Enhanced Photoelectrocatalytical Performance of Inorganic-Inorganic Hybrid Consisting BiVO4, V2O5, and Cobalt Hexacyanocobaltate as a Perspective Photoanode for Water Splitting

Thin layers of BiVO4/V2O5 were prepared on FTO substrates using pulsed laser deposition technique. The method of cobalt hexacyanocobaltate (Cohcc) synthesis on the BiVO4/V2O5 photoanodes consists of cobalt deposition followed by electrochemical oxidation of metallic Co in K3[Co(CN)6] aqueous electrolyte. The modified electrodes were tested as photoanodes for water oxidation under simulated sunlight irradiation. Deposited films were characterized using UV-Vis spectroscopy, Raman spectroscopy, and scanning electron microscopy. Since the V2O5 is characterized by a narrower energy bandgap than BiVO4, the presence of V2O5 shifts absorption edge (ΔE = ~0.25 eV) of modified films towards lower energies enabling the conversion of a wider range of solar radiation. The formation of heterojunction increases photocurrent of water oxidation measured at 1.2 V vs Ag/AgCl (3 M KCl) to over 1 mA cm-2, while bare BiVO4 and V2O5 exhibit 0.37 and 0.08 mA cm-2, respectively. On the other hand, the modification of obtained layers with Cohcc shifts onset potential of photocurrent generation into a cathodic direction. As a result, the photocurrent enhancement at a wide range of applied potential was achieved. Graphical Abstract Graphical Abstract


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, BiVO 4 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 BiVO 4 photocatalytic properties that can minimize the main drawbacks [6]. One of them is based on the formation of heterojunction. It can be nn [7] as well as n-p [8,9] junction built from BiVO 4 and another semiconductor. The most commonly tested system is based on BiVO 4 and WO 3 ; however, BiVO 4 has been already combined with, e.g., Bi 4 V 2 O 11 [10], TiO 2 [11], and MnO 2 [12]. This manuscript is focused on the photoelectrochemical properties of BiVO 4 /V 2 O 5 bulk system. The photocatalytic and antibacterial properties of such a composite have been already studied, e.g., [13,14]. Recently, BiVO 4 /V 2 O 5 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 V 2 O 5 presence positively affects charge separation efficiency and light absorption ability due to the narrower energy bandgap. BiVO 4 was additionally doped using W atoms, and it leads to one of the highest photocurrents of water oxidation obtained for BiVO 4 -based photoanodes [15]. Other authors claim that the presence of V 2 O 5 in BiVO 4 /V 2 O 5 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/BiVO 4 / V 2 O 5 composite was obtained by annealing of pure bismuth vanadate powder in Ar/H 2 gas mixture. Obtained powder exhibited photocatalytic properties towards water oxidation reaction in AgNO 3 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 (V 2 O 5 /BiVO 4 ) 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 BiVO 4 /electrolyte surface. The passivation of recombination centers and enhancement of oxygen evolution kinetics can be achieved via cocatalyst loading. BiVO 4 -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 BiVO 4 suppresses surface recombination and does not affect the kinetics of water oxidation [22]. However, appropriate cocatalyst on the top of BiVO 4 may affect both the kinetics of O 2 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 BiVO 4 modified using nickel hexacyanoferrate has been also reported [25]. Some of PBA, e.g., cobalt hexacyanoferrate can accumulate photoexcited holes from BiVO 4 , 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 BiVO 4 -based photoanodes.
In the present work, bismuth vanadate was modified in two different ways. The first one is based on a formation of the BiVO 4 /V 2 O 5 bulk heterojunction. The phase of V 2 O 5 can be formed from the excess of the V-source during the synthesis of BiVO 4 . The thin films were prepared using pulsed laser deposition technique. The role of V 2 O 5 in the photoelectrochemical water oxidation is discussed here. In order to enhance photocurrent generation in a wider range of applied potential, BiVO 4 /V 2 O 5 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 p r e s e n c e o f C o h c c o n t h e B i V O 4 a f f e c t s i t s photoelectrochemical performance as well [27]. Both ways of BiVO 4 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(NO 3 6 were supplied by Sigma-Aldrich. K 2 SO 4 , 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(NO 3 ) 3 ·5H 2 O and an excess amount of NH 4 VO 3 (the Bi:V molar ratio equals to 1:1.5) was used to obtain the BiVO 4 /V 2 O 5 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 BiVO 4 /V 2 O 5 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 cm 2 . The deposition process was performed at room temperature in an oxygen atmosphere (p O2~1 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 BiVO 4 /V 2 O 5 films. Deposition of FTO/V 2 O 5 and FTO/BiVO 4 was performed for a comparison at the same conditions, but from separate targets, (a) V 2 O 5pure oxide and (b) BiVO 4 (synthesized using equimolar amounts of Bi(NO 3 ) 3 ·5H 2 O and NH 4 VO 3 ).
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)

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 cm 2 ). 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 K 2 SO 4 . 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.

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 V 2 O 5 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 BiVO 4 /V 2 O 5 composite, the morphology is dominated by the presence of BiVO 4 grains. Elongated V 2 O 5 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 V 2 O 5 that is characterized by the lower than BiVO 4 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.

Raman Spectroscopy
The Raman spectra of the V 2 O 5 , BiVO 4 , BiVO 4 /V 2 O 5 , and BiVO 4 /V 2 O 5 /Cohcc are shown in Fig. 2. Samples showed characteristic bands of monoclinic structure BiVO 4 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 VO 4 3units, respectively. The bands at 213 and 130 cm -1 are attributed to external modes. The Raman spectrum of a film deposited from V 2 O 5 target confirms that the sputtered layer consist only V 2 O 5 [32]. In the case of the layer deposited from the BiVO 4 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 BiVO 4 /V 2 O 5 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 BiVO 4 /V 2 O 5 /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 V 2 O 5 .

UV-Vis spectroscopy
The influence of V 2 O 5 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 BiVO 4 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. BiVO 4 is characterized by allowed direct and allowed indirect transitions [3], while V 2 O 5 by direct forbidden transition [32]. The one function cannot be used in order to determine a bandgap of BiVO 4 /V 2 O 5 heterojunction. The E g of BiVO 4 was estimated to be equal to 2.45 eV. The edge seen on UV-Vis spectrum of V 2 O 5 containing film is clearly shifted towards higher wavelengths. The lower energy bandgap of V 2 O 5 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 V 2 O 5 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 V 2 O 5 and BiVO 4 , and can be omitted in a discussion.

Photoelectrochemical Performance
In order to investigate the influence of the presence of V 2 O 5 and Cohcc components on the photoelectrochemical properties of BiVO 4 -based photoanodes, the linear sweep voltammograms were recorded under the illumination of the tested photoanodes. As it is shown in Fig. 4a, both BiVO 4 and V 2 O 5 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 V 2 O 5 and BiVO 4 , 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 V 2 O 5 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 V 2 O 5 due to the Li + intercalation/ deintercalation is related to the partial reduction of V 5+ to V 4+ 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 BiVO 4 /V 2 O 5 junction; see Fig.  4b. Interestingly, the photocurrent at this range of potential was registered. It is very likely that part of photoexcited holes on BiVO 4 took part at V 2 O 5 oxidation at a specific range of applied potential (~0-0.4 V), where BiVO 4 acts as photoanode (E > E fb ), but applied potential is lower than E fb of V 2 O 5 . The LSV curve of FTO/BiVO 4 /Cohcc photoanode, presented in Fig. 4b for comparison, does not exhibit anodic hump. As it is shown, photoanode without V 2 O 5 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 V 2 O 5 almost overlapped at a low potential range that is strongly influenced by the presence of cobalt hexacyanocobaltate. Schematically, the effect of V 2 O 5 oxidation taking into account band alignment is presented in Fig. 5a. More anodic potential makes V 2 O 5 on the electrode already oxidized, and applied potential is higher than flat-band potential of V 2 O 5 , 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 V 2 O 5 . As it was evidenced (Fig. 4a), photoexcited holes from the V 2 O 5 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 V 2 O 5 [33]. BiVO 4 -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 BiVO 4 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 BiVO 4 as it is schematically showed in Fig. 5c -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 BiVO 4 /V 2 O 5 /Cohcc heterojunction photoanode was successfully synthesized. The properties of BiVO 4 -based photoanodes can be tuned via proposed modifications. The formation of heterojunction with V 2 O 5 significantly affects the values of generated photocurrent, while appropriate cocatalyst (Cohcc) makes possible to photoelectrochemical water splitting at a lower applied potential.
Acknowledgments Author KT gratefully acknowledges financial support from the Statutory Funds of Gdańsk University of Technology (DS 033209). Author M.Sz. is supported by the Foundation for Polish Science.
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