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Tailoring multilayered BiVO photoanodes by pulsed laser deposition for water splitting Sebastián Murcia-López, Cristian Fabrega, Damián Monllor-Satoca, María D. HernándezAlonso, Germán Penelas-Pérez, Alex Morata, Juan Ramon Morante, and Teresa Andreu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11698 • Publication Date (Web): 25 Jan 2016 Downloaded from http://pubs.acs.org on January 30, 2016

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ACS Applied Materials & Interfaces

Tailoring multilayered BiVO4 photoanodes by pulsed laser deposition for water splitting Sebastián Murcia-López,a* Cristian Fàbrega,a† Damián Monllor-Satoca,a‡ María D. HernándezAlonso,b Germán Penelas-Pérez,b Alex Morata,a Juan R. Morante,a, c and Teresa Andreua* a

Department of Advanced Materials for Energy, Catalonia Institute for Energy Research (IREC),

Jardins de les Dones de Negre, 1, 08930 Sant Adrià de Besòs, Catalonia, Spain. b

Repsol Technology Center, Carretera de Extremadura A-5, km 18, 28935 Móstoles, Madrid,

Spain. c

Department of Electronics, Universitat de Barcelona (UB), Martí i Franquès, 1, 08028

Barcelona, Catalonia, Spain.

KEYWORDS: pulsed laser deposition; BiVO4; WO3; water photosplitting; solar energy

ABSTRACT Pulsed Laser Deposition (PLD) is proposed as promising technique for the fabrication of multilayered BiVO4-based photoanodes. For this purpose, bare BiVO4 films and two heterojunctions, BiVO4/SnO2 and BiVO4/WO3/SnO2, have been prepared using consecutive

ACS Paragon Plus Environment

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ablation of assorted targets in a single batch. The ease, high versatility and usefulness of this technique in engineering the internal configuration of the photoanode with stoichiometric targetto-substrate transfer are demonstrated. The obtained photocurrent densities are among the highest reported values for undoped BiVO4 without oxygen evolution catalysts (OEC). A detailed analysis of the influence of SnO2 and WO3 layers on the charge transport properties due to the changes at the internal FTO/semiconductor interface is performed through transient photocurrent measurements (TPC), showing that the BiVO4/WO3/SnO2 heterostructure attains a significant decrease in the internal losses and reaches high photocurrent values. This study is expected to open the door to the fabrication of other systems based on ternary (or even more complex) metal oxides as photoanodes for water splitting, which is a promising

alternative for obtaining

materials able to fulfill the different requierements in the development of more efficient systems for this process.

1. Introduction Photoelectrochemical (PEC) water oxidation based on bismuth vanadate (BiVO4) photoanodes has undergone a significant growth as topic of study during the last years. In comparison to other more classic binary semiconductors, BiVO4 has become one of the most interesting alternatives for the development of systems capable of exploiting a larger portion of the solar spectrum owing to its relatively low band gap value (2.4 eV)1,2 and photocurrent onset potentials for water oxidation as low as 0.1-0.3 V vs RHE, similarly to TiO2,3 given the favorable valence band (VB) position. In this sense, a maximum theoretical photocurrent of around 7.6 mA cm-2 at AM 1.5G (with a solar to hydrogen conversion efficiency, STH of 9.3%)4 could be achieved by


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photoanodes based on this semiconductor, which is well above the expected values for other classic oxides such as TiO22,5 and WO3.2 However, important electron transport and low kinetic drawbacks associated to this material greatly constrain its final performance.6 Accordingly, the most commonly reported photocurrent values for undoped BiVO4 are around 0.5 mA cm-2 7-10, far from the theoretical maximum value. For this reason, major efforts are devoted to the improvement of the intrinsic carrier transport through the semiconductor and/or its charge transfer kinetics at the semiconductor/electrolyte interface. W and Mo-doping8,9 and the deposition of oxygen evolution catalysts (OEC) such as Co-Pi6,10-12 or Ni and Fe oxyhydroxides13,14 are some of the most extended strategies in this direction, leading to more promising results. Regardless of its importance, most of the recent work on BiVO4 has been addressed to the aforementioned modification methods and almost no studies have been focused on the preparation technique and/or on obtaining more feasible manufacture procedures for an eventual large scale implementation. In this regard, many of the synthesis methods are related to solution processing techniques, involving chemical or electrochemical procedures1 as metal-organic decomposition via spin-coating,4 electrodeposition9 or spray pyrolysis,6 and frequently a combination of them. Thin film vapor deposition techniques represent a reliable alternative of major interest in terms of reproducibility, control and scalability. Additionally, these methods are versatile and allow depositing different films in a consecutive manner. Few reports can be found in literature regarding the preparation of BiVO4 photoanodes by thin film vapor deposition techniques, as CVD15 and reactive sputtering.7 Pulsed laser deposition (PLD), however, has not been actively exploited in this area, despite the advantages it offers as deposition technique for obtaining high-quality films of complex oxides with controlled properties and excellent transfer


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from a single target to the substrate.16,17 As the materials are homogeneously ablated using high energy densities, the target stoichiometry can be retained in the deposited film, while the tunable atmosphere inside the chamber allows obtaining crystalline sub-stoichiometric or stoichiometric oxides18 at lower temperatures than with other physical methods. Furthermore, PLD also allows depositing multilayered films of different materials through the successive ablation of the corresponding targets,19,20 without requiring several steps or other deposition techniques. This means that multiple layer configurations, each one targeting a different functionality, such as hole blocking layer15 or guest absorber21,22 can be easily prepared using only PLD in a single batch. To the best of our knowledge, there are very few reported works on PLD deposition of BiVO4. Initially, Liu and Yan23 reported the successful preparation of organic-inorganic hybrid solar cells based on ternary bismuth oxides (including BiVO4) by PLD. Later, Rettie et al.24 focused on the preparation of epitaxial BiVO4 photoanodes, with less attention on polycrystalline films. In that case, as their films were prepared at room temperature, they obtained very low photocurrent values. In the present work, we propose PLD as a promising and easy technique for preparing bare and multilayered BiVO4-based photoanodes in single-batch processes (by means of consecutive deposition of assorted targets). For this purpose, we explore three kind of electrodes that were prepared on top of fluorine-doped tin oxide (FTO) substrates: single BiVO4, a second electrode with a previous deposit of a thin SnO2 underlayer (so called “buffer” or “hole mirror“ layer) and a heterostructure combining the SnO2 buffer layer with a thicker WO3 interlayer (acting as guest absorber). Afterwards, the photoanodes were optically, structurally and photoelectrochemically


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characterized and evaluated. Finally, a detailed analysis was performed to accomplish a better understanding of the role played by the SnO2 and the WO3 layers.

2. Experimental 2.1. Preparation of the films BiVO4 films were prepared by Pulsed Laser Deposition (PLD) in a PLD 5000 equipment (PVD Products) with a 240 nm excimer KrF (λ = 248 nm) laser and four target positions. The ceramic home-made BiVO4 target was synthesized by solid-state reaction of the respective oxides as precursors.25 For this purpose, Bi2O3 (99.999%, Sigma-Aldrich) and V2O5 (99.99%, Sigma-Aldrich) were mixed in stoichiometric proportions (1:1), ground and heated at 800ºC for 10 h. Afterwards, the BiVO4 powder was grinded, mixed with some drops of Polyvinyl alcohol (1 wt%) and pressed to form a target of around 5 cm of diameter, which was finally sintered at 800ºC for 40 h in air. Commercial SnO2 (American Elements) and WO3 (Goodfellow) sputtering targets were used without pre-treatment. Fluorine-doped tin oxide (FTO) glass (Sigma-Aldrich, TEC 7 Ω/square) was used as substrate. After cleaning (10 min sonication) with a mixture of acetone/isopropanol/water (1:1:1 %vol) and dried under N2 stream, the substrate was inserted into the PLD vacuum chamber and heated at 300ºC under O2 pressure of 200 mTorr. The depositions were carried out by ablation of the target with laser pulses at a repetition frequency of 10 Hz, with a total energy set at ~150 mJ. These optimized parameters were selected after preliminary evaluation of films prepared at different conditions, such as lower O2 concentrations and deposition temperature (included as ESI, Figure S1 and S2).


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The deposition of multiple films was easily carried out by sequential ablation of the corresponding target. BiVO4 films with a SnO2 underlayer were prepared under the same conditions. The thickness of this layer was around 10 nm, as calculated from previously optimized depositions on Si wafers and measured by ellipsometry. A BiVO4/WO3/SnO2 heterostructure was also prepared. For this purpose, after the SnO2 underlayer deposition, a thick WO3 (~3.5 µm, O2 pressure of 100 mTorr) and a BiVO4 layer were sequentially deposited (O2 pressure of 200 mTorr). The obtained photoanodes were post-annealed at 500ºC for 3h in air atmosphere. In our preliminary studies, the thermal treatment performed at these conditions proved to have a beneficial effect on the photoresponse of the electrodes, albeit crystalline films were already obtained without post-annealing treatment. In all cases, the number of pulses was the determining parameter affecting the film thickness. For this reason, preliminary calibration samples (without buffer layer) were prepared under the same deposition conditions (temperature, pressure, laser energy) by only changing the number of pulses. Additionally, a BiVO4 electrode of larger geometrical area (~10 cm2) was also fabricated in order to probe the relatively easy scalability of the technique for obtaining uniform photoanodes (Figure S3).

2.2. Characterization The crystalline phase composition was determined by X-ray diffraction (XRD) measurements in a Bruker D8 Advance diffractometer equipped with a Cu Kα (1.54051 Å) radiation source, LYNXEYE super speed detector and Ni filter. A Bragg-Brentano (θ-θ) configuration was used, with 2θ range between 20-40º and a 0.0004º s-1 step. Crystalline domain size was calculated using the Scherrer equation, after fitting the selected XRD peak with a Voigt function. Optical


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characterization was performed in a Lambda 950 UV-Vis-NIR Spectrometer (PerkinElmer) equipped with a 150 mm integrating sphere coated with Spectralon as reflectance (white) standard. Transmittance and reflectance measurements were separately carried out for each sample in a λ range of 300-800 nm, with a 5 nm wavelength step. The surface morphology of the as-prepared samples was observed by field-emission scanning electron microscopy (FE-SEM, Zeiss Auriga). The thickness of the films was measured by profilometry technique performed with a KLA-Tencor P15 equipment with low force head. A 3D image of the heterostructure was obtained with a Sensofar non-contact optical 3D profiler.

2.3. Photoelectrochemical measurements Photoelectrochemical (PEC) measurements were conducted in a three-electrode system by using a Parstat 2273 potentiostat. A three-electrode quartz cell with a Pt mesh as counter electrode and Ag/AgCl/KCl (3M) (E0 = 0.203 V vs. RHE) reference electrode was used for this purpose. The electrolyte consisted of 0.5 M Na2SO4 in a 0.1 M sodium phosphate (NaPi) buffer solution (pH ~7). The results are presented against the reversible hydrogen electrode (RHE). Cyclic voltammetries with a sweep rate of 40 mV s-1 were recorded in the dark and under simulated AM 1.5G solar light (1 Sun, 100 mW cm-2) provided by a 150W solar simulator (Peccell, PEC-L01). Chronoamperometric measurements were performed at a constant bias of 1.23 VRHE with monochromatic illumination supplied by 300W a LS Xenon Light Source (ABET technologies) coupled to an Oriel Cornerstone 260 1/4m monochromator, calibrated using a silicon diode (Gentec-EO, XLPF12-3S-H2-DO). The transient photocurrent was recorded during illumination


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and dark pulses of 80 s and 40 s, respectively. This way, the incident photon-to-current conversion efficiency (IPCE) was estimated and corrected with the light harvesting efficiency (LHE) in order to calculate the absorbed photon-to-current efficiency (APCE) for the three photoanodes under study. 3. Results and discussion 3. 1. Structural and optical characterization Polycrystalline BiVO4 films were obtained as observed from the XRD patterns (Figure 1). In all the cases, only monoclinic BiVO4 (JCPDS 014-0688) was detected with no signals related to the tetragonal phase. Some additional peaks with relatively high intensities can be identified as tetragonal SnO2 (JPCDS 041-1445), corresponding to the FTO on the underlying glass substrate. A similar spectrum was observed in the BiVO4/SnO2 sample with some differences in the intensity of peaks. For this reason, the I040/I002 ratios of BiVO4 are also depicted in the figure, indicating that the thin SnO2 layer promotes the BiVO4 growth into the [040] direction. The XRD diagram of the BiVO4/WO3/SnO2 heterostructure exhibits very intense peaks associated to the monoclinic phase of WO3 (JCPDS 072-0677). A very clear orientation in the [020] direction can be observed for WO3, suggesting a preferential growth which further leads to a more evident orientation of BiVO4 in such direction (as indicated by I040/I002 ratio). Signals related to BiVO4 are magnified in the inset graph. BiVO4 crystalline domain sizes were determined with the Scherrer equation from the [040] peak, showing that the presence of the SnO2 underlayer favors the crystallite growth of BiVO4 from ~86 to 100 nm. By contrast, the calculated size for the BiVO4/WO3/SnO2 heterostructure was


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slightly smaller (~68 nm), probably due to the different growth process onto the WO3 thick layer, although it shows even higher I040/I002 ratio.

Figure 1. XRD patterns of the BiVO4 (A), BiVO4/SnO2 (B) and BiVO4/WO3/SnO2 (C) photoanodes. Signals corresponding to SnO2 (solid squares) and WO3 (open circles) are identified.


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In our case, deposition temperatures below 200ºC led to BiVO4 amorphous films, while fully polycrystalline samples were obtained at 300ºC. Post-deposition annealing over amorphous films can also be performed as it was done in the work of Rettie et al.24 However, according to their reported results and to our preliminary work (see Figure S4), very low photocurrent values are obtained for photoanodes prepared at room temperature, despite the fact that films crystallizes after thermal treatment. When the substrate is heated during the deposition, better responses are obtained afterwards, probably because of improved substrate-to-film contact and to better surface mobility of the adatoms, which results in denser films with less crystal defects.18,26 SEM images indicate that BiVO4 and BiVO4/SnO2 samples presented very similar surface morphologies (Figure 2A and 2B). The BiVO4 layer consisted of irregular roundish grains. A cross-section image of the BiVO4/SnO2 sample revealed a BiVO4 thickness of around 198 ± 10 nm, with the two layers corresponding to the FTO and the BiVO4 well differentiated (the SnO2 layer cannot be well observed because of its small thickness). Later estimation of the films thicknesses were obtained from profilometry measurements. From these, the thickness of the BiVO4/SnO2 and BiVO4 samples were 175 and 190 nm, respectively, which are in good agreement with the SEM cross-section image. A SEM image of the BiVO4/WO3/SnO2 heterostructure can be found in Figure 2C. The surface morphology of this sample is different from the other ones. Under the selected deposition conditions, the WO3 layer presented a columnar growth (see Figure S5), commonly observed in PLD-fabricated films under relatively similar deposition conditions,27 so that the BiVO4 layer growth on top is conditioned by the WO3 columnar morphology.


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Figure 2. SEM top view of the BiVO4 (A), BiVO4/SnO2 (B) and BiVO4/WO3/SnO2 (C) samples. In the lower part, the cross-section image of BiVO4/SnO2. EDX semi-quantitative analyses were also performed, including an initial estimation of the home-prepared target composition. According to the analysis over several zones on the surface, the molar Bi:V ratio in the target was around 1 (Figure S6). A further analysis on the prepared samples indicated that a direct chemical transfer from the target to the substrates was achieved (Figure S7). The thickness of the heterostructure was measured from the 3D image (Figure S8). A mean value of 3.7 µm was obtained, which is in agreement with the nominal thickness. The improved photoelectrochemical response of BiVO4/WO3 heterojunctions with different WO3 to


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BiVO4 thickness ratios has been addressed by Hong et al.28 In their work, the composite with the thickest WO3 layer led to the best photocurrent density. The absorption coefficient (α) and the penetration depth (δ) of BiVO4 were estimated from the UV-vis optical spectrophotometry measurement of the BiVO4/SnO2 sample (Figure 3) using:29

= · ln

(1)

where T and R are the measured transmittance and reflectance respectively, and d the thickness of the film. Three different absorption situations are depicted as well, i.e. δ = 1/α, 2/α and 3/α corresponding to 63%, 83% and 96% of absorbed light. Additionally, exponential fittings were applied in the long wavelength region (460-495 nm) and extended to the shorter region where the absorption coefficient cannot be properly determined because of the low transmittance values. Considering the thickness of our BiVO4 films (~200 nm), a full absorption of the solar light is only achieved in the range of 400 to 460 nm. From there on, the absorption of light decreases up to a mere 63% at longer wavelengths (λ = 485 nm) closer to the band gap. In order to attain a complete absorption of all photons above the BiVO4 band gap energy, a film thickness larger than 0.5 µm would be needed from the optical point of view. Nevertheless, from a photoelectrochemical perspective, it is necessary to also consider the transport properties for determining the maximum thickness along which the carriers will be effectively separated, i.e. the sum of the effective diffusion length (Lp) and the width of the depletion layer (w). According to the literature, Lp values of around ~70-200 nm could be expected.12,30 Likewise, reported w values for doped and undoped BiVO4 from 3 to 36 nm can be found in literature.12,31 Thus, thicker films than 230 nm would be above the maximum limit for carrier transport and no significant improvement in terms of photocurrent would be obtained.


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Figure 3. Penetration depth (δ) and absorption coefficient (α) values vs. incident wavelength for the BiVO4/SnO2 electrode. Solid lines represent exponential data fittings. In this context, the estimated α and δ values for a single WO3 film of the same thickness than the intermediate layer deposited on the BiVO4/WO3/SnO2 heterostructure have been included in Figure S5. As can be seen in the graph, for 63% absorption, the maximum absorbed wavelength for WO3 is around 450 nm, which agrees well with its band gap value. Therefore, when the heterostructure is back-illuminated, wavelengths above 450 nm can be transmitted through the WO3 to the BiVO4 layer, which is able to absorb photons of this energy range.


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3.2. Photoelectrochemical measurements Photoelectrochemical activity of the BiVO4-based photoanodes were determined by means of cyclic voltammetric measurements under front (EE) and back (SE) illumination in a buffered phosphate solution (pH 7). The most significant differences among samples were found under EE illumination (Figure 4A) with a progressive improvement in the photocurrent as SnO2 and WO3 layers are deposited on the FTO prior to the deposition of the BiVO4 film. These values do even increase under SE illumination (Figure 4B) for the bare BiVO4 and BiVO4/SnO2 electrodes, as photons of shorter wavelengths are absorbed closer to the back contact and electrons can be efficiently extracted, thus attenuating the inherent electron transport limitations of BiVO4.1,7 In the case of BiVO4/WO3/SnO2, as the WO3 interlayer has a significant thickness in comparison to the BiVO4 layer, when the sample is back illuminated, most of the photons are absorbed in the WO3 and thus are not able to reach the BiVO4, giving rise to a current density mainly due to WO3 contribution (see Figure S9 in ESI). Still, this heterostructure leads, for instance, to a photocurrent value at 1.23 VRHE (jph1.23V) of 1.5 mA cm-2 without H2O2 and under SE illumination conditions, larger than a recently reported result by Saito et al.32 for a BiVO4/SnO2/WO3 system reaching up to 1.0 mA cm-2 with a similar electrolyte.


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Figure 4. Cyclic voltammetries obtained with the three photoanodes in 0.5M Na2SO4 with 0.1M NaPi buffer solution (pH 7) as electrolyte at several conditions: without hole scavenger with front EE (A) and back SE illumination (B); with 0.2M H2O2 as hole scavenger with EE (C) and SE illumination (D). Dotted curves represent dark current measurements.


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As previously indicated, additional bare BiVO4 films prepared with different number of pulses (i.e. different thickness) were also evaluated. The jph1.23V values extracted from the cyclic voltammetries (Figure S10) reveal that a thickness close to 200 nm is the optimum one, which in fact constitutes a good commitment between the optical performance (Figure 3) and the transport properties of the material, as it has been discussed above. Besides the effect associated to the SnO2 buffer layer or to the WO3 interlayer, promising current densities are obtained even with the BiVO4 sample. In comparison to the values reported in literature for bare BiVO4 photoanodes deposited by other techniques, PLD stands out as a very interesting alternative: for instance, jph1.23V values of 0.8-1.2 mA cm-2 have been reported for BiVO4 and BiVO4/SnO2 electrodes of the same thickness prepared by CVD and SnO2 sputtering in presence of Na2SO3 as hole scavenger, which are below our current results without hole scavenger.15 Moreover, most of the reported values for bare BiVO4 are close to 0.5 mA cm-2 or below,6,7,33 which is less than half the value obtained for our single BiVO4 electrode. The advantages of having porous or dense films have been extensively considered in literature. Porous films offer better semiconductor/electrolyte contact,31 while dense and crystalline films may present less recombination due to the decreased crystalline defects (recombination centers). One of the advantages of PLD is that it is possible to obtain a combination of both features (as particularly observed in the heterostructure): columnar morphologies consisting of crystalline particles with some porosity allowing contact with the electrolyte. In our case, the high attained crystallinity (i.e. large crystallite sizes) clearly contributes to the quality of the BiVO4 film, while the columnar growth especially observed in the heterostructure serves to improve the final performance.


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Besides the inherent drawbacks related with carrier transport, a second important limitation of BiVO4 is the slow oxidation kinetics at its surface. Therefore, most of the studies involve either the deposition of an oxygen evolution catalyst (OEC) on the surface13 or the addition of hole scavengers, such as Na2SO3,15,24 CH3OH or H2O26,34 to the electrolyte. For this reason, we performed additional voltammetric measurements in the presence of H2O2. The results are included in the lower part of Figure 4 for both illumination conditions. Significant changes in the voltammograms are observed, compared to those previously obtained in the absence of any hole scavenger. An improved fill factor (i.e. current-voltage curve shape) and current density (more than 2 mA cm-2 at 1.23 V vs. RHE for all the photoanodes under SE illumination) can be observed for all samples. Comparison with the dark current curves shows that the three electrodes present onset potentials of around 0.6 VRHE without hole scavenger, although all of them exhibit an initial photocurrent in the range of 0.4-0.6 VRHE, probably related to the effect of surface states. Particularly in the case of the heterostructure, which presents the highest dark current, this behavior resembles the voltammetry of single WO3 (see Figure S9). On the contrary, in presence of H2O2, the onset potentials suffer cathodic shifts to ~0.35-0.45 VRHE, as the hole scavenger prevents Fermi level pinning.35 In order to better elucidate the influence of the hole scavenger, a “charge extraction efficiency” at the solid/liquid interface (ηCE) was calculated from the ratio between the jph1.23V values without and with H2O2 under EE illumination as defined in Equation 2. This calculus has been proposed in literature for the estimation of the yield of hole transfer at the Semiconductor/Electrolyte interface and should become 100% in presence of a hole scavenger, as the transfer kinetics limitations are overcome.31,34,36


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=

. !"#$ %& ' . !"#$ (& '

) 100

Page 18 of 35

(2)

In that sense, similar photocurrent values without and with hole scavenger (i.e. ηCE values closer to 100%) are desirable. The ηCE values for the three electrodes are depicted in Figure 5. The extraction efficiency follows the tendency BiVO4/WO3/SnO2 > BiVO4/SnO2 > BiVO4. A lower efficiency is found for bare BiVO4, which implies that this sample presents poorer hole injection to the electrolyte at the surface in comparison to the other ones. BiVO4/SnO2 and BiVO4/WO3/SnO2, on the other hand, present higher nCE values, which means that better charge extraction processes take place at the surface as a result of the effect of the under- and interlayers. This can be understood as the improved electron collection at the internal SnO2/FTO interface leads to lower recombination with the accumulated holes at the surface, which in the end results in higher current density without hole scavenger.

Figure 5. “Charge extraction efficiency”, ηCE, and “transport efficiency”, ηt, for the three photoanodes under EE illumination.


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Besides the previous calculation, the transport efficiency (ηt) was calculated from the ratio expressed in Equation 3,10,12,34 where jabs is the photocurrent calculated by integrating the light absorption with respect to the AM 1.5G solar spectrum:

=

,-. )/01

) 100

(3)

The thus obtained separation efficiencies (ηt), calculated at 1.23VRHE under EE illumination for the three photoanodes confirm that the presence of the SnO2 and WO3 layers has a significant improvement in the transport mechanism, as also seen in Figure 5. The ratio between the photocurrent values obtained under EE and SE illumination was also calculated as figure of merit, similar to that employed in literature with IPCE for estimating diffusion lengths in mesoporous films.37,38 These results were included as ESI (see Figure S11). Both the SnO2 buffer layer and the WO3 interlayer enhance the electron transport, decreasing the difference between the responses obtained under both illumination conditions. Comparison between front and back illumination photocurrents has usually being addressed in order to qualitatively observe effects on transport properties.1,12 This way, ratios close to 100% are related to good electron transport, which in our case are obtained in the heterostructure. In this case, the hole scavenger also decreases the difference in the ratio between the samples. IPCE measurements with SE illumination under monochromatic light and polarization of 1.23 VRHE were carried out as previously indicated for the three electrodes. APCE was calculated from the IPCE and LHE values related to the optical properties (see ESI for more information). The curves at different wavelengths in presence of H2O2 as hole scavenger are represented in Figure 6. Higher efficiencies are progressively obtained in the multilayer systems


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(BiVO4/WO3/SnO2 > BiVO4/SnO2 > BiVO4), especially in the range below 405 nm. This aspect is easily understood in the heterostructure as, according to the optical characterization of WO3, photons of these wavelengths are absorbed on the WO3. In fact, most of the radiation above ~450 nm is supposed to reach the BiVO4 layer (as previously indicated), which accounts for the second maximum in the curve. Definitely, the presence of WO3 and SnO2 inter- and underlayers suppose important improvements in comparison to the APCE of the bare BiVO4.

Figure 6. APCE curves for the three electrodes at 1.23 V vs RHE under SE illumination in the presence of a hole scavenger (0.2M H2O2). The comparison of the transient current obtained during the illumination and dark periods at each wavelength revealed different behaviors among the three samples. The normalized pulses obtained under SE illumination at λ = 400 nm are represented in Figure 7. Cycles of 120 s with illumination periods of 80 s were chosen, being long enough for reaching stabilization of the transient curves while resembling short-time chronoamperommetric measurements. The transient decay profiles can give information about the photogenerated charge extraction capacity.39


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BiVO4 and BiVO4/SnO2 exhibit high initial current values (j0, determined at t = 0.5 s, for avoiding any influence of the system time constant) and a rapid initial decrease until reaching a stable photocurrent (jSS, determined at t= 80 s of illumination pulse), which was used for the IPCE calculation. It is interesting to observe more pronounced and rapid decays in the sample without SnO2 layer; moreover, a cathodic current peak is observed immediately after switching off the light, evidencing charge accumulation. This behavior might explain the lower APCE values obtained for the bare BiVO4 in comparison to the other two electrodes. A more dramatic change is seen in the shape of the transient pulses of the BiVO4/WO3/SnO2 heterostructure, with very close j0 and jss values, indicating a very stable photocurrent during all the pulse.

Figure 7. Normalized photocurrent pulses obtained from the IPCE measurements with the three materials under SE illumination (λ= 400 nm), without and with 0.2M H2O2 as hole scavenger.


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When the measurements were carried out in presence of H2O2 as hole scavenger, more stable transient curves were obtained for all the samples, which is in agreement with the behavior reported in literature for BiVO4 photoanodes.12 However, significant differences in the influence of the hole scavenger were found. The highest improvement is seen in the bare BiVO4, while a more conservative change is found in the BiVO4/SnO2 and almost insignificant in the BiVO4/WO3/SnO2 heterostructure. This is in agreement with the effect observed on the voltammetric measuremens. In order to quantify the influence of the hole scavenger, the ratio between the collected (Qc, calculated from the integration of the pulse defined by jss) and the initial charge (Qinitial, from the ideal pulse defined by j0 at t = 1s) was calculated and compared among the three photoanodes with and without hole scavenger (for more information see Figure S12). The estimated charge ratios are presented in Figure 8. According to this, the bare BiVO4 presents the lowest ratio of collected charge (~42%), which increases twofold (~86%) in presence of H2O2. This fact suggests that most of the losses are related to surface/interface processes by positive charge accumulation in this sample. On the other hand, the BiVO4/SnO2 sample presents a good collection ratio without H2O2 (~75%), which increases to 87% when the hole scavenger is added to the electrolyte. The presence of the SnO2 “buffer layer” suppresses the charge accumulation at the interface with the FTO and improves the charge collection at the surface. In the case of the BiVO4/WO3/SnO2 electrode, the influence of the hole scavenger is even less pronounced. Thus, under both conditions, the ratio of collected charge is of ~97-99%, which proves the significant charge separation efficiency attained by the heterostructure. As seen in Scheme 1, a built-in potential which favors electron transport from the surface to the back contact should be created in the BiVO4/WO3 interface. Moreover, this interface constitutes an


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energy barrier for the electrons in the conduction band to combine with trapped holes at the surface.

Figure 8. Comparison of the ratios between the collected (Qc) and initial (Q0) charges, estimated from the transient pulse at 400 nm (SE illumination) without and with hole scavenger. The Q0 value makes reference to the maximum charge that could be extracted from the initial photocurrent (j0), defined at t = 0.5 s because of instrumental limitations. 3.3. Effect of the SnO2 buffer layer In order to study the role of the SnO2 “buffer layer”, Mott-Schottky measurements were performed in dark conditions at a frequency of 20 kHz with the BiVO4 and BiVO4/SnO2 photoanodes (Figure S13). The curves were adjusted to a linear fitting; then, as the slope of the line is directly related to the number of carriers (ND), ND was calculated for the two samples (see


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ESI for details) and very similar values in the order of ~9x1018 cm-3 were obtained, which are in agreement to the values reported in literature.12,40 Concomitantly, Luo et al.41 have recently shown that Sn(IV) incorporation into BiVO4 leads to SnO2 phase segregation, indicating that Sn(IV) doping is unlikely to occur. Moreover, the length of the depletion region (w) was roughly estimated to be of around ~25 nm, which is also in the range of values proposed in literature.12,31 Therefore, SnO2 should have a negligible effect on the carrier concentration or on the depletion region. According to the results observed in the transient photocurrent measurements, the buffer layer is considered to mainly act as “hole mirror” and “passivating layer” on the FTO surface,4,15 rather than having a doping effect on BiVO4. Additionally, this layer contributes to a better quality and crystalline orientation of the BiVO4 film.

Scheme 1. Energy band diagram for the BiVO4/WO3/SnO2 heterostructure including the suggested mechanism of charge carrier transport. The scale has been maintained, except for the WO3 interlayer; a discontinuity has been drawn for illustrative purposes. 3.4. Effect of the WO3 interlayer


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A scheme of the band alignment for the BiVO4/WO3/SnO2 heterostructure has been represented in Scheme 1.42 As remarked, it is possible to observe that the band positions for this architecture are suitable for favoring the charge separation. On the one hand, both WO3 and SnO2 can act as hole mirrors; on the other hand, the built-in potential in the BiVO4/WO3 interface favors the electron separation, while the better transport properties of WO3 (higher Lp and carrier mobility)30,43 in comparison to BiVO4 contributes to decreasing the charge recombination, which agrees with the transient pulses depicted in Figure 7. Besides this, given the thickness of the WO3 interlayer, an important contribution to light absorption is expected from this material. For instance, according to the IPCE measurements, with (EE) front illumination (see Figure S14), an increase in the efficiency at shorter wavelengths is observed at expenses of a slight decrease in the IPCE from λ ≥ 450 nm. This result agrees with the expected behavior for a single WO3 photoanode, which clearly indicates that WO3 does not merely act as host scaffold but actually has a significant contribution on the charge generation at λ ≤ 450 nm. With (SE) back illumination, on the contrary, higher IPCE values in the longer wavelength region are obtained with the heterostructure in comparison to a single WO3 photoanoade of similar thickness (see Figure S14). This effect clearly comes from the contribution of the BiVO4 film, given its lower band gap value, which allows absorbing photons of 450 ≤ λ ≥ 520 nm. Although it is difficult to directly compare the heterostructure with a bare WO3 photoanode because of the different measurement conditions (different electrolytes with pH values dictated by the corresponding stability ranges of BiVO4 and WO3), the obtained results clearly show that a WO3 interlayer suppose a significant improvement for BiVO4 and BiVO4/SnO2 photoanodes. The higher photocurrent values obtained in the j-E measurements and the corresponding contribution of each semiconductor when irradiated with photons of different wavelengths


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indicate that photoanodes with enhanced properties can be obtained through the preparation of this kind of architectures. Moreover, given the instability of WO3 at pH > 4,44,45 the BiVO4 layer on the surface also confers stability to the heterostructure for working at conditions close to neutral pH.

Conclusions BiVO4 photoanodes were deposited on FTO by Pulsed Laser Deposition from a single target in a relatively easy manner. Besides the possibility of obtaining single BiVO4 films, multilayered heterostructures were also successfully prepared through the consecutive deposition of other semiconductors on the FTO substrate (SnO2 under- and WO3 interlayers), avoiding the need of using other techniques in multistep processes. Very crystalline and dense films were thus obtained, giving rise to some of the highest photocurrent values reported for bare BiVO4 photoanodes (~1.25 mA cm-2). Even more, these values increased in the heterostructures owing to the effects associated to SnO2 and WO3 layers, and the difference between the results under front and back illumination conditions importantly decreased, pointing out an improvement of the charge carrier transport properties. In this sense, a more careful analysis was performed on the BiVO4/SnO2 photoanode, indicating that the buffer layer had an important influence on decreasing the charge recombination. Finally, PLD demonstrated to be a very suitable and versatile technique for obtaining good quality electrodes for photoelectrochemical applications. Moreover, the excellent target to substrate chemical transfer was confirmed, which opens the door to the development of other complex oxides-based systems for water splitting applications.


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ASSOCIATED CONTENT Supporting Information. Preliminary synthesis results and characterization; APCE, Transient photocurrent measurements and Mott-Schottky details. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Corresponding authors: E-mail: [email protected] E-mail: [email protected] Tel: (34) 933 562 615 Present Addresses †

Present address: Department of Electronics, Universitat de Barcelona (UB), Martí i Franquès,

1, 08028 Barcelona, Catalonia, Spain. ‡

Present address: IQS School of Engineering, Universitat Ramon Llull, via Augusta, 390, 08017

Barcelona, Catalonia, Spain. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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ACKNOWLEDGMENT This work was supported by Repsol, S.A. IREC also acknowledges additional support by the European Regional Development Funds (FEDER) and by MINECO projects ENE2012-3651 and MAT2014-59961-C2-1-R.

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