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. 2022 Oct 17;5(11):13142–13148. doi: 10.1021/acsaem.2c02597

Enhanced Charge Carrier Separation in WO3/BiVO4 Photoanodes Achieved via Light Absorption in the BiVO4 Layer

Ivan Grigioni †,*, Annalisa Polo †, Maria Vittoria Dozzi †, Kevin G Stamplecoskie ‡, Danilo H Jara §, Prashant V Kamat ⊥, Elena Selli †,*
PMCID: PMC9709765  PMID: 36465258

Abstract

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Photoelectrochemical (PEC) water splitting converts solar light and water into oxygen and energy-rich hydrogen. WO3/BiVO4 heterojunction photoanodes perform much better than the separate oxide components, though internal charge recombination undermines their PEC performance when both oxides absorb light. Here we exploit the BiVO4 layer to sensitize WO3 to visible light and shield it from direct photoexcitation to overcome this efficiency loss. PEC experiments and ultrafast transient absorption spectroscopy performed by frontside (through BiVO4) or backside (through WO3) irradiating photoanodes with different BiVO4 layer thickness demonstrate that irradiation through BiVO4 is beneficial for charge separation. Optimized electrodes irradiated through BiVO4 show 40% higher photocurrent density compared to backside irradiation.

Keywords: solar water oxidation, heterojunction, ultrafast transient absorption, photoactive layer thickness, filter effect

Bismuth vanadate, BiVO4, is a promising semiconductor oxide employed in photoanodes for the oxygen evolution reaction in water-splitting devices.1,2 Its stability in contact with aqueous electrolytes,3,4 its good visible light-harvesting capability,5 and its simple preparation through cheap wet techniques6 point to this material as a possible component of future commercial photoelectrochemical (PEC) cells. Furthermore, in the last 15 years the efficiency of BiVO4-based photoanodes (in terms of current density) rapidly grew from a few microamps per square centimeter in early reports to 4–5 mA cm–2, with prolonged continuous operation of photoelectrodes modified with oxygen evolution cocatalysts.3,4,6−13 However, the fast charge recombination of BiVO4-based electrodes still hampers the efficiency of this material.2,14

A way to overcome this intrinsic flaw is to couple BiVO4 with WO3 in the WO3/BiVO4 heterojunction where visible light harvesting BiVO4 sensitizes wider band gap WO3.15 BiVO4 photoanodes based on this heterojunction achieve the highest current densities among oxide-based photoanodes.16,17 The suitable band gap alignment between the two oxides, the efficient electron and hole transport in WO3 and BiVO4, respectively, and the spacial charge separation support the high performance of WO3/BiVO4 photoanodes.18−25

In previous studies, we investigated the charge carrier dynamics in the WO3/BiVO4 system through transient absorption spectroscopy (TAS) with detection either in the visible range to observe the hole dynamics in BiVO418,26−28 or in the mid-infrared to follow the electron dynamics in WO3 and BiVO4.29 We also identified wavelength-dependent processes by tuning the excitation wavelength across the WO3 absorption edge (ca. 450 nm).18,26,29 Indeed, the type II band alignment between the two oxides (Figure 1A) allows distinct charge transfer processes leading to charge separation or recombination, depending on the excitation wavelength. Under visible light excitation of BiVO4, electrons promoted in its conduction band (CB) flow into the energetically lower-lying CB of WO3, while holes remain in the BiVO4 valence band (VB). This electron transfer process (process Ⓐ in Figure 1A) decreases charge recombination and leads to long-living charge carriers that are beneficial for PEC performance.29

Figure 1.

Figure 1

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(A) Charge carrier transfer paths involved in the WO3/BiVO4 heterojunction: light absorption in BiVO4 produces valence band (VB) holes h+ and conduction band (CB) electrons e–, which can recombine with holes (with time constants τ1 and τ2) or flow into WO3 through process Ⓐ, leading to charge separation. When WO3 is also photoexcited, electrons in its CB can recombine with holes in BiVO4 through process Ⓑ. (B) Backside (through WO3) and frontside (through BiVO4) irradiation modes of the WO3/BiVO4 heterojunction photoanodes. (C) Percent amount of 387 nm photons (the pump wavelength used in TAS experiments) absorbed by the BiVO4 layer (left ordinate) and by the WO3 layer (right ordinate) under frontside excitation vs the thickness of the BiVO4 layer in WO3/BiVO4 photoanodes.

Conversely, irradiation at wavelengths below 450 nm leads to the excitation of both oxides and opens a detrimental recombination path between the electrons photopromoted in the CB of WO3 and the holes in BiVO4 (process Ⓑ in Figure 1A). This process results in charge recombination on a ∼200 ps time scale26 and becomes more relevant with increasing WO3 layer thickness.30

Based on these dynamics, we posited that an efficient heterojunction system needs to direct charges along process Ⓐ and disfavor off-track routes such as process Ⓑ. Still, solar light includes photons energetic enough to excite WO3 (∼4% of the solar spectrum is at wavelengths below the absorption edge of bulk WO3). Therefore, a portion of photogenerated charges in WO3/BiVO4 systems may be wasted through process Ⓑ. On the other hand, BiVO4 efficiently absorbs light beyond the absorption edge of WO3, up to 520 nm, allowing us to exploit a larger fraction of the solar spectrum. Therefore, in this work we pursue the strategy of using the BiVO4 sensitizer to shield WO3 from direct photoexcitation.

We assembled a series of heterojunction electrodes with a WO3 scaffold layer of fixed thickness (ca. 150 nm) coated with BiVO4 overlayers with different thickness (15–160 nm) to tune the amount of light absorbed by BiVO4. First, a systematic PEC study allowed us to probe whether the irradiation mode (through WO3 or BiVO4, backside or frontside irradiation, respectively, Figure 1B) affects the overall PEC efficiency of the electrodes. Then, transient absorption spectroscopy (TAS) with a pump in the UV region (387 nm) and detection in the visible range was employed to assess the effects of the irradiation mode on the lifetime of photogenerated holes in BiVO4. These tests allowed us to evaluate the extent of charge recombination induced by process Ⓑ and its impact on the PEC performance of the heterojunction photoanodes as a function of the BiVO4 layer thickness.

The WO3/BiVO4 photoanodes were prepared through spin coating using fluorine-doped tin oxide (FTO) as the conductive glass substrate (see the Supporting Information). The heterojunction electrode with the thickest BiVO4 layer almost entirely absorbs 387 nm photons, the pump wavelength in TAS experiments, Figure 1C. A series of control photoanodes consisting of pure BiVO4 on FTO (without WO3 layer) with variable BiVO4 thickness was also prepared. The absorption spectra of the two electrode series are shown in Figures S1 and S2; the thickness of the BiVO4 layer was estimated using the absorption coefficient at 420 nm,19 α40 = 6.7 × 104 cm–1. XRD analyses confirm the successful synthesis of WO3 and BiVO4 (Figure S3) and FESEM images demonstrate the uniform coating of the photoanodes (Figure S4).

In order to explore the shielding hypothesis, we carried out PEC experiments on the electrodes. Figure 2A, B shows the photocurrent density generated with the WO3/BiVO4 electrodes under simulated solar light irradiation in 0.5 M Na2SO4 solution under back- and frontside irradiation at different applied potentials. The linear sweep voltammetry plots are reported in Figures S5 and S6. As a general trend, all heterojunction photoanodes outperform control pure BiVO4 electrodes (see Figure S7). Furthermore, the better light exploitation achieved with increasing the BiVO4 layer thickness drives the photocurrent increase under both irradiation conditions up to a 75 nm thick BiVO4 layer.

Figure 2.

Figure 2

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Current density generated under 1 sun simulated solar light (AM 1.5 G irradiation) at different applied potentials with the WO3/BiVO4 heterojunction electrodes in contact with a 0.5 M Na2SO4 aqueous solution under (A) frontside and (B) backside irradiation; (C) linear sweep voltammetry recorded with the best performing WO3/BiVO4 electrode (with a 75 nm thick BiVO4 layer) under backside (black line) and frontside (red line) irradiation; the vertical dashed line indicates the standard oxygen evolution reaction potential.

Under frontside simulated solar light irradiation (Figure 2A), the heterojunction photoanodes generate considerably higher photocurrent than in backside mode (Figure 2B). The best performing electrode with a 75 nm BiVO4 layer thickness (Figure 2C), when irradiated frontside shows a ca. 40% increase in the current density, from 1.0 to 1.38 mA cm–2, with respect to backside irradiation, at the formal H2O/O2 redox potential of 1.23 V vs the standard hydrogen electrode (VSHE).

We used single-wavelength efficiency measurements to gather further information on this PEC performance increase. Specifically, internal quantum efficiency (IQE, Figure 3 and Figures S8 and S9), measuring the efficiency of absorbed photons, was calculated from the incident photon to current efficiency (IPCE, see Figures S10 and S11) recorded with the WO3/BiVO4 electrodes in contact with a 0.5 M Na2SO4 solution at 1.23 VSHE. Figure 3A, B shows the IQE vs BiVO4 thickness contour plots measured under frontside and backside irradiation.

Figure 3.

Figure 3

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Internal quantum efficiency (IQE) 3D contour plots (color scale) vs the incident wavelength and the BiVO4 film thickness of the WO3/BiVO4 electrodes, obtained in contact with a 0.5 M Na2SO4 aqueous solution at 1.23 VSHE under (A) frontside or (B) backside irradiation; (C) IQE plots under backside and frontside irradiation obtained with the best performing electrode (with a 75 nm thick BiVO4 layer).

Under frontside irradiation, the IQE reaches the highest values for 70–130 nm thick BiVO4 layers in the WO3/BiVO4 heterojunction, as evidenced by the red/yellow island appearing in Figure 3A as opposed to the green plot obtained under backside irradiation, which leads to lower IQE values (Figure 3B). Notably, the largest IQE enhancement under frontside irradiation occurs below 450 nm, where WO3 absorbs light, and for BiVO4 layers thicker than 50 nm, which absorb a substantial fraction of incident light. The IQE in this irradiation mode maintains above 30% up to 450 nm for the best-performing electrodes, while it is seldom above 25% under backside irradiation, see for example the IQE traces for the WO3/BiVO4 electrode with 75 nm thick BiVO4 in Figure 3C.

At the same time, the IQE curves are similar under the two irradiation modes in the 450–520 nm range because only BiVO4 absorbs light at these wavelengths and process Ⓐ (Figure 1A) is predominantly active. Thus, these wavelength-dependent PEC analyses indicate that the performance of the WO3/BiVO4 photoanodes benefits from avoiding WO3 excitation. This condition occurs by selectively exciting BiVO4 under frontside irradiation, i.e., by shielding WO3 with BiVO4, and in both irradiation modes under excitation at wavelengths above the WO3 absorption onset (Figure 3C).

We then investigated the effect of WO3 shielding on the lifetime of photogenerated holes in the BiVO4 layer of the WO3/BiVO4 system. Previous work ascribed the transient absorption ΔA signal at 470 nm to trapped holes in BiVO4, based on experiments in the presence of hole scavengers.18,21,31 TAS proved an essential tool for studying charge carrier dynamics and diffusion in photocatalysis and photovoltaics.32−35 Therefore, TAS with detection at 470 nm was here employed to investigate the dynamics of photogenerated holes in both pure BiVO4 and WO3/BiVO4 electrode series upon backside and frontside excitation at 387 nm.

The ΔA signals recorded with pure BiVO4 electrodes were analyzed first. For this system, similar transient dynamics were obtained in the two irradiation modes. Figure S12 reports representative transient absorption spectra, while Figure S13 shows the transient decay ΔA profiles at 470 nm, which were fitted according to a biexponential decay model (eq 1).

graphic file with name ae2c02597_m001.jpg 1

In this equation, τ1 and τ2 are the lifetimes of the faster and slower decay processes typical of BiVO4, respectively, A1 and A2 are the weighted coefficients that represent the contribution of each of the two processes to the overall decay and ΔA0 is the offset (set at zero in the fitting).21 The fitting parameters for the BiVO4 electrodes (Table S1) are in line with literature reports on pure BiVO4. Regardless of the BiVO4 thickness , A1 and A2 account of ∼30 and 70% of the hole decay, respectively. The fast decay lifetime, which is associated with the recombination of trapped holes in BiVO4 with photopromoted free electrons, is independent of the BiVO4 layer thickness (τ1, ∼20 ps), because all electrodes are excited at the same pump wavelength (i.e., with the same energy excess with respect to the BiVO4 CB).21,29 On the other hand, τ2, which is ascribed to the recombination of trapped holes with trapped electrons, increases from ∼1 to 6.5 ns with increasing BiVO4 layer thickness as more holes get trapped in bulk sites.28,36,37

The decay signal of photoproduced holes in the BiVO4 layer of the WO3/BiVO4 electrodes series recorded at 470 nm under backside and frontside irradiation are reported in Figure 4 and Figures S14 and S15. Under frontside excitation, the ΔA signals decay slower than under backside excitation (Figure 4A–C). Indeed, under backside irradiation mode a significant fraction of 387 nm photons is absorbed by WO3, leading to photoexcitation of electrons into its CB (the individual WO3 layer absorbs ca. 16% of 387 photons, Figure 1C). Therefore, many photoproduced charge carriers recombine through process Ⓑ (Figure 1A). This additional recombination channel leads to the abrupt ΔA drop observed during the first 400 ps following backside photoexcitation (Figure 4A). Furthermore, under frontside irradiation the ΔA signal recorded with the WO3/BiVO4 heterojunctions becomes progressively slower and comparable with those recorded with pure BiVO4 (Figure 4D–F and Figure S13).

Figure 4.

Figure 4

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Comparison between the normalized TAS decay signals in (A–C) WO3/BiVO4 electrodes with different BiVO4 thicknesses recorded under backside (through WO3, black squares) or frontside (through BiVO4, red triangles) irradiation and in (D–F) WO3/BiVO4 (red triangles) and pure BiVO4 films (blue diamonds) with the same BiVO4 thickness recorded under frontside irradiation. The solid lines are the fitting curves according to eq 1 (BiVO4 films) or eq 2 (WO3/BiVO4 films). Excitation pump at 387 nm, TAS signal monitored at 470 nm.

In previous studies on the WO3/BiVO4 heterojunction, we evaluated the contribution of process Ⓑ to the overall ΔA decay signal by fitting the ΔA decay traces including an additional decay component in eq 1, to take into account also process Ⓑ.26 Here, we used the same approach to assess its contribution to the charge carrier dynamics in the two irradiation modes and fitted the ΔA decay with eq 2.

graphic file with name ae2c02597_m002.jpg 2

where τr accounts for the additional recombination process and Ar is its weighted contribution.

We first fitted the dynamics recorded in the WO3/BiVO4 electrodes under backside excitation. The fitting parameters are reported in Table 1. In this configuration, the WO3 layer is irradiated directly and absorbs the same amount of light in all electrodes. Therefore, we expect that process Ⓑ has a comparable effect on the charge carrier dynamics at each BiVO4 layer thickness. Indeed, this process accounts for ca. 23 ± 8% of the holes decay, with a time constant τr of ∼200 ps (Table 1).

Table 1. Fitting Parameters of the TAS Dataset Collected with the WO3/BiVO4 Electrodes under Backside Excitation at 387 nma.

BiVO4 thickness (nm) A1 (%) τ1 (ps) Ar (%) τr (ps) A2 (%) τ2 (ns)
15 30 ± 1 7.4 ± 0.8 32 ± 3 170 ± 22 38 ± 3 0.98 ± 0.07
30 26 ± 2 6.2 ± 1.1 34 ± 3 189 ± 27 40 ± 3 1.9 ± 0.2
50 24 ± 2 7.8 ± 1.2 19 ± 2 156 ± 30 57 ± 2 1.99 ± 0.11
75 22 ± 1 15.8 ± 1.8 17 ± 2 215 ± 48 61 ± 3 1.44 ± 0.07
115 23 ± 2 22 ± 3 19 ± 2 210 ± 25 58.0 ± 0.3 3.57 ± 0.11
160 18 ± 1 13.1 ± 1.7 15 ± 1 159 ± 26 67.4 ± 0.9 3.05 ± 0.11
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a

Data fitted according to eq 2.

Under frontside irradiation, the BiVO4 layer in the heterojunction electrodes shields WO3 from light absorption as the BiVO4 layer thickness increases. Indeed, the percent amount of incident 387 nm photons absorbed by the WO3 underlayer in the coupled system progressively decreases (Figure 1C). Therefore, we sought to quantify the shielding effect of the BiVO4 layer in decreasing the extent of process Ⓑ in the WO3/BiVO4 electrodes. By assuming that process Ⓑ operates with its intrinsic time constant τr regardless of the excitation mode, we fitted the decay dynamics recorded under frontside irradiation by employing the τr previously extracted from the TAS signals recorded upon excitation in backside mode (Table 1). Because of the reduced amount of charge carriers generated in WO3, the weight of process Ⓑ in terms of the Ar parameter (Table 2) decreases with increasing the BiVO4 layer thickness. Additionally, as fewer charge carriers undergo process Ⓑ, A2 increases, suggesting that a larger number of photogenerated charge carriers recombines through the slower process. A 160 nm thick BiVO4 layer almost entirely absorbs the pump (Figure 1C), preventing WO3 excitation. Due to the lack of photoexcited electrons in the CB of WO3, the electrons photopromoted in the BiVO4 CB can only recombine with trapped holes in BiVO4, or flow into WO3 CB via process Ⓐ, resulting in better charge carrier separation. Consequently, the holes photogenerated in the BiVO4 layer of the WO3/BiVO4 heterojunction live longer than those in the individual 160 nm thick BiVO4 electrode (Figure 4F). This condition is akin to selective BiVO4 excitation in WO3/BiVO4 at wavelengths beyond WO3 absorption edge, which we previously observed extending the hole lifetimes compared to individual BiVO4.26,29

Table 2. Fitting Parameters of the TAS Dataset Collected with the WO3/BiVO4 Electrodes under Frontside Excitation at 387 nma.

BiVO4 thickness (nm) A1 (%) τ1 (ps) Ar (%) τr (ps) A2 (%) τ2 (ns)
15 25 ± 1 13 ± 1 20 ± 1 170 55 ± 1 1.55 ± 0.03
30 23 ± 3 15 ± 2 27 ± 3 189 50 ± 3 2.8 ± 0.4
50 21 ± 3 19 ± 7 13 ± 5 156 66 ± 2 3.0 ± 0.3
75 23 ± 3 10 ± 3 12 ± 4 215 65 ± 2 3.1 ± 0.4
115 20 ± 1 24 ± 4 4 ± 2 210 76.3 ± 0.8 5.4 ± 0.3
160 27 ± 1 21 ± 2 - - 73.1 ± 0.3 10.6 ± 1.2
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a

Data fitted according to eq 2 using the τr values reported in Table 1.

Thus, TAS and PEC experiments suggest that light absorption by the BiVO4 layer in WO3/BiVO4 electrodes selectively suppresses process Ⓑ and promotes process Ⓐ, which leads to an increase of trapped hole lifetime in BiVO4. However, despite the promise of long-living holes in the heterojunction electrode with a 160 nm thick BiVO4 layer, it performs poorly compared to the most active heterojunction with the 75 nm thick BiVO4 layer. This latter electrode possesses an optimal matching between (i) WO3 sensitization to the visible light, (ii) photogenerated charge separation at the heterojunction, and (iii) efficient charge extraction toward the external circuit and the electrolyte. Indeed, thinner BiVO4 layers limit the electrode performance due to the low visible light absorption, while thicker films may suffer from a greater charge recombination owing to hole accumulation in the BiVO4 bulk under operando conditions.

In conclusion, we identified a shielding strategy to suppress the internal charge recombination occurring in the WO3/BiVO4 heterojunction due to WO3 excitation. Optimized light absorption in BiVO4 layers considerably suppresses this recombination channel. The best performing electrode tested in this work shows a 40% increase in the PEC performance under frontside irradiation compared to backside irradiation. Furthermore, these findings suggest that methods to suppress undesired wavelength-dependent recombination processes and optimize charge transport and surface catalysis are required to design efficient photoelectrodes based on type-II heterojunctions.

Acknowledgments

I.G. acknowledges the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie Grant Agreement 846107. This work received financial support from the MIUR PRIN 20173397R7MULTI-e project. The use of instrumentation purchased through the Regione Lombardia-Fondazione Cariplo joint SmartMatLab project (Fondazione Cariplo grant 2013-1766) is gratefully acknowledged. P.V.K. acknowledges the support of the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy, through award DE-FC02-04ER15533. This is contribution number NDRL 5380 from the Notre Dame Radiation Laboratory.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsaem.2c02597.

  • Experimental details, absorption spectra, XRD and FESEM analyses, linear sweep voltammetry tests, IQE and IPCE plots, TAS decay profiles, and fitting parameters (PDF)

The authors declare no competing financial interest.

Supplementary Material

ae2c02597_si_001.pdf (1.5MB, pdf)

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