higher when tested with sodium sulfite as a hole scavenger. Suitable oxygen evolution catalysts are therefore expected to improve the PEC performance of these materials.
1. INTRODUCTION Numerous attempts have been made to develop semiconductor materials capable of capturing solar energy for photoelectrochemical (PEC) applications.1−3 In general, an “ideal” semiconductor material should possess a band gap small enough to absorb a large portion of the solar spectrum but large enough to provide the potential necessary to drive water splitting and have suitable positioning of conduction and valence bands to straddle the proton reduction and water oxidation redox potentials. In addition, the photogenerated charge carriers must be effectively separated and transported to the material’s surface, and the material should be catalytically active for water oxidation or reduction.4−6 Among a large number of studied metal oxide materials, TiO2, α-Fe2O3, WO3, and BiVO4 have attracted the most attention of researchers.3,7−9 All of them, however, are unsatisfactory because of either a narrow spectral response range, high charge carrier recombination, or poor water oxidation kinetics. TiO2, due to its wide band gap (3.0−3.2 eV), is only able to absorb the UV portion of the solar spectrum which is less than 5% of the available solar energy.5 α-Fe2O3 has a suitable band gap of ∼2.2 eV, but the high charge carrier recombination limits its photoelectrochemical activity.8,10,11 The water splitting performances of WO3 and BiVO4 are limited by the poor charge separation and sluggish photoelectrochemical kinetics.12−16 Therefore, in order to realize the application of solar water splitting in a large scale system, two main strategies are actively pursued: (1) to use additional © 2015 American Chemical Society
technological means, such as doping with other elements, constructing appropriate heterojunctions, introducing suitable cocatalysts, etc., to improve the existing materials; (2) to discover new materials having unique and favorable properties for solar water splitting purposes. For the second approach, some noteworthy work has recently been accomplished. For example, Bi2WO6, despite its rather large band gap (∼2.8 eV), shows an earlier photocurrent onset due to its favorable conduction band edge and flat band potential position for water splitting.17−19 CuWO4 with a band gap of ∼2.3 eV shows higher stability in aqueous electrolytes at neutral pH than WO3.20−22 ZnFe2O4 with a band gap of ∼2.0 eV was incorporated into α-Fe2O3 and improved the overall photoelectrochemical performance.23,24 NiV2O6 (∼2.4 eV) was found to be capable of harvesting photons in the visible region of the solar spectrum for PEC water oxidation.25 Most recently, and somewhat related to the present study, Seabold and Neale reported on Cu3V2O8 as a potential photoanode candidate for water splitting systems.26 We were inspired by their work to investigate the other two copper vanadate compounds (CuV2O6 and Cu2V2O7). These materials have been tested as cathode materials for primary lithium batteries and active catalysts for H2SO4 and SO3 decomposition27−29 but, to the best of our knowledge, have not been systematically studied for Received: July 25, 2015 Revised: November 17, 2015 Published: November 18, 2015 27220
DOI: 10.1021/acs.jpcc.5b07219 J. Phys. Chem. C 2015, 119, 27220−27227
Article
The Journal of Physical Chemistry C
(incident light impinging on the film surface) and back-side illumination (incident light impinging on the glass/FTO substrate). A 1 mm diameter Pt wire (99.95%, Alfa Aesar) was used as the counter electrode, and a Ag/AgCl in 1 M KCl electrode (CH Instruments, CH111) was used as the reference electrode. A benchtop pH meter (Oakton) was used to check the pH of the electrolyte solutions. The electrochemical measurements were carried out using a CH Instruments 660D electrochemical workstation. All potentials reported here are versus the reversible H2 electrode (RHE):
PEC water oxidation. They were reported to have band gaps of 1.9−2.1 eV,30−32 which is very suitable for PEC water splitting. Additionally, previous work in our group showed that the dropcasting method, which has been widely used in many fields,33−35 is a very simple and fast method to prepare thin films.36 Also, the drop-casting method can reduce the waste of precursor chemicals and can easily be used to carry out the doping process and control the amount of doping elements.36−38 Herein, two n-type semiconductor materials CuV2O6 and Cu2V2O7 were synthesized for PEC water oxidation using the drop-casting method. The annealing temperature and film thickness of these two materials were optimized, and some key physical and PEC properties of the materials were investigated. The drop-cast CuV2O6 and Cu2V2O7 films reported herein are two useful additions to the expanding PEC water splitting photoanode library.
° E RHE = EAg/AgCl + 0.0591 × pH + EAg/AgCl
(2)
where EAg/AgCl ° (1 M KCl) = 0.236 V at 25 °C. A 150 W xenon lamp (Newport, model 9600) with an AM 1.5G filter (Newport) was employed as the illumination source. Full spectrum irradiation at the tested film’s position was approximately 100 mW/cm2 as measured by a thermopile detector (Newport, Models 1916C and 818-P). IPCE measurements were conducted using a full solar simulator (Newport, Model 9600, 150 W xenon lamp) and a motorized monochromator (Oriel Cornerstone 130 1/8 m). The light power intensity at a specific wavelength was measured using a hand-held optical power meter with an UV-enhanced silicon photodetector (Newport, models 1916 C and 818-UV). IPCE values were calculated using the formula 40
2. EXPERIMENTAL SECTION 2.1. Material Synthesis. Cu(NO3)2·xH2O (Alfa Aesar, 99.999%) and VCl3 (Sigma-Aldrich, 97%) were dissolved separately in ethylene glycol (Fischer, 99+%) as metal precursor solutions. In order to optimize the thickness of CuV2O6 and Cu2V2O7 films, precursor solutions with various concentrations were prepared. For CuV2O6, we prepared 85, 75, 50, 37.5, and 25 mM Cu2+ solutions and 170, 150, 100, 75, and 50 mM V3+ solutions. For Cu2V2O7, we prepared 112.5, 75, 56.25, and 37.5 mM Cu2+ and V3+ solutions. According to the matched solution concentrations, 200 μL Cu(NO3)2 solution and 200 μL VCl3 solution were mixed to form a precursor solution, which then was sonicated for 10 min. Next, 200 μL of the green precursor solution was dispensed on FTO-coated glass substrates (1.5 × 1.5 cm, Pilkington, TEC15) horizontally placed on a hot plate (Corning, PC-420D) at 180 °C. The FTO-glass had been cleaned ultrasonically for 30 min in a 1:1 volume of ethanol and detergent. The samples were dried for 30 min and then annealed in air at various temperatures from 400 to 550 °C for 2 h. 2.2. Material Characterization. X-ray diffraction (XRD) measurements were taken using a Philips X’Pert diffractometer using monochromatic Cu Kα X-rays (λ = 1.540 56 Å). Film morphology and thickness were determined using a Quanta 650 (FEI) scanning electron microscope (SEM). Energy dispersive X-ray (EDX) signals were obtained using a Quanta SEM (FEI Quanta 650) with a 15 kV accelerating voltage, and the relative ratio of Cu and V was calculated using Esprit software. The UV−vis transmission spectra were collected with a Cary 5000 UV−vis spectrometer. The absorption coefficient (α) for Tauc plots was given by α = −ln(10−absorbance)/z
(1 M KCl)
IPCE = (1240 × I )/(λ × Plight)
(3)
where I and Plight are measured photocurrent density and measured light power density at the specific wavelength and λ is the specific incident monochromatic wavelength. Mott−Schottky measurements were conducted using a CH Instruments 660D. Mott−Schottky plots show the relationship between capacitance vs applied potential assuming the ideal resistance-capacitance equivalent circuit, given by 1/C 2 = (2/εε0Nd)[Va − Vfb − kT /e]
(4)
where C is the capacitance of the space charge layer, ε is the dielectric constant of the material, ε0 is the permittivity of vacuum, e is the elemental charge, and Nd is the concentration of charge carrier. Va is the applied potential, k is the Boltzmann constant, T is the temperature in kelvin, and Vfb is the flat band potential. Values of Vfb were estimated using the linear regression of 1/C2 and Va at various frequencies. 2.4. Oxygen Evolution Detection. The O2 evolution faradaic efficiencies of these two copper vanadate films were determined by oxygen evolution measurements conducted in a three-electrode cell. An airtight design coupled with an oxygen sensor (YSI 5001) was used to measure the real-time concentration of the dissolved oxygen in the solution (50 mL of 0.1 M NaBi buffer, pH 9.2). A previous paper of our group has presented the details regarding the experimental setup for oxygen detection.25 A small rate of air that leaked into the cell (∼5 nM/L per second) was taken into account to calculate the actual oxygen generated during the measurement. The charge recorded by the potentiostat (CH Instruments 660D) was used to calculate the theoretical amount of O2 produced assuming 100% faradaic efficiency toward O2 evolution reaction. The ratio of the actual amount of O2 detected to the theoretical amount of O2 produced was defined as the O2 evolution faradaic efficiency. A high light intensity (∼300 mW/cm2) and
(1)
where z is the film thickness. VESTA software39 was used to draw the crystal structures of triclinic CuV2O6 and monoclinic Cu2V2O7. 2.3. Electrochemical and Photoelectrochemical Measurements. The electrochemical and photoelectrochemical measurements were conducted using a three-electrode cell, which had a main compartment for the working electrode and two branched compartments separated by fritted disks (Ace Glass, 10−20 μm porosity) for the counter electrode and reference electrode. A CuV2O6 or CuV2O7 film was placed in the cell with ∼0.23 cm2 for both front-side illumination 27221
DOI: 10.1021/acs.jpcc.5b07219 J. Phys. Chem. C 2015, 119, 27220−27227
Article
The Journal of Physical Chemistry C Scheme 1. Schematic Description of the Drop-Casting Method for Film Preparation
a high bias (1.58 V vs RHE) provided a better signal-to-noise ratio.
3. RESULTS AND DISCUSSION 3.1. Film Preparation and Material Characterization. Scheme 1 shows the preparation process of copper vanadate films using the drop-casting method. First, 200 μL of precursor solution mixed with Cu2+ and V3+ was drop-cast on cleaned FTO-glass which was horizontally placed on a hot plate. The precursor solution was dried at 180 °C for 30 min. The green precursor solution changed to yellow and then light green during this drying process. The samples were then annealed in air at various temperatures from 400 to 550 °C to form the copper vanadate films. As shown in Figure 1a, an XRD pattern for the triclinic CuV2O6 phase was observed after ≥400 °C annealing, and the
Figure 2. Crystal structures of 2 × 2 × 2 supercell for (a) triclinic CuV2O6 and (b) monoclinic Cu2V2O7. Thick dashed lines denote (a) the triclinic P1̅ unit cell and (b) the monoclinic C2/c unit cell. Axes a, b, and c denote the directions of lattice vectors. The brown, yellow, and red indicate copper, vanadium, and oxygen atoms, respectively. Figure 1. XRD patterns of (a) CuV2O6 and (b) Cu2V2O7 films obtained after annealing in air for 2 h at various temperatures from 400 to 550 °C. The black vertical lines correspond to (a) triclinic CuV2O6 (PDF-# 01-074-2117) and (b) monoclinic Cu2V2O7 (PDF-# 01-0782581). The asterisk symbols indicate the peaks for SnO2 (PDF-# 00046-1088).
104.00°, β = 110.45°, and γ = 46.17°. In the monoclinic Cu2V2O7 crystal structure, one V5+ ion and four oxygen ions form a VO4 tetrahedron while a Cu2+ ion is coordinated with five oxygen ions. The morphology and thickness of typical CuV2O6 (2 μm) and Cu2V2O7 (1 μm) films, which showed the highest photocurrents after optimization of film thickness, are shown in Figure 3. The SEM images revealed that CuV2O6 and Cu2V2O7 films are formed by interconnected particles grown using the drop-casting method. Cross-sectional views showed that the CuV2O6 film prepared by 75 mM Cu2+ and 150 mM V3+ precursor solutions has a ∼ 2 μm thickness, and the thickness of the Cu2V2O7 film prepared by 56.25 mM Cu2+ and V3+ precursor solutions is ∼1 μm. The particle size distributions of these two copper vanadate films based on SEM images are shown in Figure S2. The average particle sizes for CuV2O6 and Cu2V2O7 are 0.43 ± 0.05 and 0.63 ± 0.05 μm, respectively. EDX analysis indicated the expected Cu/V ratios for CuV2O6 and Cu2V2O7 films (shown in Figure S3). 3.2. Photoelectrochemical Measurements. In order to investigate the photoelectrochemical activities of CuV2O6 and Cu2V2O7 films, chopped linear sweep voltammetry (LSV) measurements (shown in Figure 4) were conducted under illumination with 100 mW/cm2 simulated solar light. According to the literature,26,32 CuV2O6 and Cu2V2O7 were expected to be stable under neutral to slightly alkaline conditions, and it is
phase is consistent with triclinic CuV2O6 synthesized by the conventional solid-state method.41 Figure 1b shows the XRD patterns for the Cu2V2O7 films annealed at various temperatures from 400 to 550 °C. The phase is consistent with the monoclinic Cu2V2O7.42 The crystallinity of the samples increases and the main diffraction peaks appear sharper by increasing the temperature. According to previous experimental and theoretical work,27,31,43,44 we drew the structures of triclinic CuV2O6 and monoclinic Cu2V2O7 using the VESTA software. Figure 2a shows the structure of a 2 × 2 × 2 supercell for triclinic CuV2O6, and the lattice parameters are a = 9.171 Å, b = 3.546 Å, c = 6.482 Å, α = 92.32°, β = 110.32°, and γ = 91.84°. Both Cu2+ and V5+ ions are octahedrally coordinated to oxygen ions to form CuO6 and VO6 octahedra. Each CuO6 group shares two edges with CuO6 groups displaced by a b-axis translation to build CuO6 linear octahedral chains, which are separated from each other by VO6 octahedral chains. For monoclinic Cu2V2O7, as shown in Figure 2b, the lattice parameters are a = 7.687 Å, b = 5.5498 Å, c = 10.09 Å, α = 27222
DOI: 10.1021/acs.jpcc.5b07219 J. Phys. Chem. C 2015, 119, 27220−27227
Article
The Journal of Physical Chemistry C
Figure 4. Chopped LSV scans for (a) CuV2O6 (2 μm, annealed at 500 °C for 2 h) and (b) Cu2V2O7 (1 μm, annealed at 450 °C for 2 h) measured in (black) 0.1 M NaBi buffer solution (pH 9.2) and (red) 0.1 M NaBi buffer solution with 0.1 M Na2SO3 (pH 9.2) under frontside illumination of AM 1.5G simulated solar light (100 mW/cm2). The insets show the onset potential. The chopping rate is about 1 time per second. The scan rate was 25 mV/s.
Figure 3. SEM images of CuV2O6 and Cu2V2O7 films on FTO substrate. (a), (b), and (c) represent the top views with various magnification times of the CuV2O6 film with 2 μm thickness. (e), (f), and (g) denote the top-view SEM images of a 1 μm thick Cu2V2O7 film with various magnifications. (d) and (h) are the cross-sectional views.
films showed a less positive onset potential and a higher photocurrent at the potential of 1.23 V vs RHE. A similar phenomenon has also been found for the Cu3V2O8 photoanode. (The photocurrent density for water oxidation of the Cu3V2O8 photoanode remained rather low (
