Photoelectrochemical performance of a spin coated TiO2 protected BiVO4-Cu2O thin film tandem cell for unassisted solar water splitting

A tandem cell consisting of a Mo-BiVO4/TiO2/FeOOH photoanode–Cu2O/TiO2/MoS2 photocathode was prepared for unassisted solar water splitting. The protective TiO2 layer was prepared by a cost-effective spin coating technique. The individual Mo-BiVO4/TiO2/FeOOH photoanode and the Cu2O/TiO2/MoS2 photocathode yielded a current density of ∼0.81 mA cm−2 at 1.23 V vs. RHE and ∼−1.88 mA cm−2 at 0 V vs. RHE, respectively under 100 mW cm−2 xenon lamp illumination. From the individual photoelectrochemical analysis, we identify the operating points of the tandem cell as 0.66 V vs. RHE and 0.124 mA cm−2. The positive current density from the operating points proves the possibility of non-zero operation of the tandem cell. Finally, a two-electrode Mo-BiVO4/TiO2/FeOOH-Cu2O/TiO2/MoS2 tandem cell was constructed and analysed for unassisted operation. The obtained unassisted current density of the tandem cell was ∼65.3 μA cm−2 with better stability compared to the bare BiVO4-Cu2O tandem cell. The results prove that the spin coated TiO2 protective layer can be a viable approach to protect the photoelectrodes from photocorrosion with better stability and enhanced photoelectrochemical (PEC) performance.


Introduction
Solar hydrogen is considered as a promising form of green energy for the future due to its potential to mitigate global warming and provide sustainable energy throughout the day despite the intermittency of sunlight. 1 The search for an efficient practical semiconductor photocatalyst material has been pursued since the pioneering work done by Fujishima and Honda in 1972. 2 The identication of a suitable single photoelectrode system with good chemical stability and long-term durability is still quite challenging without external bias. The combination of mature solar cell technology and electrolyser 3 can be an alternative approach to meet the practical requirements but the system complexity and the cost per kilogram of hydrogen produced will be high. 4 Direct conversion of solar energy using a tandem photoelectrochemical cell (PEC) is a cost-effective approach for unassisted solar water splitting. 5 In the tandem or z-scheme approach, 6 complementary semiconductors are chosen in which the photoanode or top electrode (where oxidation takes place) should be a wide bandgap semiconductor and the photocathode or the bottom electrode (where reduction takes place) should be a narrow bandgap semiconductor. 7,8 The light that is transmitted by the photoanode will be absorbed by the photocathode so that the optical absorption can be maximized to the entire incoming solar spectrum. Since two photoelectrodes are used with straddled band edges, there would be a sufficient amount of photopotential developed for the splitting of water unlike in single electrode systems. 9,10 The photoelectrodes can be chosen according to the modied contour plot devised by M. S. Prévot et al., in order to achieve a maximum theoretical efficiency. 11 In the literature, several combinations of semiconductor electrode materials have been used in tandem cell. Out of different materials, BiVO 4 (ref. 12) has oen been used as the photoanode because of its availability, favorable bandgap and the stable improved photocurrent. 13 P. Xu et al., constructed BiVO 4 photoanode with Si nanoarray photocathode. The photoanode was enhanced by Mo doping and adding co-catalyst Co-Pi. Similarly, the photocathode was p-Si grown in the form of nanoarray loaded with Pt via photo electrodeposition. The tandem cell produced ∼0.46 mA cm −2 photocurrent at zero bias condition in linear sweep voltammetry. 52 However, the stability tests of the cell was challenging because of the p-Si nanoarray degradation in aqueous solution. 14 CIGS and CZTS are similar in structure and have a variable bandgap of 1 eV to 1.7 eV. 15,16 The narrow bandgap CIGS and CZTS are excellent choices with BiVO 4 in PEC tandem cell. M. Chen et al., used BiVO 4 /NiOOH/ FeOOH as the photoanode and CIGS/CdS/Al 2 O 3 /TiO 2 /Pt as the photocathode and they achieved current density of 0.82 mA cm −2 for 30 minutes in 1 M potassium borate solution at pH 9.2. 17 Although the performance of CIGS tandem cell showed some promise, but the use of toxic materials is an issue in case of CIGS. A tandem cell consisted of BiVO 4 photoanode with CZTS/CdS/HfO 2 /Pt photocathode was prepared which produced a stable current density of ∼0.68 mA for over 10 hours. 18 In CIGS 19 and CZTS, 20 a CdS interfacial layer should be deposited to improve the transfer of charges between the electrode and electrolyte interfaces. The deposition of number of layers with ultrathin thickness is also difficult in terms of large area testing. Another choice of p-type material has been CuBi 2 O 4 which has a bandgap of about 1.3 to 1.8 eV and has a positive at band potential of 1 V vs. RHE. 21 There are several reports on BiVO 4 photoanode and CuBi 2 O 4 photocathode tandem structure proposed in the literature with considerable improvement in the PEC performance. [22][23][24] Another choice of photocathode has been Cu 2 O 25 which has narrow bandgap of 2.0 eV with a band edges suitable for reduction. P. Bornoz et al., 27 constructed the rst ever BiVO 4 -Cu 2 O tandem cell. In this work, photoelectrochemical properties with respect to the optical property of the photoelectrode was analyzed. The photocathode consisted of Cu 2 O/Al:ZnO/ TiO 2 /RuO x while the photoanode chosen was BiVO 4 with variable thickness in order to enhance the absorbance of the photocathode. The best performance was obtained for 200 nm thick BiVO 4 photoanode and the 500 nm thick 26 modied Cu 2 O photocathode. The tandem cell produced current density of ∼0.1 mA cm −2 for 4000 seconds with the STH efficiency of 0.4%. 27 Similarly L. Pan et al., enhanced the performance of Cu 2 O photocathode and constructed tandem PEC cell with BiVO 4 photoanode. Cu 2 O was protected by n-type Ga 2 O 3 overlayers for efficient charge separation and protection. The photocathode Cu 2 O/Ga 2 O 3 /TiO 2 /NiMo was combined with hydrogen treated Mo-BiVO 4 /NiFeO x co-catalyst. The tandem cell produced current density of 2.4 mA cm −2 and the highest solar to hydrogen efficiency of ∼3% was achieved for the tandem cell combinations. 28 From the above studies, it was observed that serious modications in BiVO 4 and Cu 2 O are required to obtain better performance in unassisted tandem cell operation. Particularly, the TiO 2 protective layer is important for a stable PEC operation. The ALD process 29 used in several studies is quite complex and a time-consuming process which should be replaced by a low cost process with better quality. X. Yin et al., analyzed the nanostructured BiVO 4 /TiO 2 /FeOOH electrodeposited Cu 2 O tandem cell performance. The stability of unassisted tandem cell was assessed and the current density of 0.06 mA cm −2 was achieved. The study revealed poor stability of materials as a result of Cu 2 O photocorrosion 30 since no interfacial layers are employed to enhance the PEC performance. 31 X. Fu et al., made Cu 2 O photocathode loaded with hydrogen treated Ti 3 C 2 T x MXene and tested the performance in tandem with BiVO 4 photoanode. Cu 2 O was grown on Cu foam and MXene was loaded on the Cu foam in order to improve the oxygen vacancies on the photocathode. The increased oxygen vacancies improve the charge transport to the surface and the conductivity. The tandem cell was illuminated from the Cu 2 O photocathode side contrary to the conventional illumination from the anode side. The STH efficiency of 0.55% was obtained for this tandem cell conguration. 32 Deposition of protective layer and loading co-catalysts are effective strategies for unassisted tandem PEC cell. This dual approach can reduce the overpotential of the semiconductors used in the tandem cell and therefore the STH efficiency can be improved.
In this work, we have chosen semiconductor oxide materials BiVO 4 as a photoanode and Cu 2 O as a photocathode and this combination can produce a maximum efficiency of close to 9% according to the contour plot. 11 Since both oxide materials have been individually characterized and well-documented in the literature, it is appropriate to construct a tandem cell with the aim to examine the effect of spin-coated TiO 2 protective layer on the PEC performance. Molybdenum was doped in BiVO 4 to increase the conductivity while FeOOH and MoS 2 was loaded on the photoanode and photocathode as co-catalysts, respectively. The individual performance of BiVO 4 photoanode and Cu 2 O photocathode was analyzed and the linear sweep voltammetry response was overlaid to nd the operating points of the tandem PEC cell. Finally, Mo-BiVO 4 /TiO 2 /FeOOH-Cu 2 O/TiO 2 / MoS 2 tandem structure was tested for unassisted tandem cell operation and a stable photocurrent response was obtained. No work has been hardly explored on spin coated TiO 2 protective layer for both photoanode and photocathode. We believe that the results would contribute to the further advancement in the unassisted PEC water splitting area. , sodium hydroxide (NaOH) were purchased from Avra chemicals. Thiourea (CH 4 N 2 S) from SDFCL limited. Fluorine doped tin oxide (FTO) substrates (surface resistivity ∼7 U sq. −1 ) were purchased from Sigma Aldrich. All the chemicals used in the study were of analytical grade and used without further purication.

Preparation of bismuth vanadate (BiVO 4 )/molybdenum doped BiVO 4 (Mo-BiVO 4 ) photoanode
FTO substrates were cleaned ultrasonically in acetone, isopropanol, ethanol and DI water respectively in a sequential manner for 5 minutes each. BiVO 4 solution was prepared by adding a 0.173 g of bismuth nitrate pentahydrate and 0.097 g of vanadyl acetylacetonate in the mixture of 0.6 mL of acetic acid and 4.4 mL of acetylacetone. For Mo doping, 3 at% bis(acetylacetonato)dioxomolibdenum(VI) was added in the prepared BiVO 4 solution. 33 The prepared sol was magnetically stirred at 900 rpm for 1 hour to form a homogenous solution without precipitate. The homogeneous solution was spin coated at 1000 rpm for 30 seconds on FTO substrates. The spin coated substrates were annealed at 450°C for 10 minutes. The same steps were repeated for four times to get an optimum performance. Finally, the substrates were annealed at 450°C for 2 hours in air with rate of 5°C min −1 in a muffle furnace. 34 2.3. Preparation of cuprous oxide (Cu 2 O) photocathode Cu 2 O photocathode was prepared by electrodeposition method using lactate stabilized copper sulphate solution. 35 Initially, 0.4 M copper sulphate pentahydrate was mixed in 3 M lactic acid. The pH of the solution was changed to 10 by gradually adding 10 M NaOH. Electrodeposition process was carried out under a constant potential mode using three electrode setup in which FTO substrate, Ag/AgCl (Saturated KCl) and Pt wire used as a working electrode, a reference electrode and a counter electrode, respectively. A constant potential of −0.3 V vs. Ag/ AgCl was applied for an hour to attain the desired thickness for effective optical absorption. Aer the deposition, the Cu 2 O coated FTO substrate was rinsed with DI water and dried at room temperature.

Deposition of TiO 2 protective layer
The TiO 2 protective layer was deposited on both BiVO 4 and Cu 2 O photoelectrodes. Titanium isopropoxide and isopropanol was mixed in the volume ratio of 1 : 50 in a sample vial. The solution was sonicated for 15 minutes till a homogenous solution was formed without any visible chunks of titanium isopropoxide in the solution. The prepared solution was spin coated on the as-prepared photoanodes and photocathode at an optimized speed of 2000 rpm for 60 seconds. Only one layer was spin coated to maintain the minimum thickness for the protective layer. To improve the adhesivity of the TiO 2 lm, the photoelectrodes were annealed at 200°C for 1 hour in air using a muffle furnace. 36

Deposition of iron oxyhydroxide (FeOOH)
FeOOH cocatalyst was deposited on the as-prepared photoanodes electrochemically using three electrode setup in 0.1 M iron sulphate heptahydrate (FeSO 4 $7H 2 O) electrolyte solution. BiVO 4 , Ag/AgCl (sat'd KCl), and Pt were used as the working electrode, the reference electrode and the counter electrode, respectively. A constant potential of 1.2 V vs. Ag/AgCl was applied for a period of 300 seconds. 37 Aer the deposition, the working electrode was rinsed in DI water and dried at room temperature.

Deposition of molybdenum disulphide (MoS 2 )
MoS 2 catalyst was synthesized by hydrothermal method in which a 0.242 g of sodium molybdate dihydrate (Na 2 MoO 4 -$2H 2 O) and 0.381 g of thiourea were mixed in 60 mL of DI water. 38 The solution was then transferred to 100 mL Teonlined stainless steel autoclave. The autoclave was maintained at 200°C for 24 hours. The obtained black precipitate was washed several times with DI water and ethanol followed by drying at 70°C overnight.
For the deposition of MoS 2 on Cu 2 O photocathode, 1 mg mL −1 of MoS 2 powder was mixed with n-methyl-2-pyrrolidone (NMP) and sonicated for 3 hours. Then, the supernatant solution was taken and drop-casted on Cu 2 O photocathode followed by drying at 200°C for 1 hour in air using a muffle furnace to improve the adhesion of MoS 2 on Cu 2 O surface.

Photoelectrodes characterization
The structural characterization was performed using an X-ray diffractometer, D8 Advanced, Bruker with Cu ka radiation (l = 1.5418Å). The morphology of the photoanode was analysed using Field emission scanning electron microscope (FESEM), FEI Quanta 250 FEG. Optical characterization was carried out using UV-vis spectrometer, Specord Plus in the visible range. Vibrational characteristics was carried out using the Raman microscope, Horiba XploRA™ plus with 532 nm green laser as a source (25% laser power). X-ray photoelectron spectroscopy (XPS) was measured using PHI Versaprobe III. The obtained XPS spectra was tted using CasaXPS soware.

Photoelectrochemical characterization
Photoelectrochemical characterization was analysed using a 3electrode setup in which photoanodes/photocathodes, Ag/AgCl (saturated KCl) and platinum wire were working, reference and counter electrodes, respectively. The illumination source was 300 W ozone-free Xenon lamp (Ushio, Japan) from Holmarc, India adjusted to the power intensity of 100 mW cm −2 . The active area of the photoelectrodes was restricted to 1 cm 2 . The electrolyte used for all the PEC measurements was 0.1 M Na 2 SO 4 (pH 6) and was purged with nitrogen for 30 minutes before the experiments. All the PEC measurements were recorded with respect to back-side illumination unless otherwise mentioned. A linear sweep voltammetry (LSV) was measured at a scan rate of 20 mV s −1 . An electrochemical impedance spectroscopy (EIS) was performed under illumination at a frequency range of 10 5 Hz to 1 Hz using an AC signal amplitude of 10 mV. Chronoamperometry measurement (stability measurements) of photoanode and photocathode was performed under chopped illumination at a potential of 1.23 V vs. RHE and 0 V vs. RHE for photoanode and photocathode, respectively for 2000 seconds. The potential of the working electrode was converted with respect to reversible hydrogen electrode (RHE) potential using the formula, E RHE = E Ag/AgCl + 0.059 pH + E 0 Ag/AgCl , where E Ag/AgCl is the potential of working electrode with respect to Ag/AgCl and E 0 Ag/AgCl is the standard potential of Ag/AgCl electrode which is 0.197 V.

Construction of tandem cell
Tandem cells were constructed with BiVO 4 photoanode as the top electrode followed by Cu 2 O photocathode with a xed distance of 1 cm. Upon illumination, light passed to BiVO 4 photoanode (backside illumination) rst and then unabsorbed light from BiVO 4 transmitted to the Cu 2 O photocathode (frontside illumination). All tandem measurements were performed in 0.1 M Na 2 SO 4 (pH 6) purged with nitrogen for 30 minutes before the experiments in a two electrode setup. The illuminated area of the cell was limited to 1 cm 2 . The linear sweep voltammetry (LSV) and the stability of the tandem cell was measured using a PARSTAT advanced electrochemical workstation. Stability tests of the tandem device was measured under zero bias condition.

Structural characterization
The structural characterization of BiVO 4 and modied BiVO 4 photoanodes were analysed using XRD patterns. The XRD peaks of bare BiVO 4 and the modied BiVO 4 is presented in Fig. 1(a). The signature peaks at 18.8°and 28.9°in the photoanodes represent the monoclinic-scheelite structure of BiVO 4 . 39 The results implied that the proper heat treatment of BiVO 4 converts the tetragonal structure to monoclinic, which is preferred for better photocatalytic activity. The obtained results are wellmatched to JCPDS card no. 014-6888 (ref. 40) and are indexed accordingly. No peaks related to Mo doping was observed due to minute atomic percentage of molybdenum 41 but a slight red shi in the XRD spectra was observed for Mo doped BiVO 4 . The shi in the peak was due to the lattice strain caused by molybdenum on vanadium lattice points. 42 Diffraction peaks of FeOOH was not found because of the amorphous nature of asdeposited co-catalyst. Similarly, a poor crystallization of the asdeposited TiO 2 protective layer causes no diffraction peak.
Similarly, XRD spectrum of Cu 2 O photocathodes is presented in Fig. 1(b). The obtained signature peaks at 36.8°and 43.5°for photocathodes represent [111] crystal plane orientation. All the obtained peaks corresponds to the JCPDS card no. 65-3288. 43 The obtained XRD spectra of Cu 2 O photocathodes are cubic structure. The peak at 14.4°conrmed the presence of MoS 2 in Cu 2 O/MoS 2 and Cu 2 O/TiO 2 /MoS 2 photocathodes. 44 No peak was obtained for TiO 2 because of its amorphous nature. No other extra peaks of copper oxide and metallic copper was observed in the obtained spectra. The obtained crystal structure of BiVO 4 photoanodes and Cu 2 O photocathodes are monoclinic scheelite and cubic structures which provide better photocatalytic activity.
The Raman vibrational characteristics of BiVO 4 photoanodes and Cu 2 O photocathodes are presented in Fig. 2(a) and (b). The peak at 208. 18 (Fig. S1(b) in the ESI †). The peaks were assigned to in-plane and out-plane vibration of sulphur and molybdenum atoms.

Morphological characterization
The morphology of the as-prepared photoanodes and photocathodes were analysed using FESEM and the micrographs are presented in Fig. 3. The pure BiVO 4 ( Fig. 3(a)) photoanode exhibited nanoworm-like network morphology. No signicant change in morphology was observed for molybdenum doped photoanode. The protective layer TiO 2 was deposited on BiVO 4 by cost effective spin coating method. The distribution of TiO 2 conrmed by EDS mapping (Fig. S3 in the ESI †). No dramatic change in the morphology was observed for Mo-BiVO 4 /TiO 2 / FeOOH photoanode ( Fig. 3(b)). The thickness of the Mo-BiVO 4 / TiO 2 /FeOOH photoanode layer was estimated to be ca. 706.9 nm from cross sectional FESEM micrograph ( Fig. S2(a) in ESI †). The EDS mapping presented in Fig. S2 † proves that all the layers are conformally coated on the photoanode. FESEM micrograph of Cu 2 O photocathode are presented in Fig. 3(c) and (d). The observed morphology of the photocathodes are nanospheres like structures (Fig. 3(c)). The MoS 2 coating was found as sheets like structure throughout the photocathode (Fig. 3(d)). EDS  mapping (Fig. S4 in the ESI †) conrms the presence of all layers on Cu 2 O photocathode. The thickness of the photocathode was estimated to be ca. ∼1.7 mm (Fig. S2(b) in ESI †) from the cross sectional FESEM micrograph. The EDS chemical elemental analysis of the Mo-BiVO 4 /TiO 2 /FeOOH photoanode and Cu 2 O/ TiO 2 /MoS 2 was further supported by the XPS analysis shown in Fig. S20 to S22 in the ESI. †

Optical characterization
The optical characterization of photoanode and photocathode was analysed by UV-vis spectrometer and the results are presented in Fig. 4. The absorbance spectra shows a typical optical absorption of BiVO 4 with the absorption band edge starts from 500 to 550 nm for different composition of BiVO 4 samples ( Fig.  4 (a)). The optical bandgap was determined by constructing Tauc's plots (ahn = k(hn − E g ) n ) using an indirect optical transition (n = 2) for BiVO 4 . From the Tauc plot shown in Fig. 4(

Photoelectrochemical characterization
In order to conrm the effective operation of the tandem cell, individual photoelectrodes must be checked for the PEC activity. Hence, BiVO 4 photoanodes and Cu 2 O photocathodes were tested in 3-electrode conguration under illumination. The photoelectrochemical behaviour of the BiVO 4 photoanodes and Cu 2 O photocathodes was tested in 0.1 M Na 2 SO 4 (pH 6). The chopped linear sweep voltammetry (LSV) of BiVO 4 photoanode and Cu 2 O photocathode is presented in Fig. 5(a) and (b).
3.4.1 Photoanode. The dark current density of all the photoanodes are almost negligible. The prepared photoanodes provided anodic response under illumination suggesting that the materials were of n-type ( Fig. 5(a)). PEC parameters such as the onset potential and current density at water oxidation potential (1.23 V vs. RHE) are critical observation from LSV curves. There was a signicant cathodic shi of onset potential observed when bare BiVO 4 was doped with molybdenum and deposited with FeOOH and TiO 2 layers. The results indicate that addition of dopants, co-catalysts and protective layer improved the conductivity, reduced the surface defects and enhanced interfacial charge transfer. 46 The onset potential of bare BiVO 4 estimated from the LSV curve was ∼0.55 V vs. RHE. The bare BiVO 4 photoanode at water oxidation potential yielded a current density of ∼0.296 mA cm −2 . The obtained current density was very low compared to the theoretical current density of BiVO 4 . 47 Hence, modication on the surface of BiVO 4 photoanode were performed. The onset potential of the Mo-BiVO 4 was cathodically shied to ∼0.40 V vs. RHE and an enhancement in the current density of ∼0.465 mA cm −2 was observed as a result of the improvement in the conductivity of the photoanode. In addition to doping, the TiO 2 thin lms and FeOOH co-catalysts were deposited to protect the surface and to reduce the overpotential of BiVO 4 , respectively. The photocurrent density of Mo-doped BiVO 4 /TiO 2 photoanode was slightly better than Modoped BiVO 4 but lower than Mo-doped BiVO 4 /FeOOH. As pointed out in the literature, a single overlayer on BiVO 4 considerably minimizes the surface defects and as a consequence, the surface recombination is substantially suppressed compared to charge carrier kinetics. 48 We assume that the TiO 2 protective layer served its purposes as a protective layer to minimize surface defects. On the other hand, the Mo-BiVO 4 / FeOOH photoanode did the same protective role in addition to improve the reaction sites over TiO 2 because of its catalytic nature. The trend in the increase in the current density was followed as: BiVO 4 (∼0.296 mAcm −2 ) < Mo-BiVO 4 (∼0.465 mAcm −2 ) < Mo-BiVO 4 /TiO 2 (∼0.517 mA cm −2 ) < Mo-BiVO 4 / FeOOH (∼0.70 mA cm −2 ) < Mo-BiVO 4 /TiO 2 /FeOOH (∼0.81 mA cm −2 ). A four-fold increase in photocurrent was observed for Mo-BiVO 4 /TiO 2 /FeOOH compared to bare BiVO 4 due to the deposition of 2 overlayers of which the TiO 2 layer served as a protective layer and the FeOOH layer improved the kinetics of charge carriers. The stability test also suggested that the photoanode BiVO 4 /TiO 2 /FeOOH photoanode retained the current with a slight decay during the testing time of 2000 seconds ( Fig. S7(a) Tables S1, S2 and S7 in ESI, † respectively.
We also performed PEC tests of photoanodes in the presence of a hole scavenger (0.1 M Na 2 SO 4 + 0.1 M Na 2 SO 3 ) to measure the actual performance of the prepared photoanodes. To assess the actual performance of the prepared photoanodes, we calculated charge separation efficiency, injection efficiency and applied bias photon-to-current efficiency (ABPE). The results are shown in Fig. S8 in the ESI. † The photoanode coated with both the TiO 2 protective layer and FeOOH co-catalyst outperformed all other prepared photoanodes in terms of charge separation, injection and ABPE. The charge separation efficiency, the amount of photogenerated charge reaching the surface of the photoelectrode, was calculated to be 62% (Fig. S8(b) in ESI †) for Mo-BiVO 4 /TiO 2 /FeOOH. The injection efficiency (Fig. S8(c) in ESI †) is the amount of photogenerated carriers reaching the surface and get injected into the solution. The Mo-BiVO 4 /TiO 2 / FeOOH photoanode exhibited ∼80% injection efficiency. The ABPE (Fig. S8(d) †) reached 0.16% at 0.9 V vs. RHE for Mo-BiVO 4 / TiO 2 /FeOOH photoanode. The Mo-BiVO 4 /TiO 2 photoanode yielded better charge separation efficiency than the samples with Mo-BiVO 4 /FeOOH due to proper band level alignment of the former than the latter. 49 On the other hand, the charge injection efficiency was better for Mo-BiVO 4 /FeOOH than the Mo-BiVO 4 /TiO 2 photoanodes because of the reduction in overpotential generated by the FeOOH co-catalysts. 50 Overall, the results suggest that both the protective and co-catalyst layers can yield better photoelectrochemical performances for BiVO 4 photoanodes.
3.4.2 Photocathodes. Similar PEC studies were conducted for Cu 2 O photocathodes to examine their photoelectrochemical properties. The LSV of Cu 2 O photocathodes is shown in Fig. 5(b). All Cu 2 O based photocathodes displayed cathodic response under illumination conrming the p-type nature of photocathodes. The current density of bare Cu 2 O reached to ∼−0.612 mA cm −2 at 0 V vs. RHE with an onset potential of  (Fig. S7(b) in ESI †). The photocurrent retained at ∼−1.36 mA cm −2 for Cu 2 O/TiO 2 /MoS 2 photocathode during the testing time. On the other hand, the unprotected bare Cu 2 O photocathode's photocurrent declined quickly during the stability test due to instability of Cu 2 O photocathode in aqueous solution. The LSV response of best performing photoanode and photocathode was tested for frontside and back-side illumination and the response is presented Fig. S9 in the ESI. † The current density at water oxidation and reduction potentials, onset potential of BiVO 4 photoanodes and Cu 2 O photocathodes are listed in Table 1.
3.4.3 Electrochemical impedance spectroscopy (EIS) studies. Electrochemical impedance spectroscopy (EIS) is a powerful tool to analyse the charge carrier kinetics of the photoelectrodes. The EIS experiment was carried out under illumination with respect to the water oxidation potential (1.23 V vs. RHE) and water reduction potential (0 V vs. RHE) for photoanode and photocathode, respectively. The EIS result of photoanode is shown in Fig. 6(a). As noticed from the spectra, it is obvious that the charge transfer resistance followed the order as: Mo-BiVO 4 /TiO 2 /FeOOH < Mo-BiVO 4 /TiO 2 < Mo-BiVO 4 /FeOOH < Mo-BiVO 4 < BiVO 4 from the shape of the semicircle. The obtained EIS spectra was tted with different resistances and capacitances from which an equivalent circuit was constructed using Scribner's Z View soware. The tted values indicated a smaller charge transfer resistance for Mo-BiVO 4 /TiO 2 /FeOOH and thereby supported the maximum current obtained for this photoanode from the LSV response. The results also conrmed that the addition of both TiO 2 and FeOOH minimized surface defects and also supported the ndings of high charge injection efficiency and separation efficiency of the same photoanode. The EIS spectra of Cu 2 O photocathodes is shown in Fig. 6(b). The results revealed the charge transfer resistance in the following order: Cu 2 O/TiO 2 /MoS 2 < Cu 2 O/TiO 2 < Cu 2 O/MoS 2 < Cu 2 O. The minimum R s and R ct was obtained for Cu 2 O/TiO 2 / MoS 2 which was the best performing photocathode among Cu 2 O photocathodes as inferred from the LSV response. The results proved that the addition of TiO 2 protective and MoS 2 cocatalyst layers facilitate better charge transfer from the electrode to electrolyte. The equivalent circuit of photoanode and photocathode was similar to Randle's circuit and was presented in Fig. S10 in the ESI. † The tted results of different resistances and capacitances for both photoanode and photocathode were tabulated in the ESI Table S5. † The Mott-Schottky analysis was performed in the dark condition at 1 kHz frequency for both BiVO 4 based photoanodes and Cu 2 O based photocathodes. The corresponding Mott-Schottky plots of photoanodes and photocathode are illustrated in Fig. S11 and S12 † and the at band potential values are displayed in Table S6 in the ESI, † respectively.

Tandem cell measurements.
Aer analysing the PEC properties of individual photoanode and photocathodes, we analysed overlaying the individual LSV response of the Mo-BiVO 4 /TiO 2 /FeOOH photoanode and Cu 2 O/TiO 2 /MoS 2 photocathode as shown in Fig. 7. From the overlay plot, we obtained from the intersection point the upper limit of the operating potential (0.66 V vs. RHE) and operating current density (0.129 mA cm −2 ) for the tandem cell. The intersection point was also obtained for Cu 2 O/TiO 2 /MoS 2 photocathode ltered with Mo-BiVO 4 /TiO 2 /FeOOH photoanode which were 0.58 V vs. RHE and 0.095 mA cm −2 . The non-zero points provide the possibility of unassisted tandem cell operation. The energy band diagram of the proposed tandem cell which consisted of Mo-BiVO 4 /TiO 2 / FeOOH-Cu 2 O/TiO 2 /MoS 2 is presented in Fig. 8. As noticed from the band diagram, the band positions are favoured for both the photoanode and photocathodes in terms of both electron and hole transports to the electrolyte for reduction and oxidation reactions, respectively.
The LSV response of the tandem cell was recorded from −0.2 to 1.3 V and the result is presented in Fig. 9(a) The stability test of the tandem cell was conducted and the result is shown in Fig. 9(b). The stability curve of Mo-BiVO 4 / FeOOH-Cu 2 O/MoS 2 was not stable for rst 500 seconds and it started to decay for every 500 second cycle. On the other hand, the photocurrent stability of Mo-BiVO 4 /TiO 2 /FeOOH-Cu 2 O/  TiO 2 /MoS 2 produced a stable current density of ∼65.3 mA cm −2 with a slight decay at the end of 3000 seconds (∼53.9 mA cm −2 ). Despite the observed current density was low for TiO 2 protected tandem structure, the photoelectrodes were very stable enough during the operating window of 3000 seconds. The results revealed that TiO 2 deposited by spin coating method could be an alternative and economical approach to protect the photoelectrodes in aqueous environment.

Post-PEC analysis
Post-PEC analysis such as XRD (Fig. S14 †), SEM with EDS ( Fig. S15 to S19 †), and XPS ( Fig. S20 to S22 †) was performed in order to understand the physical and chemical state of Mo-BiVO 4 /TiO 2 /FeOOH photoanode and Cu 2 O/TiO 2 /MoS 2 photocathode (See ESI †). The crystallinity of the prepared photoelectrodes retained the same XRD spectra aer the PEC tests suggesting that there is no change in the crystalline phase of the prepared photoelectrodes. From the EDS studies, we observed that the chemical state of co-catalyst FeOOH intensity was decreased and also the chemical state of MoS 2 was signicantly decreased indicating the poor adhesion of co-catalysts on the photoelectrode which will be improved in the future work. Similarly, XPS analysis also supported the EDS ndings that there is a signicant loss of both co-catalysts aer the PEC test. In addition, we observed the presence of both Cu + (Cu 2 O) and Cu 2+ (CuO) peaks in the XPS spectra. The result indicates the possibility of CuO also in the sample aer PEC test. The reason could be exposure of Cu 2 O on the side edges and/or the protective layer deposition may be not conformal in addition to some porosity. Despite the change in chemical state of copper, the TiO 2 protected photocathodes performed better than the bare photoelectrodes. We will explore and optimize the deposition parameters of protective layer in our future work.

Conclusions
Thin lms BiVO 4 photoanode and Cu 2 O photocathode tandem cell protected by spin-coated TiO 2 was demonstrated for unassisted solar water splitting. The photoanode consisted of Mo-BiVO 4 /TiO 2 /FeOOH yielded better photocurrent than bare BiVO 4 . The improvement in photocurrent for Mo-BiVO 4 /TiO 2 / FeOOH was attributed to (i) the TiO 2 layer which protected the surface from defects and (ii) the FeOOH layer which improved the kinetics of charge carriers. The results were further supported by EIS spectra and charge separation and injection studies. The photocathode consisted of Cu 2 O/TiO 2 /MoS 2 showed better PEC performance compared to bare Cu 2 O because of moderate protection of Cu 2 O from photocorrosion and an improvement of charge carrier kinetics by MoS 2 . The tandem cell made of Mo-BiVO 4 /TiO 2 /FeOOH-Cu 2 O/TiO 2 /MoS 2 produced a stable current density of ∼65.3 mA cm −2 at zero bias with better stability and the retention percentage of photocurrent was 83.6% in the unassisted stability test. The co-catalyst dissolution aer the PEC test due to improper adhesion will be further explored. The results suggested that spin-coated TiO 2 could be an alternative viable approach for achieving moderately better PEC performance compared to unprotected photoelectrodes. The quality of TiO 2 can be further explored by varying the spin speed, concentration, and annealing temperature to better optimize the lms.