Introduction

The scientific community has a moral obligation to take up efforts to reduce emissions of greenhouse gases. Photoelectrochemical water splitting is one of the most promising methods of hydrogen generation which is of great importance considering climate changes [1]. Sunlight energy conversion to environmentally desired fuel requires efficient photocatalysts. Since 1972, when Fujishima and Honda published their milestone paper [2], many different materials were tested as photoanodes for water photooxidation. Among them is bismuth vanadate, which exhibits high absorption coefficient [3] and a relatively narrow energy bandgap [4]. However, BiVO4 suffers from poor kinetics of water oxidation and low mobility and separation efficiency of charge carriers [5]. There are many strategies of enhancement of the BiVO4 photocatalytic properties that can minimize the main drawbacks [6]. One of them is based on the formation of heterojunction. It can be n-n [7] as well as n-p [8, 9] junction built from BiVO4 and another semiconductor. The most commonly tested system is based on BiVO4 and WO3; however, BiVO4 has been already combined with, e.g., Bi4V2O11 [10], TiO2 [11], and MnO2 [12]. This manuscript is focused on the photoelectrochemical properties of BiVO4/V2O5 bulk system. The photocatalytic and antibacterial properties of such a composite have been already studied, e.g., [13, 14]. Recently, BiVO4/V2O5 junction has been characterized as a promising photoanode for water splitting. The various geometries of the junction (e.g., bulk, layer on layer) have been studied [15]. The V2O5 presence positively affects charge separation efficiency and light absorption ability due to the narrower energy bandgap. BiVO4 was additionally doped using W atoms, and it leads to one of the highest photocurrents of water oxidation obtained for BiVO4-based photoanodes [15]. Other authors claim that the presence of V2O5 in BiVO4/V2O5 junction enhances mobility of charge carriers measured as transit time for the migration of photoexcited electrons to counter electrode, what was confirmed using intensity-modulated photocurrent spectroscopy [16]. The Bi/BiVO4/V2O5 composite was obtained by annealing of pure bismuth vanadate powder in Ar/H2 gas mixture. Obtained powder exhibited photocatalytic properties towards water oxidation reaction in AgNO3 solution. The films of resulting material were tested as photoanodes, but only for hole scavenger photoelectrooxidation [17]. Significant enhancement of water oxidation photocurrent has been achieved for inverse opal heterostructure (V2O5/BiVO4) modified using NiOOH due to the specific morphology, presence of the heterojunction, and electrode/electrolyte interface modified by cocatalyst [18].

The formation of a junction may positively affect the electron/hole pairs separation on the interface between the components as well as, in some cases, enhancing photoabsorption ability. Nevertheless, such a way of modification does not affect the surface recombination that takes place on the BiVO4/electrolyte surface. The passivation of recombination centers and enhancement of oxygen evolution kinetics can be achieved via cocatalyst loading. BiVO4-based photoanodes are commonly modified using oxygen evolution catalysts, i.e., FeOOH/NiOOH [19], cobalt phosphate (CoPi) [20], and cobalt borate [21]; however, the catalytic role of cocatalyst is not the most important. It is claimed that the presence of CoPi on BiVO4 suppresses surface recombination and does not affect the kinetics of water oxidation [22]. However, appropriate cocatalyst on the top of BiVO4 may affect both the kinetics of O2 evolution reaction and inhibits surface recombination [23]. Thus, the discussion about the role of oxygen evolution cocatalysts in photoelectrochemical water splitting is still open.

Prussian blue analogues (PBA) are interesting group of oxygen evolution catalysts. Generally, the cobalt center coordinated by the nitrogen atoms from the cyanide group in PBA is essential to obtain efficient electrocatalyst [24]. However, the enhancement of photocurrent generation of water oxidation by BiVO4 modified using nickel hexacyanoferrate has been also reported [25]. Some of PBA, e.g., cobalt hexacyanoferrate can accumulate photoexcited holes from BiVO4, positively affecting the efficiency of photocurrent generation [26]. Thus, PBAs which exhibit electrocatalytic properties in oxygen evolution reaction can be successfully utilized to modification of electrode/electrolyte interface in BiVO4-based photoanodes.

In the present work, bismuth vanadate was modified in two different ways. The first one is based on a formation of the BiVO4/V2O5 bulk heterojunction. The phase of V2O5 can be formed from the excess of the V-source during the synthesis of BiVO4. The thin films were prepared using pulsed laser deposition technique. The role of V2O5 in the photoelectrochemical water oxidation is discussed here. In order to enhance photocurrent generation in a wider range of applied potential, BiVO4/V2O5 photoanodes were modified by cobalt hexacyanocobaltate (Cohcc) synthesize during two-step method. The choice of Cohcc is based on previous reports showing its catalytic activity towards water oxidation. Among tested cobalt hexacyanometalates, cobalt hexacyanocobaltate exhibited the lowest overpotential of water oxidation [24]. We have also reported previously that the presence of Cohcc on the BiVO4 affects its photoelectrochemical performance as well [27]. Both ways of BiVO4 modification enhance the efficiency of water splitting in a different way. In both cases, significant enhancement of anodic photocurrent has been demonstrated.

Experimental

Chemicals

FTO (fluorine-doped tin oxide) glass slides were purchased from Sigma and used as electrode substrates. Chemicals of analytical grade, Bi(NO3)3·5H2O, NH4VO3, V2O5, K3[Co(CN)]6 were supplied by Sigma-Aldrich. K2SO4, acetone, and isopropanol were supplied by POCH. The metallic Co target (TK 8900) for magnetron sputtering was purchased from Quorum. Triple distilled water was used for all electrochemical experiments.

Layer Preparation

The high-temperature solid-state chemical reaction using of Bi(NO3)3·5H2O and an excess amount of NH4VO3 (the Bi:V molar ratio equals to 1:1.5) was used to obtain the BiVO4/V2O5 powder via annealing at 500 °C for 5 h. The resulting mixture was homogenized and pressed into a pellet, and annealed again at 500 °C for 4 h. The BiVO4/V2O5 pellet acted as a target for the pulsed laser deposition (PLD) technique. PLD was performed using a laser (Nd:YAG) equipped with a 4th harmonic generation module emitting 6 ns pulses at 266 nm (4 pulses per 1 s). The energy density of the laser was established at about 6.5 J cm2. The deposition process was performed at room temperature in an oxygen atmosphere (pO2 ~1 10-2 mbar). Deposition took 60 min. Then, the samples were annealed in a tube furnace (PRC 55 L/1300 M, Czylok) for 2 h at 450 °C in air atmosphere (heating rate 2 °C/min) in order to obtain crystalline BiVO4/V2O5 films. Deposition of FTO/V2O5 and FTO/BiVO4 was performed for a comparison at the same conditions, but from separate targets, (a) V2O5 – pure oxide and (b) BiVO4 (synthesized using equimolar amounts of Bi(NO3)3·5H2O and NH4VO3).

The Cohcc deposition was performed according to the procedure reported in our previous report [27]. Briefly, it was a two-step method that consists (1) sputtering of a metallic cobalt using magnetron sputtering and (2) electrooxidation of Co film in 0.05 M K3[Co(CN)6] + 0.1 M KCl electrolyte. As a result, FTO/BiVO4/V2O5/Cohcc electrodes were obtained.

Research methods

The surface morphology was examined using scanning electron microscopy (SU3500, Hitachi). Raman spectra were recorded by a confocal micro-Raman spectrometer (InVia, Renishaw) with sample excitation, by means of an argon ion laser emitting at 514 nm and operating at 5% of its total power (50 mW). The transmittance of the samples was measured by UV-Vis spectrometer (Lambda 35, Perkin-Elmer). The spectra were registered in the range of 300–700 nm, with a scanning speed of 120 nm min− 1. The calibration of UV-Vis spectrometer was performed using bare FTO substrate.

The electrochemical and photoelectrochemical studies of materials were conducted using the AutoLabPGStat 302 N potentiostat-galvanostat system (Methrom, AutoLab) in the one-compartment three-electrode cell with a quartz window, where photoanode served as a working electrode (geometric surface area of ~0.5 cm2). The Pt mesh with the high surface area was used as a counter electrode, while Ag/AgCl (3 M KCl) as a reference electrode. The electrochemical tests were carried out in deaerated 0.2 M K2SO4. Scan rate during photoelectrochemical measurements was equal to 20 mV s- 1 A xenon lamp (LOT-QuantumDesign) equipped with AM 1.5 filter with a light intensity of 100 mW cm− 2 was used as the light source.

Results and Discussion

Scanning Electron Microscopy

In order to investigate the surface morphology of annealed films, the SEM was employed. The SEM images of four films are presented in Fig.1. The film of V2O5 deposited using pulsed laser deposition technique crystallizes in the form of coarse, longitudinal crystals. Bismuth vanadate exhibits completely different morphology. The films are built from small grains with ~100–200 nm diameter. In the case of BiVO4/V2O5 composite, the morphology is dominated by the presence of BiVO4 grains. Elongated V2O5 crystals are not present in the SEM image. As can be observed, the grains forming the layer seem to be partially melted on the edges. It may be related to the presence of V2O5 that is characterized by the lower than BiVO4 melting point (690 °C [28] and 940 °C [29], respectively). Films modified by cobalt hexacyanocobaltate are evenly covered by regular, cubic-shaped crystallites characteristic for metal hexacyanometallates [30]. The zoom of the ideal cube of Cohcc is shown in Fig. 1 inset.

Fig. 1
figure 1

SEM images of FTO/V2O5, FTO/BiVO4/ FTO/BiVO4/V2O5, and FTO/BiVO4/V2O5/Cohcc

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Raman Spectroscopy

The Raman spectra of the V2O5, BiVO4, BiVO4/V2O5, and BiVO4/V2O5/Cohcc are shown in Fig. 2. Samples showed characteristic bands of monoclinic structure BiVO4 in 828, 709, 367, and 327 cm- 1 [31]. A single band at 828 cm– 1 is assigned to symmetric stretching V-O while the band at 709 cm– 1 is attributed to the antisymmetric stretching V-O. The bands at 367 and 327 cm- 1 are assigned to the symmetric and antisymmetric deformation V-O vibrations in VO43- units, respectively. The bands at 213 and 130 cm– 1 are attributed to external modes. The Raman spectrum of a film deposited from V2O5 target confirms that the sputtered layer consist only V2O5 [32]. In the case of the layer deposited from the BiVO4 target with V-source excess, Raman spectrum of resulting layer consists of band characteristic for both bismuth vanadate and vanadium pentaoxide. No other phases are detected, confirming the formation of pure BiVO4/V2O5 junction. Raman spectroscopy has been applied also in order to affirm the chemical structure of Cohcc. As it is shown in Fig. 2, the spectrum of BiVO4/V2O5/Cohcc exhibits new bands in comparison of the film without Cohcc. The set of Raman bands at 2150–2230 cm- 1 is characteristic for C ≡ N stretching vibration [33]. The presence of at least two bands at this region suggests that Co centers coexist at different oxidation states. The Raman band of Co-CN vibration at about 490 cm- 1 are not clearly detected [34], probably due to the overlapping with bands coming from V2O5.

Fig. 2
figure 2

Raman spectra of V2O5, BiVO4, BiVO4/V2O5, and BiVO4/V2O5/Cohcc

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UV-Vis spectroscopy

The influence of V2O5 on the optical properties of tested films was investigated using UV-Vis spectroscopy in a transmittance mode, see fig. 3. The spectrum of bismuth vanadate film is characterized by an absorption edge typical for monoclinic BiVO4 deposited onto transparent conductive oxide substrate [35]. The energy bandgaps were estimated from the absorption edges. The Tauc plot was not utilized here due to the difficulty to choose an appropriate type of an electron transition. BiVO4 is characterized by allowed direct and allowed indirect transitions [3], while V2O5 by direct forbidden transition [32]. The one function cannot be used in order to determine a bandgap of BiVO4/V2O5 heterojunction. The Eg of BiVO4 was estimated to be equal to 2.45 eV. The edge seen on UV-Vis spectrum of V2O5 containing film is clearly shifted towards higher wavelengths. The lower energy bandgap of V2O5 enhances the absorption ability of the film in a visible range of electromagnetic radiation. The shift of absorption edge of about 0.25 eV is achieved due to the presence of V2O5 phase. It is not a very significant change; however, it should positively affect the photoactivity of investigated photoanodes. As it was reported, the absorbance of Prussian blue analogues is negligible due to the very low absorption coefficient [36] in comparison with absorption coefficients of V2O5 and BiVO4, and can be omitted in a discussion.

Fig. 3.
figure 3

UV-Vis spectra of BiVO4 and BiVO4/V2O5

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Photoelectrochemical Performance

In order to investigate the influence of the presence of V2O5 and Cohcc components on the photoelectrochemical properties of BiVO4-based photoanodes, the linear sweep voltammograms were recorded under the illumination of the tested photoanodes. As it is shown in Fig. 4a, both BiVO4 and V2O5 act as n-type semiconductors in contact with an aqueous electrolyte. Registered photocurrent measured at 1.2 V vs Ag/AgCl was equal to 90 and 370 μA cm- 2 for V2O5 and BiVO4, respectively. There is a significant difference of onset potential, where photocurrent is detectable. Photocurrent of water oxidation was generated at the much lower potential in the case of bismuth vanadate. Such an effect was expected due to the differences of flat-band potential of two photoanode materials [37]. The curve of V2O5 consists also a dark current anodic hump at 0.2–0.6 V. It was previously reported that at this range of applied potential, electrochemical oxidation with simultaneous Na+ desorption occurs, what was confirmed using electrochemical quartz crystal microbalance and in situ Raman spectroscopy [38]. This phenomenon should be rather confined to the surface or a few crystallographic planes of the studied thin film than to the bulk oxide because current response is recorded at relatively high sweep rates. The effect of cation intercalation/deintercalation in aqueous electrolytes is even more pronounced for K+ cations [39, 40]. It was reported that electroactivity of V2O5 due to the Li+ intercalation/deintercalation is related to the partial reduction of V5+ to V4+ centers [41, 42], and it occurs in the aqueous electrolyte as well [43]. The similar effect of dark anodic hump was observed here in the case of BiVO4/V2O5 junction; see Fig. 4b. Interestingly, the photocurrent at this range of potential was registered. It is very likely that part of photoexcited holes on BiVO4 took part at V2O5 oxidation at a specific range of applied potential (~0–0.4 V), where BiVO4 acts as photoanode (E > Efb), but applied potential is lower than Efb of V2O5. The LSV curve of FTO/BiVO4/Cohcc photoanode, presented in Fig. 4b for comparison, does not exhibit anodic hump. As it is shown, photoanode without V2O5 generates lower photocurrent at a more anodic potential. Lack of heterojunction makes bulk e-/h+ recombination easier to occur. However, curves recorded for photoanodes with and without V2O5 almost overlapped at a low potential range that is strongly influenced by the presence of cobalt hexacyanocobaltate. Schematically, the effect of V2O5 oxidation taking into account band alignment is presented in Fig. 5a. More anodic potential makes V2O5 on the electrode already oxidized, and applied potential is higher than flat-band potential of V2O5, thus photoexcited holes from both components take part in water photooxidation as it is outlined in a Fig. 5b. There is a significant enhancement of photocurrent generation due to the bulk heterojunction formation and photocurrent reached ~1 mA cm- 2 at 1.2 V. There are three possible reasons for increase in photoactivity. The first one is related to the wider range of absorbed radiation due to the narrower energy bandgap of V2O5. As it was evidenced (Fig. 4a), photoexcited holes from the V2O5 can take part in water photooxidation. The second reason for enhancement is observed due to the heterojunction formation. The internal electric field is generated on the interface between components of photoanode. It significantly enhances e-/h+ pairs separation and inhibits bulk recombination. The third one is related to the higher mobility of h+ of V2O5 [33]. BiVO4-based photoanodes required to apply high anodic potential to photooxidize water on its surface efficiently [44]. The common method of onset potential shift towards cathodic direction is based on the BiVO4 modification with oxygen evolution catalysts. Indeed, the presence of Cohcc, which exhibits catalytic properties, makes tested photoanode active in a much wider range of applied potential. Thus, photocurrent of water oxidation is generated at potential more anodic than the flat-band potential of BiVO4 as it is schematically showed in Fig. 5c (effect of V2O5 oxidation is probably overlapped). Photocurrent recorded by photoanode with and without Cohcc equalizes over ~1.1 V vs Ag/AgCl (3 M KCl). It is possible to achieve photocurrent on the same range because catalyst (Cohcc) does not affect the absorption ability of investigated photocatalyst. Thus, the formation of BiVO4/V2O5 junction increases photocurrent at an anodic range of potential, but surface recombination processes and more anodic location of flat-band potential of V2O5 adversely shift onset potential of photocurrent generation in comparison with bare BiVO4. On the other hand, this effect can be eliminated due to the presence of cobalt hexacyanocobaltate on the surface of photoanode.

Fig. 4
figure 4

The linear sweep voltammetry curves of a) FTO/BiVO4 and FTO/V2O5, and b) FTO/BiVO4/V2O5, FTO/BiVO4/Cohcc (based on previous report [32]) and FTO/BiVO4/V2O5/Cohcc recorded under intermittent simulated solar light illumination

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Fig. 5
figure 5

The schematic presentation of photocurrent generation by a), b) BiVO4/V2O5 and c) BiVO4/V2O5/Cohcc

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Conclusions

Four different photoanode arrangements: FTO/BiVO4, FTO/V2O5, FTO/BiVO4/V2O5, and FTO/BiVO4/V2O5/Cohcc were systematically examined using UV-Vis spectroscopy, Raman Spectroscopy, scanning electron microscopy, and electrochemical measurement under intermittent simulated solar light illumination. The phase of V2O5 can be formed from the excess of the V-source during the synthesis of BiVO4, and no additional reagents are required. Pulsed laser deposition technique allows obtaining BiVO4/V2O5 bulk heterojunction. The photoelectrochemical measurements showed that it is possible to photooxidize V2O5 component by photoexcited BiVO4 at some range of applied potential. Applying a higher potential makes BiVO4/V2O5 a more efficient photoanode for water oxidation (than bare BiVO4 and V2O5) due to the wider utilization of the solar spectrum, improvement of hole mobility, and enhancement of bulk e-/h+ separation due to the formation of internal electric field on the BiVO4/V2O5 heterojunction. Thus, V2O5 improve the photoelectrochemical water splitting, but only at anodic range of applied potential. The BiVO4/V2O5/Cohcc photoanode achieved a significantly improved photocurrent density of about 1 mA cm- 2 at 1.2 V vs Ag/AgCl (3 M KCl) than the individual components: BiVO4 (0.37 mA cm-2) and V2O5 (0.90 mA cm-2). However, the enhancement was achieved in a whole range of tested potential, because the presence of Cohcc significantly shifts onset potential of photocurrent generation towards a cathodic direction. In conclusion, a highly efficient BiVO4/V2O5/Cohcc heterojunction photoanode was successfully synthesized. The properties of BiVO4-based photoanodes can be tuned via proposed modifications. The formation of heterojunction with V2O5 significantly affects the values of generated photocurrent, while appropriate cocatalyst (Cohcc) makes possible to photoelectrochemical water splitting at a lower applied potential.