Article pubs.acs.org/JPCC
Origin of the Different Photoelectrochemical Performance of Mesoporous BiVO4 Photoanodes between the BiVO4 and the FTO Side Illumination Shuang Xiao,†,‡ Haining Chen,† Zhengshi Yang,§ Xia Long,† Zilong Wang,† Zonglong Zhu,† Yongquan Qu,‡ and Shihe Yang*,† †
Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China Joint School of Sustainable Development and Center of Applied Chemical Research, Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an, China § Department of Physics and Astronomy, University of Kentucky, Lexington, Kentucky 40506, United States ‡
S Supporting Information *
ABSTRACT: Understanding charge separation and charge transport in mesoporous semiconductor films is crucial to designing high efficiency photoelectrochemical water splitting cells. In the present work, we systematically study the origin of the higher photoelectrochemical performance of mesoporous BiVO4 film under FTOside illumination (F-illumination) than that under BiVO4-side illumination (Billumination). Via intensity-modulated photocurrent spectroscopy in conjunction with modeling simulation of electron diffusion inside mesoporous BiVO4 films with different thicknesses, we find that the F-illumination is more tolerant to recombination than the B-illumination, leading to a higher charge separation efficiency of the former. Specifically, we have identified a trap-free electron transport region of BiVO4 vicinal to the FTO substrate and a trap-limited transport region farther away under F-illumination, whereas only a trap-limited transport exists under B-illumination. Simulated results accord well with the experimental data and further provide a deep insight of the detailed electron transport behavior: it is the higher electron density in the region proximal to the FTO under F-illumination that has led to the greater recombination tolerance than under B-illumination. Such a photogenerated electron transport characteristic in mesoporous films is expected to be common for other semiconductors and will inspire practicle strategies for designing high efficiency semiconductor nanostructure-based photoelectrochemical devices.
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INTRODUCTION Solar energy, which can be converted into a variety of other energy forms including chemical fuels, wind, heat, and tide, is the primary source that sustains all lives in our planet. Mimicking the solar-to-chemical energy conversion machinery of nature is one of the most promising strategies to meet the challenges of fossil fuel shortage and environmental problems.1,2 One way to realize this goal is through photoelectrochemical (PEC) water splitting in a controllable fashion with artificial devices, which aim to convert the energy of solar photons into a usable energy stored in the chemical bonds of, for example, H2, thereby completing the solar-to-chemical energy conversion.3−6 After decades of research, the key challenge remains to be the design and construction of a robust and efficient PEC device yet at an acceptable cost.7 BiVO 4 is one of the most promising materials for photoelectrochemical water splitting because of its high chemical stability, abundant storage in earth crust, relatively low bandgap (∼2.4 eV), and suitable valence/conduction band edges.8−14 Strides have recently been made to promote the light-harvesting efficiency, shorten the hole diffusion distance, and reduce the surface recombination rate of BiVO4, by doping © 2015 American Chemical Society
with appropriate atoms, employing potent oxygen evolution catalysts, and tailoring nanoscale structures. Impressively, high efficiencies of the BiVO4-based PEC cells have been achieved (4.2 mA cm−2 at 1.23 V vs RHE).10 However, there is still much room for innovation because this current is still well below the theoretical photoinduced current density of a 2.4 eV bandgap material, viz., 7.5 mA cm−2.11 Commonly, the pristine BiVO4 PEC cells are tested under illumination incident from the FTO side (F-illumination) for the larger current density than incident from the BiVO4 side (B-illumination).15−18 However, the F-illumination has two notable drawbacks: (1) the incident light is filtered by the FTO, leading to a ∼20% solar energy loss and (2) it is incompatible with tandem devices, in which a narrower bandgap semiconductor needs be joined to the wider bandgap BiVO4 in such a way that the light should be incident to the latter layer before entering the former layer. It has been demonstrated previously that the hole transport inside BiVO4 (diffusion length ∼100 Received: August 3, 2015 Revised: September 24, 2015 Published: September 25, 2015 23350
DOI: 10.1021/acs.jpcc.5b07505 J. Phys. Chem. C 2015, 119, 23350−23357
Article
The Journal of Physical Chemistry C
Characterization. The morphologies of BiOI and BiVO4 deposited on FTO were characterized by scanning electron microscopy (SEM, JEOL 6700F). Crystal structure identification was determined by X-ray diffraction (XRD, Philips PW1830 X-ray diffractometer, λ= 1.5406 Å, 298 K, Cu Kα radiation). The valence of elements was measured by X-ray photoelectron spectroscopy (XPS, PerkinElmer model PHI 5600 XPS system, Mo Kα radiation). Ultraviolet−visible (UV− vis) absorption and transmission spectra were measured using an UV−vis spectrophotometer (PerkinElmer, model Lambda 20). For diffuse reflectance mode, BaSO4 was taken as the reference. Photoelectrochemical Measurement. Photocurrent measurements utilized simulated solar illumination obtained by a 450 W Xe lamp with AM 1.5 global filter (Oriel solar simulator, calibrated to 1 sun). Photoelectrochemical (PEC) measurement was performed using the IM6x electrochemical workstation (ZAHNER-Elektrik GmbH & Co.) in a conventional three-electrode cell with a 0.5 M KPH buffer + 1 M Na2SO3 (pH = 7.5) mixed solution as the electrolyte. The asprepared BiVO4 was employed as the working electrode, while a platinum wire and Ag/AgCl (Sat.) were utilized as the counter and reference electrode, respectively. Linear scan voltammetry (LSV) was employed to measure the photocurrent of as-prepared samples at a scan rate of 10 mV/s. For one thickness, seven samples were tested to obtain a statistical result. All results in this work were presented against the reversible hydrogen electrode (RHE) for comparison by applying the Nernst equation20
nm) is no longer a limiting factor to the efficiency when BiVO4 is made mesoporous with a feature size of 76 nm.10,14,19 Consequently, the electron transport properties in BiVO4-based devices become crucial because they can limit the charge separation efficiency of BiVO4-based PEC cells and may lead to different PEC efficiencies for the different modes of illumination defined above.14,15,17,18 We believe that understanding the mechanism of electron transport inside BiVO4 mesoporous nanostructures under light illumination through systematic studies will help us to find ways to further improve the PEC efficiency of BiVO4-based cells. In this work, we systematically study the electron transport properties of nanostructured BiVO4 films with different thicknesses under different modes of illumination by combining PEC testing and intensity-modulated photocurrent spectroscopy (IMPS). For a fair comparison, (1) different thicknesses of mesoporous BiVO4 films are synthesized but with a similar feature size, and (2) surface recombination due to poor surface catalytic reaction kinetics is excluded by adding hole scavengers.10 To complement the experimental measurements of electron transport properties, simulation based on the multiple-trapping (MT) model is performed, providing a deeper insight into the relationship between the illumination mode, electron transport, and PEC efficiency of BiVO4-based PEC cells. The different PEC performance under the different modes of illumination is attributed to (1) the dominant fast trap-free electron diffusion mode proximal to FTO under Fillumination and to (2) the higher recombination probability under B-illumination than under F-illumination. Also, the origins of recombination tolerance under F-illumination and of favorable electron diffusion toward the electron collector under B-illumination are exposed through the simulation studies with BiVO4 films of different recombination characteristics and thicknesses.
E RHE = EAg/AgCl + 0.059 pH + 0.1976 V
(1)
Incident photon-to-electron conversion efficiency (IPCE) measurements were carried out on a Jobin-Yvon Triax 190 monochromator at a constant bias of 0.9 V vs RHE. IMPS were taken by the Zahner Zennium C-IMPS system with a purple light-emitting diode (λ = 421 nm). Detailed simulation methodology is in the Supporting Information.
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EXPERIMENTAL SECTION Synthesis of BiVO4 Electrode. First, BiOI electrodes were synthesized on the FTO substrate by the electrodeposition method.10,12 Bi(NO3)3·5H2O (0.04 M) (Merck) was dissolved in 50 mL of 0.4 M KI (RDH) solution after its pH was adjusted to 1.7 by adding HNO3 (VWR). The resulting solution was mixed with 20 mL of 0.23 M 1,4-benzoquinone (SigmaAldrich) in ethanol and stirred until well mixed. Electrodeposition was carried out in a conventional three-electrode cell in a CHI 660D electrochemical workstation. A 1 cm × 2 cm fluorine-doped tin oxide (FTO) glass working electrode, an Ag/AgCl (Sat.) reference electrode, and a platinum counter electrode. For 750 and 550 nm thick samples, electrodeposition was performed by cyclic voltammetry from 0 to −1.1 V and −0.9 V at a 5 mV/s scan rate. For 450, 250, and 150 nm thick samples, electrodeposition was performed potentiostatically at −0.13 V vs Ag/AgCl for 240, 80, and 30 s. All of the processes were performed at room temperature. The as-prepared BiOI electrodes were purged thoroughly by ultrapure water (UPwater, 18 MΩ) and ethanol. Vanadylacetylacetonate (0.2 M) (VO(acac)2, Sigma-Aldrich) dissolved in dimethyl sulfoxide (DMSO, Sigma-Aldrich) was drop casted onto the BiOI electrode with a volume of 200, 150, 120, 100, and 80 μL for 750, 550, 450, 250, and 150 nm thick sample each. Then, they were heated in a box furnace at 420 °C for 1 h at a ramping rate of 2 °C/min. Excess V2O5 on the electrode was removed by immersing in 1 M NaOH for 30 min. The resulting BiVO4 electrode was washed with UP-water.
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RESULTS AND DISCUSSION Mesoporous BiVO4 Photoanodes. Mesoporous BiVO4 was synthesized by two-step methods. First, BiOI nanoflakes were deposited on the FTO substrate by electrodeposition. Then VO(acac)2 DMSO solution was dropped on their surface, followed by heating treatment in air at 420 °C for 1 h to get the final mesoporous BiVO4.10,12 The SEM images show the top (Figure 1a) and side (Figure S1a−c) views of the as-prepared sample, which is composed of flake-like crystals with a thickness of around 20 nm. After the thermal annealing treatment, as shown in Figure 1b, nanoflakes were converted into a wormlike porous structure composed of the nanoparticles about 120 nm in size. The XRD pattern in Figure 1c confirms the full conversion of the BiOI phase to BiVO4 phase. High-resolution XPS spectra on Bi 4f (Figure S2a) and V 2p (Figure S2b) of the BiVO4 sample inform the entity of Bi3+ and V5+ ions in the oxides reported previously.17 The bandgap of mesoporous BiVO4 is estimated as 2.5 eV from the UV−vis absorption spectrum (Figure S3). In addition, the film thickness exhibits little change before and after the thermal annealing procedure, as shown in Figure S1. Therefore, the thickness of BiVO4 films can be tuned by changing the BiOI film thickness, which in turn can be 23351
DOI: 10.1021/acs.jpcc.5b07505 J. Phys. Chem. C 2015, 119, 23350−23357
Article
The Journal of Physical Chemistry C
about the performance limits of the different illumination modes to guide the future PEC designs. Toward this end, we tuned the film thickness of mesoporous BiVO4 and changed the illumination mode to elucidate the relationship between light absorption, electron transport, and charge separation. As mentioned above, the particle sizes in all of the BiVO4 films were made similar and uniform to minimize the influence of other factors such as hole diffusion on the electron collection efficiency. What is more, we employed Na2SO3, which has an extremely fast oxidation kinetics, as the hole scavenger to neutralize the influence of surface recombination of holes.10,12,21 The typical photocurrent density−potential curves (J−V curves) of 450 nm BiVO4 are shown in Figure 2a. A relatively low onset potential (0.2 V vs RHE) and a rapid increase of photocurrent in the range of 0.3−0.7 V lead to the photocurrent plateaus at a relatively low bias value (∼0.9 V). In the following, we select the current density at 0.9 V, where no dark electrocatalytic reaction occurs but the photocurrent plateaus, to comparatively examine the film thickness dependent current density obtained under the F-illumination and under the B-illumination. First, when the film thickness is below 450 nm, both the F-illumination current density (jf) and the B-illumination current density (jb) increase in the same way with the film thickness (Figure 2b, region I). In this region, light absorbance dominates the current density: thicker films absorb more light and thus produce a higher current density. However, when the film thickness is increased to beyond 450 nm, the jf and jb show different changing trends (Figure 2b, region II). Specifically, jb goes down with increasing film thickness, whereas jf increases monotonically with film thickness and finally levels off at about 1.8 mA/cm2. The corresponding J−V curves with different thicknesses under different illumination modes can be found in the Supporting Information (Figure S4a−e).
Figure 1. Top-view SEM images of (a) BiOI nanoflakes and (b) mesoporous BiVO4. (c) XRD patterns of BiOI and BiVO4.
controlled by varying electrodeposition conditions (see details in the Experimental Section). For our systematic studies, we mainly chose the film thicknesses of 250, 450, and 750 nm because the film quality and morphology at these thicknesses are nearly identical, which is a prerequisite for making meaningful comparisons. Film Thickness Dependent Photocurrent Density. Previous experiments showed that for BiVO4 photoanodes in PEC the FTO-side illumination exhibits a larger photocurrent (jf) than that under the BiVO4-side illumination (jb).10,11,14 The difference was imputed to the poor carrier transport resulting in the more efficient electron collection from near the interface with the FTO region than from the far-off regions. More quantitative studies are warranted to untangle the light distribution and electron transport and have a better idea
Figure 2. Effect of film thickness on photocurrent efficiency. (a) J−V curves of 450 nm BiVO4 in darkness (black curve), under B-illumination (red curve), and under F-illumination (blue curve). (b) Dependence of current density at 0.9 V on the BiVO4 film thickness under different illumination modes. (c) Lambert−Beer simulation of the light intensity in BiVO4 films of different thicknesses under different illumination modes (reflective loss neglected). (d) APCE values at 420 nm for different thicknesses of the BiVO4 films under different illumination modes. The inset plot is the APCE ratio of B-illumination to F-illumination. The scan rate is 10 mV/s. 23352
DOI: 10.1021/acs.jpcc.5b07505 J. Phys. Chem. C 2015, 119, 23350−23357
Article
The Journal of Physical Chemistry C According to Lambert−Beer’s law, εL = lg(1/T), where ε is light absorption coefficient; L is film thickness; and T is film transmittance. The exponential attenuation of the incident light with thickness is reflected in the simulation result presented in Figure 2c for films with different thicknesses under different modes of illumination. Under the F-illumination, obviously, most of the incident light is absorbed in the near FTO region (
