Innovative multifunctional hybrid photoelectrode design based on a ternary heterojunction with super-enhanced efficiency for artificial photosynthesis

Electrochemical cells for direct conversion of solar energy to electricity (or hydrogen) are one of the most sustainable solutions to meet the increasing worldwide energy demands. In this report, a novel and highly-efficient ternary heterojunction-structured Bi4O7/Bi3.33(VO4)2O2/Bi46V8O89 photoelectrode is presented. It is demonstrated that the combination of an inversion layer, induced by holes (or electrons) at the interface of the semiconducting Bi3.33(VO4)2O2 and Bi46V8O89 components, and the rectifying contact between the Bi4O7 and Bi3.33(VO4)2O2 phases acting afterward as a conventional p–n junction, creates an adjustable virtual p–n–p or n–p–n junction due to self-polarization in the ion-conducting Bi46V8O89 constituent. This design approach led to anodic and cathodic photocurrent densities of + 38.41 mA cm–2 (+ 0.76 VRHE) and– 2.48 mA cm–2 (0 VRHE), respectively. Accordingly, first, this heterojunction can be used either as photoanode or as photocathode with great performance for artificial photosynthesis, noting, second, that the anodic response reveals exceptionally high: more than 300% superior to excellent values previously reported in the literature.


Results
photoelectrode synthesis and characterization. One of the components targeted in this work is Bi 4 O 7 , and its synthesis on a large scale and high-purity grade is known to be a challenge. But, also known is that this oxide usually appears as an impurity during processing of other bismuth oxides 29 , and this was the approach considered here through controlling synthesis parameters. Briefly, particulate composite-like bismuthand vanadium-containing photoelectrode films, simply abbreviated as BVO, were produced from high-purity Bi 5 O(OH) 9 (NO 3 ) 4 and NH 4 VO 3 reagents, followed by deposition on transparent conducting indium tin oxide (ITO) coated glass, as described in the Experimental section. The films were characterized in terms of X-ray diffraction (XRD), Raman spectroscopy (RS), scanning electron microscopy (SEM), and diffuse reflectance spectra (DRS). Figure 1 shows the XRD pattern from the prepared BVO material, the data of which reveals presence of the three phases intended in this work: Bi 46 V 8 O 89 (ICSD 415113), Bi 3.33 (VO 4 ) 2 O 2 (COD 1507777) and Bi 4 O 7 (ICSD 51778). Indeed, getting the two former phases involved a careful control of the synthesis process, which showed a clear influence on the stoichiometry of the end product. That is, by using acetic acid (pH ≈ 2), formation of the Bi 46 V 8 O 89 phase, which is self-doped with Bi 5+ , was verified. Furthermore, when ammonium hydroxide was added to change the pH to 8, a decomposition of part of Bi 46 V 8 O 89 into Bi 4 V 2 O 11 was favored. Besides these bismuth-and vanadium-containing photoactive phases, diffraction peaks corresponding to the ITO substrate were also identified in Fig. 1 (ICSD 85084). Refinement of the XRD data by applying the Rietveld method allowed assessing the crystallographic parameters of these phases, and the results are presented in Table S1, remaining totally comparable with data from the literature 26,28,31 .
In order to get additional information about the BVO ternary heterostructure, Raman spectroscopy was performed, and the result is shown in Fig. 2. The data was fitted using a theoretical method based on the location of the curvature maxima in concave-down (CMCD) spectral regions of the recorded Raman spectrum 32 . The fitting curve of the measured spectrum (solid line) as well as the decomposed contributions into individual Lorentzian components (dashed lines) are also shown in Fig. 2. The peaks position for each Raman active mode (natural wavenumber, cm -1 ), obtained by the aforementioned CMCD method, and the assigned vibrational   33 . In particular, the peak at 94.03 cm -1 is caused by the Bi (3-x)+ and Bi 5+ species 34 . The bands at 347.02 cm -1 and 467.43 cm -1 are assigned to the ν 2 and ν 4 symmetric flexion modes of the VO 4 tetrahedron, respectively 33,35,36 37 . The presence of these bands suggests the existence of the two bismuth oxide vanadate phases, which is a consequence of the OHaction in the aqueous solution during synthesis control, as we mentioned above.
The following reaction (a) represents the synthesis process under acidic pH conditions, using acetic acid as a solvent for the reagent dilution. (a) The Bi 5 O(OH) 9 (NO 3 ) 4 compound is formed after mixing of bismuth nitrate and bismuth oxide, having the used reagent in the present work about 79% of Bi 2 O 3 dosage. After reacting with NH 4 VO 3 , it would result in the formation of Bi 46 V 8 O 89 . However, for a conversion of ~ 100% of Bi 46 V 8 O 89 , as described in the previous reaction, the crystallization process would have to be necessarily performed at higher temperatures (≥ 700 °C) 26 ; nevertheless, as the correction was carried out under a basic pH condition, according to the reactions (b) and (c), and the crystallization of the BVO film was performed at 450 °C, it was already expected that other crystalline phases would be formed during the thermal decomposition 26,27 . Furthermore, it is known that corner sharing BiO − 3 e VO − 3 ions could give rise to mixed oxides 38-40 (Bi 3.33 (VO 4 ) 2 O 2 and Bi 4 O 7 ) by the action of hydroxyl radicals from the NH 4 OH base used in the synthesis process, as can be seen by the following reactions (b) and (c): After such reactions, citric acid was used in order to stabilize the bismuth metallic ions during the drying process. On the other hand, in order to provide additional support for the good correspondence between the observed results from XRD data and Raman spectroscopy, complementary structural analysis has been considered, which involves two fundamental parameters that can be used to get further structural information regarding the synthesized phases, as is the case of the average crystallite size (D) and structural micro-strain (ϵ). Both parameters were estimated from the XRD pattern using the Williamson-Hall equation: 41,42 where K is a proportionality constant related to the particle geometry, λ is the wavelength, θ is the Bragg's angle and β hkl represents the full-width at half-maximum of the diffraction peaks. The results are shown in Table 1.  , the consequence of which is a reduced average crystallite size for the latter. Due to the bismuth deficiency, the lattice oxygen ions (O 2-) undergo a charge transfer with the Bi 3+ ions on the surface of the BVO heterostructure. Therefore, due to the ions' mobility, we consider that there is a double valence fluctuation scheme involving the change of two electrons on the surface. The double valence variation process, as described in the following reaction (c), occurs due to the dimerization of the ligand holes (h + ) caused by direct ligand-ligand hybridization, which is expected in perovskite-type structures in which there is an oxygen accumulation nearby. This process occurs, in particular, when the spin density of Bi 3+ is small and it participates in the double valence fluctuation.

(c)
Under thermal evacuation, the bismuth oxide with the Bi 3+ (6p°) electronic configuration is transformed into its Bi 5+ (6s°) state by double valence flotation 43,44 . For complementary information, SEM image of the films is presented in Figure S1, the result of which revealed the BVO material to consist of islands of self-assembled nanoplates, forming nano-porous sponges. In addition, the cross-sectional view, shown in Figure S2, indicated to be dealing with films showing a thickness of about ~ 10 m.
On the other hand, the DRS data represented through the transmittance (T) and reflectance (R) spectra, whose results are shown in Figure S3a-b, revealed that the displacement of the bands occurred after the deposition of the BVO film on the ITO. The BVO film was quite rough on the surface due to the higher reflectance, compared to the ITO glass. According to the literature, and some data collected in our Laboratory, values of the direct band-gap energies for the model (individual) compounds are: 1.89 eV for Bi 4 27,28,30 . These values are comparable to those estimated in this work from Figure S3c, except for Bi 4 O 7 , where the synthesis procedure was determinant in reducing its value. In any case, these results indicate that all the three components in the BVO heterojunction can be individually excited by visible light to produce the reactive species, that is to say, electrons and holes needed to trigger water splitting.
Regarding the main interest spectrum, i.e. Kubelka-Munk absorbance (F(R)), we proceeded with a Gaussian deconvolution of the absorption bands, the results of which are shown in Fig. 3. Five absorption bands were identified, which presented wavelength displacements towards the region of the visible spectrum for the BVO film. Such a displacement is caused by defects in the crystalline structure of the BVO, and it is expected to promote  www.nature.com/scientificreports/ donor (or acceptor) states between the energy bands, giving rise to certain charge trap states between the bands. The charge carriers trapping in these states could play a crucial role in the optical and photoelectrochemical properties of the BVO film 45 .
The adequate alignment of the conduction (CB) and valence (VB) bands applying in this situation is briefly outlined in Fig. 4 (with additional details given later).
The results presented in the following refer to data extracted from Mott-Schottky, photoelectrochemical performance and impedance spectroscopy measurements, aimed at getting insights into the materials chemistrycontrolled electrical and electrochemical responses from these heterojunction-structured BVO films. All these experiments were conducted using a standard three-electrode cell configuration 46 , with an Ag/AgCl (3.0 M KCl) reference electrode, a platinum metalized FTO substrate as the counter electrode, and the BVO film acting as the working electrode (as described in Methods). For the sake of a direct comparison, the electrochemical performance and impedance data were collected under both dark and illumination conditions 46 .
Mott-Schottky data. The conductivity characteristics of the heterojunction-structured BVO material were partly elucidated by the Mott-Schottky electrochemical characterization, and interpreted through the mathematical model described by the equation below: from which a linear behavior between the inverse of the square capacitance (Cp -2 ) of the space-charge layer and the applied potential (V) is predicted. In this expression, N D is the number of donor ions, k B the Boltzmann's constant, ε the dielectric constant of the semiconductor, A the area of the electrode, ε 0 the dielectric permittivity of free space, T the absolute temperature, e the electronic charge and V fb the flat-band potential 47,48 .
The Mott-Schottky results from the BVO film are illustrated in Fig. 5, and the nature (n or p) of conductivity from each component is there identified. The data reveal that, at low frequencies, in the range of 1-100 Hz (Fig. 4, left-hand side), the photoelectrode mostly shows n-type conductivity (straight lines with positive slope) for potential values < + 1.2 V RHE . For values > + 1.2 V RHE , however, an inversion of polarity is observed, which is related to the contact of n-Bi 4 O 7 , n-Bi 3.33 (VO 4 ) 2 O 2 and n-Bi 46 V 8 O 89 , resulting in the formation of a spatialcharge region at the Bi 3.33 (VO 4 ) 2 O 2 /Bi 46 V 8 O 89 interface of this minority h + -charged junction so as reversing the conductivity of Bi 46 V 8 O 89 to p-type. For high frequencies, in the range of 1-100 kHz (Fig. 4, right-hand side), a modification to mostly p-type conductivity occurs (straight lines with negative slope) towards low potential values, with a polarity inversion occurring at + 0.8 V RHE while increasing the potential. In this case, the inversion layer acts in the opposite way, because when the p-Bi 46 V 8 O 89 structure is in contact with the p-Bi 3.33 (VO 4 ) 2 O 2 one, the spatial charge region at the interface of the junction will be charged with a minority of negative (e -) charges, and the conductivity of Bi 46 V 8 O 89 will be of n-type.
Regarding particularly Bi 4 O 7 , an additional relevant information seen in this Figure is the polarity change this component shows at 100 Hz and 100 kHz, indicating formation of a dipole moment at the Bi 4 O 7 /Bi 3.33 (VO 4 ) 2 O 2 interface, with apparently a minority of positive (h + ) charges at 100 Hz, and a minority of negative (e -) charges at 100 kHz. According to the literature, the inversion layer cannot be detected for thin films with a single semiconductor 11 . This suggests that this process occurs at the interface between the materials constituting the heterojunction, revealing that in the inversion layer the BVO film may act as a photocathode. For this reason, besides the originally-planned tests for photoanode applications (the results of which are presented in the next section), behavior of the Bi 4 O 7 /Bi 3.33 (VO 4 ) 2 O 2 /Bi 46 V 8 O 89 heterojunction as photocathode was also checked and analyzed through current-potential curves ( Figure S4). The BVO heterostructure exhibited a notable photocurrent typical    Fig. 6. Photoactivity of this working electrode is clear, noting that, under light irradiation, the extracted data of anodic (peak value) photocurrent density is + 38.41 mA cm -2 (+ 0.76 V RHE ). Furthermore, the photocurrent density values obtained for the BVO heterostructure were significantly higher than the values for Bi 46 V 8 O 89 , indicating that the formation of the heterojunction improves the charges separation and transport processes. Notice that the result for water oxidation is actually telling, as revealing to be 393% greater than that reported, for instance, for one of the best photoanodes based on parent W-BiVO 4 /V 2 O 5 films, which achieved a high but comparatively-quite lower photocurrent density of + 7.79 mA cm -2 (+ 1.23 V RHE ) 49 . These results, showing photoactivity of the BVO film either as photocathode or as photoanode are outstanding because revealing the bifunctionality of this heterojunction-structured photoelectrode for water splitting into H 2 and O 2 . In addition, the highest (peak) value we obtained for the HC-STH (half-cell solar-to-hydrogen efficiency) of the BVO film was about 19.75% at + 0.65 V RHE ( Figure S5). All these results demonstrate that the photoelectrode we built here can promote the photoelectrochemical oxidation of water in O 2 , and is also photoactive in the reduction reaction of water to produce renewable H 2 fuel.
We would like to point out that in the region of the visible spectrum, the photoelectrode showed a light harvesting efficiency (LHE) from 36.67 to 47.23% and separation charges (η sep ) around 48%, at applied potentials from 0.5 to 0.8 V RHE . Traditionally, the thermodynamic potential required for the water decomposition reaction is nearly 1.23 V RHE . Because of the superpotentials at the cathode and anode, a voltage higher than 1.6 V is required for the electrolyzer to operate under a current density of around 1 A cm -2 . In this case, almost 33.5-40% of electricity can be lost, resulting in a low overall energy conversion efficiency 50,51 . In practice, in order to minimize the energy loss, water electrolysis should be performed in highly acidic or alkaline conditions 52 . However, in this work, the data presented in Fig. 7 (representing the Tafel's plot), suggest that the action of superoxide ions (O 2-), resulting from the first redox step, can act in the second redox step (where a drop in photocurrent density occurs) in order to increase the potential energy necessary for the kinetic process of separation/transfer of photogenerated charges. Therefore, water splitting can be conducted in a sustainable manner in an electrochemical reaction.
The excess potential (η) was estimated by the intersection of the anodic and cathodic inclinations and, according to the Tafel's kinetic-electrochemical model, is represented by the Eq. (3), where J is the current density, α represents the excess of anodic and cathodic potential, and β is the Tafel's slope of the anodic (or cathodic) reaction 53 .
This mathematical model is based on the kinetics and thermodynamics of the reactions involved on the surface of the photoelectrode, making possible to predict the potential and density of corrosion current by extrapolating the anodic and cathodic slopes 54 . In general, the higher the value of η the better the anti-corrosion properties. 54 From the Tafel's plot shown in Fig. 7 it was possible to obtain some parameters, which are summarized in Table 2.
In particular, the photoelectrode under illumination reached a value of η = 2.07 V RHE , well above the aforementioned minimum potential. In addition, values of the operational potential (V op ) required for water splitting at a given current density (J) were obtained by the Eq. (4) 55 , and are also listed in Table 2. www.nature.com/scientificreports/ The A parameter is the electrode area and R p represents the polarization resistance, which is obtained by the Eq. (5) 56 .
Notice that the values of V op and η are close each other, showing excellent agreement with the expected data from the Tafel's plot towards the explored current densities. At the operating point, the efficiency for energy conversion ( PCE = V op × J op /P ) was estimated to be around 9.3%. Regarding the high density of photogenerated current (high HC-STH efficiency) we have reported, a reasonable approach was described in this manuscript, that is, when Bi 46 V 8 O 89 comes into contact with the Bi 4 O 7 /Bi 3.33 (VO 4 ) 2 O 2 interface, an adjustable virtual p-n-p or n-p-n junction is created, with the rectifying contact acting afterward as a conventional p-n junction. For this reason, photogenerated charges can be efficiently separated by the ternary heterostructure, thus providing such a high density of photogenerated current. At the microscopic level, moreover, the following mechanisms may be also taken into consideration. It is well-known that the efficiency of the photogenerated e -/h + pairs separation is a critical factor for the photoelectrochemical performance 39 . Because of the Bi 3+ and Bi 5+ coexistence in the present BVO material, there is a scenario with bridges for local charge transfer and to reinforce the photogenerated electrons flow. Furthermore, Bi 5+ should act as a charge trap for electrons, thus delaying the recombination of electrons and photogenerated holes 39,57 . For the same reason (the ions coexistence), and with Bi 4 O 7 acting as a photosensitizer, singlet oxygen is formed after excitation by a triplet-triplet energy transfer step. This this kind of electrophilic oxygen plays also an important role in increasing the efficiency in the photoelectrochemical oxidation process, leading to an improvement in the HC-STH efficiency for energy conversion 38,57 .  www.nature.com/scientificreports/ Further insights into the feature of electrical transport process across the photoelectrode can be theoretically also obtained via electrochemical impedance spectroscopy (EIS). The measurements were carried out with a 20 mV alternating-current (AC) potential, together with a direct-current (DC bias) potential at + 1.23 V RHE . The measured impedance data were processed using Nyquist plots (as observed in Figure S6), from which incidence of three semicircles was noted and simulated using an appropriate professional software (NOVA 2.1.3-Metrohm Autolab BV, Netherlands). The impedance data shown in Figure S6 have been fitted by the equivalent circuit represented in Figure S7, where a non-ideal Debye model involving three resistance-constant phase elements (Ri-Qi) networks, all connected in series, with each set of R and Q elements linked in parallel, plus a series resistance (Rs) coupled, has been considered, being Rs the resistance of the used solution (Na 2 SO 4 , 0.6 M). In practice, materials whose impedance data follow the non-ideal model show depressed semicircles below the real axis in Nyquist plots for (1-n)/2 angles. For n = 1, such a model reduces to the classical Debye's scenario. In our work, we verified n values to be in the range of 0.69-0.96, approaching to unity for the high-frequency semicircle. Table 3 summarize the electrochemical parameters derived from Nyquist plots, according to the impedance measurements performed on the BVO photoelectrode.
The equivalent values we finally found for capacitance fall in the range of the expected values for the different contributions we have identified, in line with similar results reported in the literature (Ref. 59 cited). In particular, the R 1 -Q 1 network, attributed to the polycrystalline photoelectrode, with C 1 values in the order of 10 -8 F, should arise from the grain-boundary contribution. Observing the electrical response arising from the corresponding bulk would likely need to expand the measurements to higher frequencies (i.e., above 100 kHz). The values of Q 2 and Q 3 are also compatible in magnitude order (significantly higher than expected for grain-boundary or bulk effects) with the designated interface and diffusion contributions. The constant phase element in parallel with the resistors (R 1 , R 2 and R 3 ) can be converted into the capacitances (C 1 , C 2 and C 3 ) through the Eq. (6), 58 where C is the resulting capacitance, Y 0 is the admittance value of the constant phase element, R is the resistance value and n is the exponent of the constant phase element 58 .
As expected for charge transfer (R 1 ) and diffusion (R 3 ) processes, results shown in Table 3 indicate that presence of light is responsible for an increase in conductivity. Also observed in this table, R 2 is quite high under illumination, demonstrating that there is less recombination of electrons and, thus, enhanced chemical capacitance effect (C 2 ) at the BVO/electrolyte interface. The lifetime of the photoinjected electrons ( τ 2 ) can be and was estimated using the simple expression represented by the Eq. (7). 59 The combination of these two electrical parameters produces a very high value of lifetime τ 2 (also presented in Table 3) for the photoinjected electrons, accounting for the high values we in fact also observed for the HC-STH efficiency ( Figure S5). Furthermore, the excellent photoelectrochemical performance is the result of the exceptional electrical conductivity level of the ITO and the used active area of the photoelectrode ~ 0.2 cm 260 .
Finally, the onset potential of the BVO film was also determined by processing the photocurrent data in a J 2 versus applied potential plot (as shown in Figure S8), at which point the potential where the current starts to increase, that is, a reaction begins to occur, is identified. The obtained value was 0.33 V RHE , much lower than the values found in bismuth-and vanadium-based films such as Bi 46 11 . This result confirms that the ternary heterojunction formed in our BVO film improved the charge-transfer kinetics through the photoelectrode/electrolyte interface, that is, without the use of dopants or cocatalysts.

Discussion and concluding remarks
Due to the intimate contact between Bi 46 V 8 O 89 and Bi 3.33 (VO 4 ) 2 O 2 nanoplates, a sufficiently high internal polar electric field is created between both phases, resulting in the formation of a minority electron inversion layer (Fig. 8a) or minority holes (Fig. 8b) at the interface, as verified from the Mott-Schottky data (Fig. 5). The difference between the effective working functions of both semiconductors is the driving force required to achieve the electronic equilibrium through a transfer of charges between them and the creation of a spatial-charge region at the heterojunction interface. Then, charges of the same polarity are repelled from the interface, while those of the opposite polarity are attracted to it. Accordingly, when the ionic-conductor Bi 46 V 8 O 89 (with an inherent-like ability to form dipoles) is in contact with Bi 3.33 (VO 4 ) 2 O 2 , most of the electrons will be depleted from the interface between them, while the minority electrons (or minority holes) will be attracted to this interface, thereby  (Figs. 8a, b). In Bi 3.33 (VO 4 ) 2 O 2 there is, therefore, an accumulation of minority electrons (Fig. 8a) or minority holes (Fig. 8b)  In summary, due to a self-polarization-like event of the Bi 46 V 8 O 89 ionic-conductor, and depending on the applied potential and considered frequency of operation, the nature of the conductivity across this designed ternary Bi 4 O 7 /Bi 3.33 (VO 4 ) 2 O 2 /Bi 46 V 8 O 89 heterojunction can be modified, becoming either a virtually p-n-p type heterojunction at low frequencies: 1-100 Hz (Fig. 8b), or a n-p-n type heterojunction at high frequencies: 1-100 kHz (Fig. 8a). These findings are exclusive, as they open a new design strategy to be considered for construction of multifunctional, high-efficiency hybrid photoelectrodes for sustainable artificial photosynthesis.  characterization. The morphology of the BVO film was investigated by scanning electron microscopy (SEM, microscope model VEGA3 TESCAN) operated at 5.0 kV, coupled to an energy dispersive spectroscopy (EDS) mapping device (Oxford Instruments, Model 51-ADD0007) with 20 kV accelerating voltage. The crystalline phases of the films were determined using an X-ray diffractometer (XRD 6,000, Shimadzu). The data were collected from 10 to 80° at a step-width of 0.2°, scan-speed of 2.0° min -1 , sampling pitch of 0.02° and preset time of 5 s, at 40 kV, 30 mA, using CuKα radiation (λ = 1.540560 Å). Silicon was used as an external standard sample. Structure refinement through Rietveld was performed using the GSAS + EXPGUI program package 63,64 . Further information on the structural phases involved was extracted from Raman spectroscopy. The measurements were performed towards the 40 to 1,500 cm -1 region, using a Horiba LabRAM HR Evolution spectrometer, with a 532 nm laser line. The system was calibrated with a 520.7 cm -1 silicon wafer, and the LabSpec 6 software was used for data collection within a spectral resolution of 1 cm -1 . The diffuse reflectance spectra (DRS) were collected within a resolution of 0.1 nm by using a UV-Vis spectrophotometer (UV-2501 PC Shimadzu), equipped with an ISR 240A integration sphere (and UVPC program), while BaSO 4 was used as reference material (100% transmission). The direct band-gap energies were calculated via the following Tauc equation: where A is a constant, hν the light energy, E g the optical band-gap energy and α refers to the absorption coefficient. The fraction of transmitted light was measured as reflectance, and the Kubelka-Munk radiative transfer model was employed to extract the α parameter. This model is described by the equation below: where F(R) is the Kubelka-Munk function, S the scattering coefficient and R stands for the absolute reflectance. If the scattering coefficient is wavelength independent, then F(R) is proportional to α and the Tauc's plots can be made using F(R) instead of α 65,66 . photoelectrochemical measurements and electrochemical impedance spectroscopy. The photoelectrochemical measurements were carried out using a potentiostat AUTOLAB Potentiostat-Galvanostat PGSTAT204. The system uses a standard three-electrode cell with an Ag/AgCl (3.0 M KCl) reference electrode, a platinum metalized FTO substrate as a counter electrode, a working electrode with irradiation area of 0.2 cm 2 , and a scan-rate of 20 mV s -1 . A Na 2 SO 4 aqueous solution (with 0.5 M and pH = 6.6) was used as electrolyte. The prepared BVO film, tested as the working electrode, was connected to a copper tape in order to measure the photoactivity. The current-potential curves were recorded in the dark and under back illumination. For converting the obtained potential vs. Ag/AgCl to RHE (Reversible Hydrogen Electrode), the following expression was used: The V o Ag/AgCl (KCl 3 M) = 0.197 at 25 • C.
When the BVO photoelectrode behaved like a photoanode, the applied-bias-compensated half-cell solar-tohydrogen (HC-STH) efficiency was calculated from the following equation: On the other hand, when the BVO photoelectrode behaved like a photocathode, the equation below was used: where V RHE is the applied potential, J dark and J light are the photocurrent densities in the dark and under irradiated light, respectively, P light is the power density of the irradiated light, while V O2/H2O and V H + /H2 represent the equilibrium potentials of oxygen (+ 1.23 V RHE ) and hydrogen (0 V RHE ) evolution, respectively. Finally, the faraday efficiency is symbolized by η 67-69 . www.nature.com/scientificreports/ The electrochemical impedance spectroscopy was performed using the AUTOLAB Potentiostat-Galvanostat PGSTAT204 equipped with the FRA32M module. The standard three-electrode cell configuration was also used, together with the Na 2 SO 4 aqueous solution (0.5 M) as electrolyte. The impedance measurements were carried out under a + 1.23 V RHE DC potential bias, with an amplitude of 0.01 V RMS , where RMS is the root mean square, in the frequency range of 100 mHz to 100 kHz. An Xe Lamp (λ > 450 nm, 100 mW cm -2 ) was used as the light source. The measured data were processed using Nyquist plots and fitted using the NOVA 2.1.3 software (Metrohm Autolab BV, Netherlands).
It was used as a source of light a 300 W Xenon arc lamp (Newport Ozone Free Model 6,258). The equipment used to measure the intensity (power density) of the irradiated light was a Newport model 1916-R optical power meter equipped with an UV sensor (Newport UV 818-UV/DB). An AM 1.5 filter in front of the light output was also used as the sun simulator.
Mott-Schottky data acquisition. The spectra were collected using also the AUTOLAB Potentiostat-Galvanostat PGSTAT204 equipped with the FRA32M module, and a cell configuration with three electrodes: a reference electrode of Ag/AgCl (3.0 M KCl), a platinum metalized FTO substrate as counter electrode wire and a working electrode (the BVO film) with 0.2 cm 2 area in the dark, applying a potential from -0.7 to + 0.7 V Ag/AgCl in the frequency range of 1 Hz to 100 kHz, using a step of 0.03 V Ag/AgCl and amplitude of 0.01 V RMS . The measured spectra were fitted using the NOVA 2.1.3 software (Metrohm Autolab BV, Netherlands). Again, the Na 2 SO 4 solution (0.5 M) was used as electrolyte for these electrochemical measurements.