Duc N Nguyen

Mariam Fadel

Pascale Chenevier

Vincent Artero

Phong D Tran

Water-Splitting Artificial Leaf Based on a Triple-Junction Silicon Solar Cell: One-Step Fabrication through Photoinduced Deposition of Catalysts and Electrochemical Operando Monitoring

whether they are published or not. The documents may come

Main Text Introduction

Solar water splitting represents a potential technological solution for the large-scale production of H 2 fuel in the future. Great efforts are being devoted to develop efficient components, namely hydrogen evolving catalysts, oxygen evolving catalysts and light harvesters, which are required to construct solar water splitting devices. Important progresses have been achieved in the last decade in assembling these components to make bias-free operational devices. [START_REF] Rongé | Monolithic cells for solar fuels[END_REF][START_REF] Tembhurne | A thermally synergistic photo-electrochemical hydrogen generator operating under concentrated solar irradiation[END_REF][START_REF] Cheng | Hydrogen from Sunlight and Water: A Side-by-Side Comparison between Photoelectrochemical and Solar Thermochemical Water-Splitting[END_REF][START_REF] Kim | Fully solution-processable Cu 2 O-BiVO 4 photoelectrochemical cells for bias-free solar water splitting[END_REF] The first solution relies on metal oxide photocatalysts like SrTiO 3 , which usually have large band gap and therefore harvest only UV or near UV photons of the solar spectrum. As a result, these photocatalysts offer low solar to H 2 conversion yield ( STH ), e.g. of ca. 1%. [START_REF] Wang | Scalable water splitting on particulate photocatalyst sheets with a solar-to-hydrogen energy conversion efficiency exceeding 1%[END_REF][START_REF] Goto | A Particulate Photocatalyst Water-Splitting Panel for Large-Scale Solar Hydrogen Generation[END_REF] However, they are cost effective, (photo)chemically stable, e.g. against the chemical corrosion and are appropriate for integrating within a large-sized device. Domen and coll. demonstrated the safe operation of a 100 m 2 solar farm based on 625 cm 2 photocatalytic sheets obtained by drop-casting SrTiO 3 :Al nanoparticles on a glass substrate. The system showed a  STH of 0.76% under the natural sun light. [START_REF] Nishiyama | Photocatalytic solar hydrogen production from water on a 100 m 2 scale[END_REF] Such metal oxides can also be used as photoelectrode materials in tandem photoelectrochemical cells (PEC), sometimes after dye-sensitization and proper combination with catalysts. [START_REF] Windle | Earth-Abundant Molecular Z-Scheme Photoelectrochemical Cell for Overall Water-Splitting[END_REF][START_REF] Sherman | A Dye-Sensitized Photoelectrochemical Tandem Cell for Light Driven Hydrogen Production from Water[END_REF] Low band-gap semiconductors such as silicon, III-V materials or organic semiconductors can also be exploited to prepare PEC prototypes. The second solution consists of using an appropriate solar cell to power a water electrolysis system in a decoupled (wired) configuration. This approach provided the most important progress in the field in term of  STH performance. [START_REF] Wang | Scalable water splitting on particulate photocatalyst sheets with a solar-to-hydrogen energy conversion efficiency exceeding 1%[END_REF][START_REF] Luo | Water photolysis at 12.3% efficiency via perovskite photovoltaics and Earth-abundant catalysts[END_REF][START_REF] Gao | Solar Water Splitting with Perovskite/Silicon Tandem Cell and TiC-Supported Pt Nanocluster Electrocatalyst[END_REF][START_REF] Lin | Solar-powered overall water splitting system combing metal-organic frameworks derived bimetallic nanohybrids based electrocatalysts and one organic solar cell[END_REF] Jaramillo and coll. reported on the use of a InGaPGaAsGaInNAs(Sb) triplejunction solar cell to power a dual proton exchange membrane water electrolyser made of Pt/C and Ir/C water electrolysis catalysts. [START_REF] Wang | Scalable water splitting on particulate photocatalyst sheets with a solar-to-hydrogen energy conversion efficiency exceeding 1%[END_REF] The wired device showed a remarkable  STH of 30%.

Highly efficient photoabsorbers such as silicon, [START_REF] Schüttauf | Solar-to-Hydrogen Production at 14.2% Efficiency with Silicon Photovoltaics and Earth-Abundant Electrocatalysts[END_REF][START_REF] Eftekharinia | Unassisted Water Splitting Using Standard Silicon Solar Cells Stabilized with Copper and Bifunctional NiFe Electrocatalysts[END_REF] III-V semi-conductor junctions, [START_REF] Tembhurne | A thermally synergistic photo-electrochemical hydrogen generator operating under concentrated solar irradiation[END_REF][START_REF] Wang | Scalable water splitting on particulate photocatalyst sheets with a solar-to-hydrogen energy conversion efficiency exceeding 1%[END_REF] or hybrid organic-inorganic perovskites [START_REF] Luo | Water photolysis at 12.3% efficiency via perovskite photovoltaics and Earth-abundant catalysts[END_REF] or their tandem combination [START_REF] Gao | Solar Water Splitting with Perovskite/Silicon Tandem Cell and TiC-Supported Pt Nanocluster Electrocatalyst[END_REF] were also used. These materials are known to be unstable in the presence of aqueous electrolytes, so that a decoupled configuration, in which the solar cell stays in the air to minimize corrosion while only the two water electrolysis catalyst electrodes are in contact with the electrolyte solution is usually preferred. Despite being efficient and based on two existing mature technologies, such wired solar cell + electrolysis systems involves high capital expenditures that may prevent their adoption by the market for decentralized hydrogen production. [START_REF] Eftekharinia | Unassisted Water Splitting Using Standard Silicon Solar Cells Stabilized with Copper and Bifunctional NiFe Electrocatalysts[END_REF] An operational wireless device combining the solar cell and the two catalytic electrodes in a monolithic architecture would represent a more practical solution. [START_REF] Rongé | Monolithic cells for solar fuels[END_REF] Such a configuration requires burying the photoabsorbers within the architecture to avoid contact with the aqueous electrolyte and prevent their corrosion.

Following the initial demonstrations by Miller and Rocheleau (Table S1), [START_REF] Rocheleau | High-Efficiency Photoelectrochemical Hydrogen Production Using Multijunction Amorphous Silicon Photoelectrodes[END_REF] Nocera and coll.

reported on the creation of an artificial leaf based on a triple junction amorphous Si solar cell (3jn-a-Si) coated with a NiMoZn alloy H 2 -evolving catalyst and a CoPi O 2 -evolving catalyst. [START_REF] Reece | Wireless Solar Water Splitting Using Silicon-Based Semiconductors and Earth-Abundant Catalysts[END_REF][START_REF] Surendranath | Interplay of oxygen-evolution kinetics and photovoltaic power curves on the construction of artificial leaves[END_REF][START_REF] Cox | Ten-percent solar-to-fuel conversion with nonprecious materials[END_REF] This wireless monolithic device displays a  STH of 2.5% when being conditioned for operation in a pH 9.2 borate buffer. While this 2011 report revived the field of monolithic devices, limited data are available regarding the actual limitations in terms of light harvesting, catalysis and interface thereof within the system. 1 19-20 In terms of scaling-up the fabrication of the devices to deploy the technology, another limitation comes from the multistep assembly of the device, which requires a photo-assisted electrodeposition of the CoPi catalyst on the anode-side followed by an electrodeposition of the NiMoZn catalyst on the cathode-side of the 3jn-a-Si solar cell.

Over years of intensive development, several promising H 2 -evolving and O 2 -evolving catalysts made of earth-abundant-elements are available. Among those, bimetallic sulfides like cobalt molybdenum sulfide [START_REF] Merki | Ni ions promote the catalytic activity of amorphous molybdenum sulfide films for hydrogen evolution[END_REF][START_REF] Chen | Silicon Decorated with Amorphous Cobalt Molybdenum Sulfide Catalyst as an Efficient Photocathode for Solar Hydrogen Generation[END_REF][START_REF] Karikalan | Cobalt molybdenum sulfide decorated with highly conductive sulfur-doped carbon as an electrocatalyst for the enhanced activity of hydrogen evolution reaction[END_REF] or cobalt tungsten sulfide [START_REF] Tran | Novel cobalt/nickel-tungsten-sulfide catalysts for electrocatalytic hydrogen generation from water[END_REF][START_REF] Yang | Amorphous nickel/cobalt tungsten sulfide electrocatalysts for high-efficiency hydrogen evolution reaction[END_REF] are promising noble metal free H 2evolving catalysts whereas their oxide/hydroxide counterparts are active O 2 -evolving catalysts. 24- 27 Even, few of these bimetal sulfides or oxides were reported to be efficient bifunctional or Janus catalysts for driving the overall water splitting. [START_REF] Yadav | Highly efficient porous morphology of cobalt molybdenum sulfide for overall water splitting reaction[END_REF][START_REF] Ray | Amorphous Phosphorus-Incorporated Cobalt Molybdenum Sulfide on Carbon Cloth: An Efficient and Stable Electrocatalyst for Enhanced Overall Water Splitting over Entire pH Values[END_REF] These materials can be prepared as thin films on a conducting or a semiconducting electrode via electrochemical or photo-assisted electrochemical deposition. For example, some of us reported that, using a solution of [Co(WS 4 ) 2 ] 2-in a pH 7 phosphate buffer, a cobalt tungsten sulfide layer (denoted hereafter as CoWS) active for the H 2 -evolution can be grown at an onset cathodic potential of +0.1V vs.

RHE (here and after, we arbitrarily define onset (over)potentials as the (over)potential values at which the modulus of the current density exceeds 0.1 mA.cm -2 ). [START_REF] Tran | Novel cobalt/nickel-tungsten-sulfide catalysts for electrocatalytic hydrogen generation from water[END_REF] From the same solution, a cobalt tungsten oxide layer (denoted hereafter as CoWO) being active for the O 2 -evolution can be obtained at an onset anodic potential of 1.3V vs. RHE [START_REF] Nguyen | Electrodeposited Amorphous Tungsten-doped Cobalt Oxide as an Efficient Catalyst for the Oxygen Evolution Reaction[END_REF] (see also figure S1). In a pH 7 phosphate buffer these catalysts operate with respective onset overpotentials of -0.

Results and discussion

The commercial 3jn-aSi solar cell is composed of three amorphous Si junctions where each junction consists of an intrinsic Si (i: a-Si) layer sandwiched by a p-type (p: µc-Si) layer and a ntype (n: a-Si) layer. The front-side of the solar cell is protected by an indium-doped tin oxide (ITO) layer. The whole solar cell stands on a stainless steel substrate (figure S3). Hereafter, we denote the solar cell as ITO3jn-a-SiSteel. For manipulation, it was first cut in small species of 15 ×17 mm 2 . The cell edge was then carefully cleaned with a 1M HCl solution to remove any broken Si part that might have negative impact onto the cell operation. The cell edge was then covered by an epoxy resin or just a commercial steel paint. The working area on both the ITO front-side and the Steel back-side was limited to 10×10 mm 2 (figures S4, S5. See SI for detailed preparation of the cell). We employed a two-electrode configuration to verify the actual photovoltage that was generated when the cell was illuminated with simulated sunlight (100 mW.cm -2 ; Xe lamp filtered with AM 1.5G filter ) in air without being immersed in any electrolyte solution (see SI for details). A stable photovoltage of 1.95 V was recorded (figure 1a, inset showing the evolution of photovoltage recorded using the chopped light mode) which was lower than the value of 2.2 V expected from the product specifications. [START_REF] Xunming | In Optimization of a-SiGe based triple, tandem and single-junction solar cells[END_REF] This observation was confirmed through the V oc of 2.02 V obtained through the characteristic current-voltage plot. The fill factor was recorded at 34.6%, also lower than the expected 67.2% (figure 1b), reflecting the lower performances of large commercial samples (here 30 ×20 cm 2 ) compared to small lab-scale samples. However, the H 2 /O 2 molar ratio was found to range between 4 : 1 and 6 : 1, thus much higher than the expected 2 : 1 molar ratio for overall water splitting. This result suggests that electrodeposition of the CoWO catalyst continues to compete with O 2 evolution to a greater extent than the corresponding reductive processes. In the online gas chromatography analysis, we observed the maximal rate of H 2 evolution after 20 minutes of operation. However, this value is convoluted with the few-minutes time lag between actual gas evolution on the device and its quantification in the gas chromatograph. We thus conducted an optimization of catalyst deposition time between 1 to 20 min. After catalyst deposition, the resultant monolithic device was taken out, intensively washed with DI water and then naturally dried in air. The resulting monolithic devices were again tested for H 2 evolution in a pH 7 phosphate buffer solution being free of [Co(WS 4 ) 2 ] 2-(Table 1 andfigure S6). Formation of bubbles during photoelectrochemical water splitting may affect the performance of the device through an increase of the backscattering losses, thereby decreasing the photocurrent density generated. [START_REF] Bhanawat | How do bubbles affect light absorption in photoelectrodes for solar water splitting?[END_REF] Noteworthy, the electrolyte was continuously bubbled in our setup with Ar as the carrier gas used for gas chromatographic detection. The resulting convection of the electrolyte was continuously detaching H 2 and O 2 bubbles, potentially adhering to the catalytic layers, therefore limiting this back-scattering effect and allowing to reach a steady state in terms of performances.

The H 2 evolution rate was found to increase with the catalyst deposition time until the latter reached 10 minutes (Table 1, entries 1-4). A marginal improvement was found when the time was increased to 20 min (Table 1, entry 5) and, in that case, we sometimes observed the detachment of the CoWO catalyst layer from the ITO surface. Thus, we decided to set the catalyst deposition time for 10 minutes for the fabrication of all devices.

We then optimized the chemical composition of the catalyst deposition bath in order to achieve the best monolithic device. Actually, in situ formation of the [Co(WS 4 ) 2 ] 2-species is expected regardless the Co 2+ / [WS 4 ] 2-molar ratio employed. [START_REF] Müller | Delocalized molecular orbitals in the trimetallic thioheteroanion [S 2 WS 2 CoS 2 SW 2 ] 2-: spectroscopic and cyclic voltammetric results[END_REF] S7). Hereafter, we focus on monolithic devices prepared under these optimized conditions.

We also measured the performances of the device in pH 0.3 (0.5 M H 2 SO 4 ), 9.2 (borate buffer) and 14 (1M NaOH) electrolytes. While deterioration of the catalytic layers occurred within 20 min. of exposure to pH 0.3 and 14 electrolytes in the dark (Figure S8), the performance of the artificial leaf in pH9.2 borate buffer (Figure S8) was observed to be approximately half of that measured in neutral phosphate buffer. This lower performance could be the result of the instability of the CoWS catalyst, as previously shown for amorphous molybdenum sulfide. [START_REF] Tran | Coordination polymer structure and revisited hydrogen evolution catalytic mechanism for amorphous molybdenum sulfide[END_REF] For this reason, we report in the following the data we measured in neutral phosphate electrolyte which has been also advocated by Costentin and Nocera for a safe and deployable technology. 36 (Figure S12). The chemical composition of the deposit was further characterized by XPS. In the W4f core spectrum, a doublet with W4f 7/2 binding energy of 35.36 eV was found, indicating the presence of the W 6+ oxidation state (Figure S13). [START_REF] Nguyen | Electrodeposited Amorphous Tungsten-doped Cobalt Oxide as an Efficient Catalyst for the Oxygen Evolution Reaction[END_REF][START_REF] Chakrabarti | Temperature-Dependent Non-linear Resistive Switching Characteristics and Mechanism Using a New W/WO 3 /WO x /W Structure[END_REF] In the Co2p spectrum, the main and satellite Co2p 3/2 were observed at 781.3 and 785.9 eV that suggested the presence of the Co 2+ oxidation state. [START_REF] Zhao | Quasi-Two-Dimensional Earth-Abundant Bimetallic Electrocatalysts for Oxygen Evolution Reactions[END_REF][START_REF] Zang | Cobalt Molybdenum Oxide Derived High-Performance Electrocatalyst for the Hydrogen Evolution Reaction[END_REF] P 2p3/2 was found at 132.9 eV, being consistent with a phosphate species adsorbed onto cobalt oxide surface. [START_REF] Nguyen | Electrodeposited Amorphous Tungsten-doped Cobalt Oxide as an Efficient Catalyst for the Oxygen Evolution Reaction[END_REF][START_REF] Cobo | A Janus cobalt-based catalytic material for electrosplitting of water[END_REF][START_REF] Laudadio | Impact of Phosphate Adsorption on Complex Cobalt Oxide Nanoparticle Dispersibility in Aqueous Media[END_REF][START_REF] Palma | Atomic layer deposition of cobalt phosphate thin films for the oxygen evolution reaction[END_REF] Sulfur was not found. Thus, all analysis performed on CoWO deposited onto ITO show full similarity with the same material deposited on FTO [START_REF] Nguyen | Electrodeposited Amorphous Tungsten-doped Cobalt Oxide as an Efficient Catalyst for the Oxygen Evolution Reaction[END_REF] and indicate that CoWO is an amorphous cobalt tungsten oxide having surface-bound phosphate species. Figure S14 shows that, similarly to those deposited on FTO, the CoWO films deposited on ITO display a good stability when achieving continuous water oxidation in phosphate buffer.

To gain more insights into the light absorption properties of this catalytic film, we fabricated CoWO films using electrodeposition on FTO substrates at 2 mA.cm -2 current density corresponding to the value measured in wired configurations. CoWO films deposited on FTO are quite diffusive [START_REF]We preferred FTO to ITO substrates as they allow increased reproducibility of the measures in terms of scattering[END_REF] and show a characteristic absorption peak at 380nm. To correct absorption measurements from light scattering, the absorbance at 550 nm, where CoWO films are fully transparent, was substracted from the absorbance measured at 380nm (Figure S15). In parallel, the thickness of the films was determined from SEM cross section images. Although with large data dispersion due to the inhomogeneityof the film over the whole sample as evidenced by color gradients in the CoWO films in Figure S9, Figure S15 reveals a linear correlation of absorbance and film thickness with deposition time, allowing to determine a thickness of ~30 nm for films deposited over 10 min as in the optimized monolithic device.

On the steel surface, the CoWS deposit composed of small spherical nanoparticles with average size of 100 nm was observed (Figure 3c,d,S16). The deposit was found compact and fully covering the steel surface. The EDX elemental mapping shows a homogeneous distribution of Co, W, O, P, and S elements over the deposit (Figures S17,S18). XPS analysis showed Co2p, W4f and P2p signatures similar to those recorded for the CoWO layer (Figure S13). A significant difference was the appearance of the S element, with two characteristic S2p

Evaluation of the performance of the monolithic device for overall solar water splitting

Figure 4a shows the actual evolution rates of H 2 and O 2 gases generated over illumination time using the optimized monolithic device in the pH 7 phosphate buffer solution. The molar H 2 /O 2 ratio is 2:1, thus confirming the overall water splitting process. When the light illumination was stopped, e.g. after 3 hours of operation, the generation of H 2 and O 2 gases ended immediately and the accumulated amount of these gases remained constant (figure 4b). We thus conclude unambiguously that H 2 and O 2 were products of the overall solar water splitting process driven by the CoWOITO3jn-a-SiSteelCoWS monolithic device. The average solar-to-hydrogen conversion yield ( STH ) of 1.9% was calculated based on 3 hour experiments. We note that the current performance is close to that reported by Nocera and coll. ( STH of 2.5%) for a similar 3jn-a-Si solar cell loaded with CoPi OER and NiZnMo HER catalysts. [START_REF] Reece | Wireless Solar Water Splitting Using Silicon-Based Semiconductors and Earth-Abundant Catalysts[END_REF] However, our current 14 device operates under more benign conditions, e.g. pH 7 phosphate buffer versus pH 9.2 borate buffer in the case of Nocera`s device. The stability of the optimized monolithic device was then investigated in a series of solar water splitting experiment. Each experiment consisted of 3 hours light illumination. Between experiments, the device was kept in the same solution in the dark for 72 hours. The device performance was found to decrease over time, reaching  STH of 0.8 % after 4 cycles, corresponding to 12 hours of operation in total (Figures 4c andS20). Adding [Co(WS 4 ) 2 ] 2-to the operating phosphate medium allows to partly restore the device performance with  STH of 1.4 %. These result demonstrated the progressive degradation of the monolithic device but also its capability to heal with additional [Co(WS 4 ) 2 ] 2-catalyst precursor.

We finally examined the operation of this optimized monolithic device at higher irradiation power. To this end, the device was first conditioned under the standard simulated sunlight until the water splitting rate, quantified by an online gas chromatography, reached a steady value.

Then, a Fresnel lens was used to concentrate the light illumination reaching the device to 2.2 sun.

Immediately, the water splitting rate increased reaching a steady value 2.2 time higher than that obtained under 1 sun illumination (Figure 4d), thus with unmodified  STH of 1.9%. This result indicates that the water splitting rate of the optimized monolithic device is still limited by the supply of photons to the the ITO3jn-a-SiSteel solar cell not by the catalytic ability of the CoWO-OER and CoWS-HER catalysts or the light harvester/catalysts interfacing.

Aforementioned, the device performance slightly decreased over the operation time, e.g. 18 % of the initial H 2 evolution rate was lost after 3 hours (Figure 4a). In order to gain insights into the performance degradation, we first characterized the catalyst layers after 3 hours of the catalytic operation. The CoWO layer showed no obvious change in surface morphology as revealed by FE-SEM analysis (Figure S21). Whereas, the CoWS layer experienced an obvious change in morphology. The spherical CoWS nanoparticles (~100 nm in diameter) transformed into walllike objects suggesting a partial dissolution of the material during the H 2 -evolving process (Figure S22). A similar structure conversion from amorphous structure to wall-like structure under H 2 -evolving conditions has been demonstrated in the case of molybdenum sulfide. [START_REF] Lee | Chemical and Phase Evolution of Amorphous Molybdenum Sulfide Catalysts for Electrochemical Hydrogen Production[END_REF][START_REF] Xi | Structural Transformation Identification of Sputtered Amorphous MoS x as an Efficient Hydrogen-Evolving Catalyst during Electrochemical Activation[END_REF] Unfortunately, attempts to compare the film thickness by a cross-section SEM analysis was not successful since the CoWO and CoWS films were too thin. In order to detect the catalyst partial dissolution, the chemical composition of the phosphate solution was analyzed using ICP-OES at different time intervals. Both dissolved Co and W were found, thus clearly confirming the partial dissolution of catalysts (Figure S23). However, for this monolithic device operation, it is not possible to easily distinguish the cathodic CoWS catalyst dissolution to the anodic CoWO catalyst dissolution, if the latter also occurs, nor to quantify the degree of redeposition of these catalysts. Indeed, efficient redeposition of cobalt, manganese and nickel oxides from lowconcentration metal salts in electrolyte causing the healing capability of these catalysts has been previously reported. [START_REF] Thorarinsdottir | Self-healing oxygen evolution catalysts[END_REF] However the situation experienced here is significantly more complex as we both deal with two distinct OER and HER catalysts and with bimetal catalysts with the possibility for monometallic phases to redeposit in addition to the initial bimetallic phases. The combination of these two phenomena prevented the modelling of the data shown in Figure S23.

Such dissolution/ redeposition processes likely alter the chemical composition of the CoWO and the CoWS layers over time, thus contributing to the decrease of performance of the monolithic device. This data is also consistent with the possibility to heal the deposits upon addition of [Co(WS 4 ) 2 ] 2-in the electrolyte as discussed above (Figures 4c andS20). To get more insights into these processes, it is however necessary to have additional information on the electrochemical potential at which they occur. This prompted us to develop methodologies allowing to monitor both the current density flowing through the device, as well as the electrochemical potential provided to the ITO and Steel electrodes by the 3jn-a-Si solar cell.

Investigation on operation of monolithic device: Half-cell assessment

In order to gain insights into the leaf operation, we first probed the operation of the two halfcells, namely the SteelCoWS cathode and the CoWOITO3jn-a-Si photoanode, in a pH 7 phosphate buffer electrolyte. The SteelCoWS cathode was found to catalyze for the H 2 evolution with a moderate onset overpotential of 250 mV, being close to that recorded for a similar film deposited on FTO substrate (figure 5a). [START_REF] Tran | Novel cobalt/nickel-tungsten-sulfide catalysts for electrocatalytic hydrogen generation from water[END_REF] To assay the CoWOITO3jn-a-Si photoanode, an Ohmic contact was applied to the stainless steel base. Under the illumination of 1 sun light, the photoanode showed an onset photopotential of -0.66 V vs. RHE and a photocurrent density of ca. 9.0 mA.cm -2 at 1.23V vs. RHE applied potential (figure 5b, red bold trace). Such performance proved superior to that previously reported for a CoPiITO3jn-a-Si photoanode operated in a pH 9.2 borate buffer (onset potential of -0.4 V vs. RHE and steady photocatalytic current of ca. 4 mA.cm -2 at +0.55 V vs. RHE), [START_REF] Reece | Wireless Solar Water Splitting Using Silicon-Based Semiconductors and Earth-Abundant Catalysts[END_REF] evidencing the advantage of the CoWO layer to promote catalytic O-evolution as compared to CoPi. A control measurement using a pristine ITO3jn-a-Si photoanode without decoration of a CoWO layer showed similar (-0.39 V vs RHE) onset photocurrent potential but significantly lower photocurrent density of less than 1.44 mA.cm -2 at 1.23V vs. RHE (figure 5b, red dotted trace).

Investigation on operation of monolithic device: Operando analysis

Next, we designed an experiment aimed at probing the evolution of both the photovoltage and the photogenerated current circulating between the CoWOITO anode-side and the SteelCoWS cathode-side of the monolithic device when it was illuminated. To this end, a biopotentiostat configuration was used. The CoWO-OERITO anode side of a monolithic device was interfaced with a pH 7 phosphate buffer electrolyte and exposed to light whereas its corresponding SteelCoWS cathode side was exposed to air. To simulate this cathode, another SteelCoWS electrode was fabricated by etching out the CoWOITO3jn-a-Si component from a monolithic device (see the supporting information for more details). The simulating SteelCoWS cathode was immersed in the electrolyte but not exposed to light (figure 6a). The photoanode was used as a working electrode and the cathode as both counter and reference electrode. The potentiostat was then operated in chronoamperometric mode with an applied potential to the photoanode of 0

V vs the cathode. This reproduces the wired configuration already reported by Nocera and colleagues, [START_REF] Reece | Wireless Solar Water Splitting Using Silicon-Based Semiconductors and Earth-Abundant Catalysts[END_REF] but allows monitoring the current flowing between both electrodes. Upon illumination a photocurrent of ~2 mA.cm -2 was initially measured, in line with the recorded 1.9% STH efficiency of the system. This current density is however not stable with time and slowly decreases to 1-1.5 mA.cm -2 over a 3h period (figure 6a). potential of 2.04 V vs. RHE and a cathodic potential of 0.10V vs RHE were recorded and found stable over 3 hours of measurement (figure S24b, red and black dash traces respectively). Thus, the photovoltage generated in this case was 1.94 V, quite similar to the value expected from and SteelCoWS electrode, respectively. These potentials were found to be stable over 24 hours (figure 7b, purple and blue traces). In total, the ITO3jn-a-SiSteel solar cell generated a stable photovoltage of 1.9 V between the CoWO ITO and SteelCoWS electrodes, as for the CoWOITO3jn-a-SiSteelCoWS monolithic device (figure 1). This experiment allows to conclude that the contact with an aqueous electrolyte is the major cause for the degradation over time of the photopotential, and hence the photocurrent, of the CoWOITO3jn-a-SiSteelCoWS artificial leaf.

The degradation of this fully wired device over several tens of min. was related to slow and limited corrosion of the catalysts in the HER/OER process (the photovoltage generated by the 3jn-a-Si cell remains constant). Accordingly, dissolved Co and W species were found in the phosphate electrolyte during the system operation as revealed by the ICP-OES analysis. Thus the accelerated degradation observed for the partially wired configuration (figure 6b) can be assigned to the degradation of the ITO3jn-a-SiSteel solar cell, causing a voltage drop and therefore a drop of the overall solar water splitting performance. In other words, the CoWO/ITO layer do not perfectly protect the inner 3jn-a-Si light harvesting component against corrosion.

Actually, the oxidative corrosion of a native Si electrode could occur at an extremely low O 2 dissolved concentration, e.g. 15 ppb. [START_REF] Hou | Bioinspired molecular co-catalysts bonded to a silicon photocathode for solar hydrogen evolution[END_REF] Thus, in the case a crack, a pin-hole or just a non-perfect coverage present on CoWO ITO layer, the electrolyte-exposed Si layer would be quickly oxidized by the O 2 which evolves from nearby CoWO catalyst. We posit that the clamp configuration of the partially wired configuration we used can be responsible for the formation of such defects due to mechanical constrains, which in turns highlights the interest of the compact architecture of the monolithic artificial leaf. Still, this indicates that the deposition of a thicker ITO layer may be a solution to obtain more stable devices.

However, after an initial jump to ~2 mA.cm -2 , the photocurrent density generated in the separated photovoltaic-electrocatalytic, fully wired configuration rapidly decays to 0.2 mA.cm - 2; It then appear to be quite stable with less than 10% decrease within 24 h (figure 7b). As observed in the semi-wired configuration with potential sensing at the surface of the photoanode, the photocurrent is significantly lower than that of the full artificial leaf. This ten-fold lower value is in line with the solar-to-H 2 conversion yield  STH of 0.23 % determined from the quantification of evolved H 2 gas, allowing to confirm a faradic efficiency of the water splitting catalysts close to 100% (figure 7c). Reasons for such an inactivation of ~90% of the device is still under investigation but may be related to the fabrication of an ohmic contact with the ITO layer. Illumination was provided by a simulated 1 sun light.

Conclusion

The development of the artificial leaf technology based on amorphous silicon triple junction solar cells requires the establishment of simple fabrication procedures. We report here on a novel method based on a simple [Co(WS 4 ) 2 ] 2-electrolytic solution to simultaneously drive the photodeposition of O 2 -evolving and H 2 -evolving catalysts onto the respective anodic and cathodic sides of a triple junction Si solar cell. This rather simple solution process without a requirement of any high-tech facility allowed to create a complete CoWO ITO3jn-a-SiSteelCoWS monolithic device for the overall solar water splitting application. The current best device offered a solar-to-H 2 conversion yield of 1.9% when being operated under benign conditions, e.g. in a pH 7 phosphate buffer solution, thus quite similar to previously reported performances. As observed for other hybrid PEC devices, [START_REF] Wadsworth | Interplay between Light Flux, Quantum Efficiency, and Turnover Frequency in Molecular-Modified Photoelectrosynthetic Assemblies[END_REF][START_REF] Nishiori | Parallels between enzyme catalysis, electrocatalysis, and photoelectrosynthesis[END_REF] this STH yield is preserved when the irradiation intensity was increased, providing solutions for easy upscale of this technology with simple light management systems (e.g. mirrors and lenses) without increasing the amount of the expensive active material. The device performance slightly decreased over 10 hours of operation time due to the partial dissolution of the catalysts that caused changes in their chemical composition and morphology. Such an aged device could recover most of its initial performance by adding the [Co(WS 4 ) 2 ] 2-catalyst precursor into the phosphate operating condition. However, the main cause for long term degradation of the performances of the device are due to the instability of the silicon semi-conductor core due to exposure to the electrolyte, particularly enhanced by mechanical constrains or invasive modification of the ITO layer. Additional efforts should now focus on enforcing the electrochemical protection of this transparent catalyst/ITO layer.

ASSOCIATED CONTENT

Figure 1 .

 1 Figure 1. (a) Photovoltage generated over time (Inset shows the evolution of voltage generated

Figure 2 .

 2 Figure 2. (a) Schematic representation of the catalysts assembling process; (b) real-time and (c)

Figure 3 .

 3 Figure 3. SEM images collected on the ITO front-side and Steel back-side of a pristine solar

Figure 4 .

 4 Figure 4. Assaying performance of an optimized monolithic device ( STH = 1.9%) in a pH 7 KPi

Figure 5 .

 5 Figure 5. Current-voltage plot of a Steel CoWS cathode (left) and a CoWO ITO3jn-a-Si

Figure 6 .

 6 Figure 6. Schematic illustration of bipotentiostat configurations employed for evaluating the

Figure 1 .reported in figure 7 .

 17 Figure 1. Adding 0.5 mM [Co(WS 4 ) 2 ] 2-into the phosphate electrolyte changed the chemical

Figure 7 .

 7 Figure 7. (a) Schematic illustration of tripotentiostate configuration employed to track anodic &

  However, varying this ratio adds up the excess Co 2+ or [WS 4 ] 2-, which could alter the deposited catalyst performance. Using a 1 mM [WS 4 ] 2-deposition bath free of Co 2+ did not result in any deposit on the ITO3jn-a-SiSteel solar cell. The resultant device showed a negligible performance (Table1, entry 10). A device showing  STH of 1% was obtained when using a deposition bath composed of 0.5 mM Co 2+ (no

	[WS 4 ] 2-

; Table

1

, entry 9). In this case, deposition of a cobalt-oxide-based O 2 evolution catalyst (e.g. CoPi having onset deposition potential of +1.1V vs. NHE

[START_REF] Kanan | In Situ Formation of an Oxygen-Evolving Catalyst in Neutral Water Containing Phosphate and Co 2+[END_REF]

) and a cobalt-based H 2 evolution catalyst (e.g. H 2 -CoCat having onset deposition potential of -0.2 V vs. RHE)

[START_REF] Cobo | A Janus cobalt-based catalytic material for electrosplitting of water[END_REF]

could be expected on the ITO front-side and the steel back-side the solar cell, respectively. For other baths composed of both [WS 4 ] 2-and Co 2+ , the resultant devices showed  STH performance in the range of 1.4-1.9% (table 1, entries 4, 6-8). The highest  STH performance (1.9%) was achieved when the Co 2+ / [WS 4 ] 2-molar ratio was exactly 1/2, corresponding to the stoichiometry of the [Co(WS 4 ) 2 ] 2-precursor (Figure

Table 1 .

 1 Performance of resultant monolithic device in function of chemical deposition bath and catalyst deposition time employed for device assemblage

		Catalyst deposition bath*		Performance of
	Entry	Co 2+ (mM)	[WS 4 ] 2-(mM)	Catalyst photodeposition time (minutes)	resultant monolithic device ( STH, %)**

ACKNOWLEDGEMENTS

This work was supported by the National Foundation for Science and Technology Development (NAFOSTED, project 103.99-2019.328) and the Agence Nationale de la Recherche (Labex ARCANE, CBH-EUR-GS, ANR-17-EURE-0003). Duc N. Nguyen thanks the French Embassy in Hanoi for an excellence grant. We thank Dr. Truong Quang Duc from Tohoku University (Japan) for XPS measurement support.

In order to understand if this decrease of performance is due to a decrease of the photovoltage generated by the 3jn-a-Si cell, we then created an electrical contact on the top of the ITO layer. This contact was made using silver paste and was not immersed into the electrolyte (see the Supporting Information). A second channel of the potentiostat allowed monitoring the voltage between the ITO layer and the steel electrode in open-circuit potential mode (figure 6b). The photocurrent initially jumped to ~2 mA.cm -2 as previously observed but then decays to 0.5 mA/cm -2 within a few seconds and continues to drop over the time of the experiment. The photovoltage, initially measured as expected from Figure 1 at 2.05 V, continuously dropped during the experiment, which can explain the observed drop of current density as well (figure 6b). When repeating the bipotentiostat measurement for several devices, we obtained reproducible results with the same short life-time issue of the device. We assigned this discrepancy with regard to the stability observed over 12h for the whole cell to some detrimental side effect of the electrical connections made to monitor current and potential. This is heavily supported by the fact that, in this second monitoring mode, the current density is much lower than for the first one (figure 6a), suggesting that the contact on ITO generates defects that widely propagate through the whole layer and inactivates most of the device.

To identify the reasons for such a decrease in current density, we turned back to a fully wired configuration where the ITO front-side of the ITO3jn-a-SiSteel cell was wired to a fluorinedoped-tin-oxide (FTO) electrode while the steel back-side was wired to a Ag/AgCl reference electrode, both being immersed in a pH 7 phosphate buffer solution (see figure S24a and the supporting information for experimental details). This configuration indeed allows to measure potentials and currents to each electrode independently and simultaneously. An actual anodic Supporting Information. Experimental details, materials, methods and additional supporting figures and tables. The supplementary information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Authors * Dr. Vincent Artero; vincent.artero@cea.fr ; * Dr. Phong D Tran; tran-dinh.phong@usth.edu.vn

Notes

The authors declare no conflict of interests.


Kun Yang

Matthieu Koepf
email: matthieu.koepf@cea.fr

Vincent Artero
email: vincent.artero@cea.fr

Revisiting amorphous molybdenum sulfide's activity

come

Over the course of evolution, diazotrophic organisms have selected a specialized family of enzymes i.e the nitrogenases that are able to reduce dinitrogen (N 2 ) to ammonia (NH 3 ) via multiple electron and proton transfer under physiological conditions. The reaction occurs at highly preserved (hetero)metal sulfide cofactors such as the FeMoCo. 1,2 Synthetic transition metal sulfides can be viewed as analogues of these cofactors and are appealing candidates for developing catalysts for the electro-driven N 2 reduction reaction (NRR). 3 Iron and molybdenum derivatives have been particularly scrutinized as the latter metals are involved in the FeMoCo which supports the most active form of the enzyme. 1 Early examples of synthetic mimics incorporating these transition metals were indeed reported to exhibit some activity towards N 2 reduction. [4][5][START_REF] Liu | Proc. Natl. Acad. Sci[END_REF] Recent theoretical studies emphasized the potential of molybdenum sulfide derivatives for NRR, [START_REF] Azofra | [END_REF][8][9] which was quickly confirmed experimentally. [10][11][12][13][14][15] Meanwhile, several other families of metal-sulfide have been tested and showed promising results for the electrocatalytic reduciotn of N 2 .

16─18

The reported activity of crystalline MoS 2 towards NRR in aqueous conditions, strongly arouse our curiosity since our group has been involved in the study of related catalytic materials for photo-and electro-driven proton reduction over the last years. [19][20][21] Specifically, the reports of X. Sun et al. 11 and Lou and al., 13 highlighting enhanced NRR performances of defect-rich crystalline MoS 2 derivatives when compared to their defect-free counterparts, prompted us to investigate the potential of amorphous molybdenum sulfide (a-MoS x ) for electro-driven N 2 reduction. Lacking long-range crystalline order, a-MoS x is composed of chains of trigonal molybdenum {Mo 3 S 7 } and {Mo 3 S 6 } clusters connected via shared disulfide ligands. 21,22 It has demonstrated superior activity for proton reduction when compared to crystalline MoS 2 , due to the increased accessibility of the Mo centers of the individual clusters after partial elimination of the terminal disulfide ligands under reductive conditions. 21,22 It is tempting to compare the resulting unsaturated Mo sites to the "surface defects" of crystalline MoS 2 , thus, a-MoS x appeared to us as an ideal candidate for promoting NRR. To probe this hypothesis we adapted classical conditions from the literature and investigated the potential of this material for driving N 2 reduction in aqueous conditions. Thin films of a-MoS x particles were deposited with or without Nafion ionomer onto 1 cm 2 glassy carbon electrodes (loading:

0.3 mg a-MoSx .cm ─2
). The latter were used as working electrodes (WE) in a classical three electrodes configuration, for over 20 h-long potential-controlled electrolysis in N 2 -saturated aqueous electrolytes. Unless otherwise indicated, during the electrolysis the WE was polarized at a ─0.3 V vs. the reversible hydrogen electrode (RHE) in 10 mL of the selected electrolyte, and aliquots were taken at fixed times for analysis. The concentration of ammonia in these samples was determined using the salicylate colorimetric method. 23 Due to the instability of a-MoS x particles in solutions of pH ≥ 7 for long electrolysis times, we used either 0.1 M HCl (pH = 1) or 0.1 M potassium phosphate buffer (KPi, pH = 5.8) as electrolytes.

During the electrolysis, N 2 was continuously fed into the electrolyte at a rate of 10 mL.min

─1

. The NRR measurements were completed with a set of 5 control experiments (Table 1) allowing to define This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins the background concentration of adventitious ammonia inherent to our experimental conditions. All the experiments (control and NRR) were conducted at least two times to build the datasets discussed in the following. Due to the overall low concentration of NH 3 typically measured in these experiments, small variations of the concentration of adventitious ammonia can significantly affect the absorbance of the samples in the salicylate assay, even at time zero (t = 0) where virtually no ammonia is expected (ESI, Figure S2). Because the apparent 'zero' ammonia concentration measured for each experimental series appears random and never null, all the datasets discussed below are corrected for the absorbance at t = 0 for each series. Negative concentrations, thus, indicate a decrease of the apparent ammonia concentration in the electrolyte relative to t = 0. Furthermore, during the course of the electrolysis, the total volume of electrolyte decreases due to the sampling. This induces an overestimation of any variation of the apparent ammonia concentration between successive measurements.

To compensate for this effect we corrected all the data reported in the following as explained in the ESI. First, let us consider the evolution of the ammonia concentration measured over time for the control series HCl-1/KPi-1 -HCl-5/KPi-5 (Figure 1; ESI, Figure S8a-j). No strong trends appear in any series. We observe a random distribution of the concentrations with respect to time zero with equal chances of measuring an increase or a decrease of the apparent ammonia concentration over time in both 0.1 M HCl and 0.1 M KPi, as illustrated by the frequency distribution histograms (ESI, Figure S9). Overall more than 95% of the experimental values remain within ± 10 μM of the initial ammonia concentration for up to 25 hours in all the series (ESI, Table S3). In the following, thus, any variation of the ammonia concentration falling within this range will be considered as mere fluctuation of the experimental background concentration of adventitious NH 3 .

We next turned to controlled electrolysis experiments run in N 2 -saturated electrolyte over a-MoS x thin-films (series HCl-6/KPi-6 and HCl-7/KPi-7, Figure 2). Here again, the concentration of ammonia measured in both 0.1 M HCl and 0.1 M KPi electrolyte remains generally close to the range of the experimental background. Furthermore, no traces of hydrazine could be detected in the electrolyte using the method of Watt and Chrisp (detection limit 5 μM), 24 in any of the conditions tested.

Therefore, a-MoS x does not exhibit the expected activity towards direct electro-driven N 2 reduction.

A more detailed analysis of the series is, however, interesting. Controlled electrolysis run on Nafionless films in both electrolytes (series HCl-6, KPi-6) as well as controlled electrolysis conducted over Nafion-containing films in 0.1 M HCl (series HCl-7) lead to an essentially random evolution of the ammonia concentration over time. The dispersion of the individual values remains strictly within the range of the experimental background and a-MoS x is clearly inactive for NRR in these conditions. Surprisingly a small but steady increase in the ammonia concentration is, however, observed during the controlled electrolysis run in N 2 -saturated 0.1 M KPi, on films incorporating Nafion (series KPi-7). The overall quantity of NH 3 released appears slightly dependent on the bias applied, with slightly higher NH 3 concentrations reached after long electrolysis run at ─0.3V vs. RHE (ESI, Figure S10). In addition, we note a clear correlation between the quantity of ammonia detected in the electrolyte and the total charge passed during the electrolysis for KPi-7 series, in strong contrast with HCl-7, as shown on Figure S11 (ESI). These observations suggest that Nafion-coated a-MoS x promotes a sluggish electro-driven release of ammonia in 0.1 M KPi under N 2 . In order to probe whether traces amount of nitrite (NO 2 ─ )

or nitrate (NO 3 ─ ) resulting from the hydration of NO x impurities present in the feeding gas may be the source of ammonia detected in our experiments, 25 we determined their concentration using the Griess assay. 27 As shown in Table S6 (ESI), the variation of the concentration of both anions remained under the detection limit of the assays (~ 1 μM) in 0.1 M KPi buffer under constant N 2 feeding (10 mL.min

─1

) for at least 24 h. The accumulation of NH 3 observed for KPi-7 series, thus, likely originates from another source of nitrogen, however, we cannot exclude the direct reduction of neutral NO x contaminants on a-MoS x . 27 Additional experiments, such

Please do not adjust margins Please do not adjust margins S1). The contrast is really striking with respect to the rates reported for defect-rich MoS 2 ( ; in 0.5 M LiClO 4 at ─0.3 V vs. RHE) recently described by Liu et al. 14 The different nature of the electrolytes may partly explain these observations, however, the magnitude of the difference between the rates of ammonia production determined for a-MoS x and those reported for defect-rich MoS 2 or amorphous MoS 3 is surprisingly large for related forms of molybdenum sulfide. The comparison with amorphous MoS 3 is particularly relevant. In-depth spectroscopic investigations of MoS 3 films prepared in a similar manner as described by Liu et al., strongly suggested that the latter is also composed of chains of interconnected trigonal {Mo 3 S x } clusters. 28 Thus, MoS 3 and a-MoS x are likely closely-related forms of molybdenum sulfide. In fact, noticeable differences between the rates of ammonia production reported for supposedly identical forms of molybdenum sulfide catalysts are found through the literature (ESI, Table S1). Considering the issues recently raised for other families of catalysts, 25,27 a careful re-evaluation of the activity of molybdenum sulfides for NRR appears timely to rationalize the apparent discrepancy found between studies. Next, we investigated the activity of a-MoS x towards other Ncontaining substrates of the nitrogenases leading to the release of ammonia under (non-physiological) turnover conditions: hydrazine (N 2 H 4 ), hydroxylamine (NH 2 OH), nitrate (NO 3 ─ ), nitrite (NO 2 ─ ), and azide (N 3 ─ ). 1,29 The electro-reduction of these substrates was first tested under potential-controlled electrolysis on pristine a-MoS x films, in argon-saturated 0.1 M KPi electrolyte (pH = 5.8) with an applied potential of ─0.2 V vs. RHE (ESI; Figure S12). During the initial screening, only hydroxylamine and azide resulted in a clear accumulation of ammonia in the electrolyte. From cyclic voltammetry measurements, the onset potentials of the catalytic processes can be estimated at 0.38 V and 0.36 V vs. RHE for NH 2 OH and N 3 ─ respectively (ESI; Figure S13). Unfortunately, NH 2 OH interfered with both the salicylate assay and selective ammonia electrode measurements, impeding the accurate quantification of NH 3 produced during the electrolysis. Such issues were not faced with azide, which allowed us to investigate the activity of a-MoS x for its reduction in more details. A significant and steady increase in the concentration of ammonia is observed during the electrolysis of a 10 mM solution of sodium azide in 0.1 M KPi, at ─0.2 V vs. RHE (ESI; Figure S14, series N 3 RR-3). The ammonia concentration reaches values (> 200 μM) well above the experimental background (± 10 μM) after 4h of electrolysis, and there is a quasi-linear relationship between the total charge passed through the films and the quantity of ammonia produced as shown on Figure S15a (ESI). This strongly contrasts with the limited increase in NH 3 concentration (ca. 40 μM) measured after a 4h-long electrolysis run under similar conditions on bare glassy carbon electrode (ESI; Figure S14, series N 3 RR-1). Importantly a-MoS x did not demonstrate any intrinsic chemical reactivity towards azide leading to the release of ammonia in the electrolyte at room temperature, in the absence of an applied bias (ESI; Figure S14, series N 3 RR-2). a-MoS x thus, exhibits a clear activity for the electro-driven reduction of azide to ammonia.

In the enzymatic process the azide anion (N 3 ─ ) is known to be reduced in a stoichiometric manner to ammonia and dinitrogen via an unusual two-electron three-proton transfer process, whereas hydrazoic acid (HN 3 ) reduction follows a sixelectron six-proton pathway leading to the equimolar release of ammonia and hydrazine. 30 With a pKa of 4.6, hydrazoic acid (HN 3 ) accounts for about 16% of the azide species partition in our experimental conditions (pH 5.8) and we must consider both HN 3 and N 3 ─ as potential substrates for a-MoS x . Only negligible amounts of hydrazine could be detected during the electrolysis (< 20 μM after 21h, ESI, Table S7) which suggests that the direct reduction of N 3 ─ is highly favoured on a-MoS x over that of HN 3 . This seems quite reasonable in view of the much stronger coordination to unsaturated Mo centers expected for the azide anion as compared to a neutral hydrazoic acid ligand. Considering an ideal two-electron reduction process, we can calculate an average FE of ca 50 ± 10% for azide reduction with a production rate of 3. The competitive production of hydrogen in these conditions was confirmed using gas chromatography analysis. For this, a pristine a-MoS x film was polarized at ─0.2 V vs. RHE for 6h, in 0.1 M KPi while continuously monitoring the quantity of hydrogen produced. After 2h of electrolysis 1 M sodium azide solution was added to reach a concentration of 10 mM at the cathode. It led to a sharp increase in the current density and a large drop in the production rate of hydrogen as shown on This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins ‡ It is worth noting that the FE of ammonia production calculated in the latter experiment (71%) is higher than the average FE (50 ± 10%) determined for electrolysis run in in the presence of 10 mM of azide from the beginning. This observation suggests a partial poisoning of the a-MoS x catalytic sites by long term exposure to N 3 ─ ligand under non-catalytic conditions (no applied bias). Despite having highlighted a partial Nitrogenase-like activity for a-MoS x, this study demonstrates that the NRR activity is not intrinsic to all molybdenum sulfide derivatives, and that increasing the density of surface defects in these materials is not a universal strategy to enhance their activity towards NRR. This work was supported by the French National Research Agency (ANR-15-IDEX-02, Labex ARCANE, CBH-EUR-GS, ANR-17-EURE-0003, and ANR-20-SODR-0004-03).

We are grateful to Adina Morozan, Jennifer Fize, Duc Nguyen Ngoc and Louis Dubrulle for their contributions to this work.

Table 1 :

 1 Experimental conditions used for control (HCl-1/KPi-1 ─ HCl-5/KPi-5) and NRR (HCl-6/KPi-6, HCl-7/KPi-7) experiments, in 0.1 M HCl and 0.1 M KPi (pH = 5.8) solutions.

a

  direct purge, no electrodes/electrochemical set up involved. b open circuit potential: complete three electrode setup without connecting the potentiostat.

Figure 1 :

 1 Figure 1: Evolution of the apparent NH3 concentration over time (left in HCl; right in KPi) for all control groups. The distinct filling patterns within each series indicates measurements executed on samples collected from independent runs.

as 15 NFigure 2 :

 152 Figure 2: Evolution of the apparent concentration of NH3 in the electrolyte during potential-controlled electrolysis in N2-saturated 0.1 M HCl (left) or 0.1 M KPi buffer (pH = 5.8, right) using bare (HCl-6/KPi-6) or Nafion-coated (HCl-7/KPi-7) a-MoSx films deposited on glassy carbon electrodes. The working electrode was polarized at -0.3 V vs. RHE. The distinct filling patterns within each series indicates measurements executed on samples collected during independent runs.

Figure 3 .Figure 3 :

 33 Figure 3. Before addition of the azide in the electrolyte, the FE calculated for the hydrogen evolution reaction (HER) reaches about 70 ± 20% (t = 0 to 2h), whereas after the addition of NaN 3 the FE for HER sharply decreases to ca 2 ± 20% (t = 2h to 6h). Concomitantly a large quantity of ammonia is formed and a

Conflicts of interest

There are no conflicts to declare.

Notes and references


J Toupin

H Strubb

S Kressman

V Artero

N Krins

Ch Laberty-Robert

CuO photoelectrodes synthesized by the sol-gel method for water splitting

Keywords: CuO photoelectrode, sol-gel, TiO 2 protecting layer for CuO photoelectrode, water splitting

Highlights

 Sol-gel based CuO photoelectrode has values of -0.94 mA.cm -2 at pH = 8 and 0 V vs. RHE.  CuO/TiO 2 photoelectrodes have been synthesized by the sol-gel chemistry coupled with dip-coating.  These photoelectrodes exhibit -0.5 mAcm -2 at pH = 7 and 0 V vs. RHE.  These photoelectrodes are 100% stable over 600 s.

Introduction

The need for clean, safe, and sustainable energy has always been one of the most important challenges for the future, since worldwide energy demand increases continuously. An augmentation of 37% in the next 20 years is indeed envisioned [START_REF] Dudley | BP Energy Outlook 2035[END_REF] . New energy carriers have to meet severe criteria being renewable, environment-friendly and, high-energy dense. Among the different alternative processes, producing hydrogen fuel directly from inexhaustible sunlight with minimal environment impacts appears to be one of the cleanest and the most advantageous technologies to solve both energy and climate crisis [START_REF] Lewis | Powering the planet: chemical challenges in solar energy utilization[END_REF][START_REF] Yilanci | A review on solar-hydrogen/fuel cell hybri energy systems for stationary applications[END_REF] . Photoelectrochemical (PEC) water splitting research was pioneered by Fujishima and Honda in 1972 [START_REF] Fujishima | Electrochemical Photolysis of water at a semiconductor electrode[END_REF] , and since this date, many studies have been carried out to find the most efficient semi-conducting materials and the adequate nanostructure to obtain efficient photoelectrode materials. The later must fulfill two criteria: an efficient absorption of sunlight and electronic bands correctly positioned to trigger the water splitting reactions [START_REF] Van De Krol | Principles of Photoelectrochemical cells[END_REF] .

In addition, they should include components with low toxicity, be earth abundant, and film processing should be low cost as well as easy to scale up [START_REF] Wadia | Materials Availability Expands the Opportunity for Large-Scale Photovoltaics Deployment[END_REF] . Cost efficiency is of particularly importance since PEC cells need to compete with alternative approaches that consist in wiring traditional photovoltaic panels to commercial watersplitting electrolysers. [START_REF] Dincer | Technical, environmental and exergetic aspects of hydrogen energy systems[END_REF] In this context, photoanode materials should have a band gap of  2.1 eV, a valence band (VB) that is more positive than the apparent potential for oxygen evolution, E app (O 2 /H 2 O) [START_REF] Van De Krol | Principles of Photoelectrochemical cells[END_REF] . These materials must also be stable under oxidative conditions and illumination in aqueous electrolytes. Some metal oxides 8 , including TiO 2 [START_REF] Courtin | New Fe2TiO5-based nanoheterostructured mesoporous photoanodes with improved visible light photoresponses[END_REF] , -Fe 2 O 3 10 , WO 3 11 , and BiVO 4 8a, 12 , whose VB consists of O 2p orbitals, meet all these requirements. Many ptype semiconductors including GaP, Cu 2 O, CuO and p-Si are investigated as photocathode materials for H 2 evolution, as they exhibit adequate band gap comprise between 1.1 to 2.1 eV, and a conduction band (CB) more negative than the apparent potential for H 2 evolution, E app (H 2 /H 2 O). The stability of photocathode materials in aqueous electrolyte is very important as well.

Based on consideration of cost, earth abundance and electrochemical potential, both Cu 2 O and CuO have been identified as promising materials for solar energy conversion [START_REF] Wadia | Materials Availability Expands the Opportunity for Large-Scale Photovoltaics Deployment[END_REF] . In particular, their conduction band edges are -0.7 V and -0.2 V vs. RHE, respectively 13 14 . Cu 2 O is a ptype conductor with a direct band gap of 2 eV [START_REF] Paracchino | Highly active oxide photocathode for photoelectrochemical water reduction[END_REF] . It is thus able to absorb visible light and has a theoretical photocurrent of 14.7 mA.cm -2 under standard AM1.5 irradiation. Due to its excellent properties, Cu 2 O has been studied by several groups with experimental photocurrents above 7 mA.cm -2 in optimized device configurations [START_REF] Lin | Cu2O|NiOx nanocomposite as an inexpensive photocathode in photoelectrochemical water splitting[END_REF][START_REF] Paracchino | Synthesis and Characterization of High-Photoactivity Electrodeposited Cu2O Solar Absorber by Photoelectrochemistry and Ultrafast Spectroscopy[END_REF][START_REF] Zhang | Carbon-Layer-Protected Cuprous Oxide Nanowire Arrays for Efficient Water Reduction[END_REF] . CuO is a ptype semiconductor with a direct band gap of 1.21.3 eV, implying that it is theoretically capable of generating a much higher photocurrent of up to 35 mA.cm -2 . In comparison to Cu 2 O, only very few reports exist on the use of CuO as the active light-harvesting material for photocatalytic water splitting [START_REF] Lim | Sol-gel deposited Cu2O and CuO thin films for photocatalytic water splitting[END_REF][START_REF] Lin | Cu2O|NiOx nanocomposite as an inexpensive photocathode in photoelectrochemical water splitting[END_REF][START_REF] Jin | 5.1% Apparent quantum efficiency for stable hydrogen generation over eosin-sensitized CuO/TiO2 photocatalyst under visible light irradiation[END_REF][START_REF] Liu | Hierarchical CuO/ZnO "corn-like" architecture for photocatalytic hydrogen generation[END_REF][START_REF] Pai | Preparation and characterization of porous Nb2O5 photocatalysts with CuO, NiO and Pt cocatalyst for hydrogen production by light-induced water splitting[END_REF] .

Electrochemical deposition appears to be the most common method used for the formation of Cu 2 O thin film photocatalysts 13,16b, 22, 23, 17 . An attractive alternative method that is much less commonly explored is the deposition by dip-coating from solgel precursor solutions, followed by thermal annealing. Similar to electrodeposition, solgel-based dip-coating deposition presents the advantage to be low cost and scalable.

Additionally, a fairly good control of the stoichiometry, film thickness and nanostructure can be achieved. To the extent of our knowledge, there are only few reports on the deposition of CuO thin films by sol-gel process [START_REF] Hench | The sol-gel process[END_REF][START_REF] Jia | Polymer-assisted deposition of metal-oxide films[END_REF][START_REF] Lim | Sol-gel deposited Cu2O and CuO thin films for photocatalytic water splitting[END_REF] .

In this work, we report on the deposition of CuO thin film by dip-coating from sol-gel precursor solutions. We show that purephase CuO thin films can be obtained at low temperature. Additionally, we found that the PEC performance of solgel deposited CuO thin film strongly depends on the synthesis conditions. Finally, a very thin layer of solgel TiO 2 film (100 nm) was deposited onto the best CuO films to enhance the stability of these photoelectrodes. This work not only shows the potential of CuO films as efficient photocathode materials for hydrogen evolution reaction (HER), but also provides an easy, low-cost, and scalable strategy to prepare bilayer CuO/TiO 2 hydrogen-evolving photoelectrodes for PEC applications. The solution previously described is deposited onto FTO substrate (Solems) with a resistivity of 100 .cm -1 .

Experimental Section

Prior to dip-coating, the substrate is cleaned by sonication in ethanol for 5 minutes. The cleaned substrate was then dipped in the sol-gel solution and withdrawn at a speed of 2.5 mm.s -1 under dried air (RH = 5-8 %). After dip-coating, the film was treated on a hot-plate at 330°C for 1 min, before being annealed at a temperature comprised between 360°C and 450°C for 30 min to degrade the structuring agent and induce the crystallization of CuO. To increase the total film thickness, several layers were deposited. Between each layer, a treatment under air at 330°C for 1 min was applied to avoid the dissolution of the film during the deposition of the successive layers. This operation was repeated 5 times to reach a thickness of 500 nm (when 1.5 M-sol is used), and of 100 nm (for 0.3M-sol).

TiO 2 layer synthesis

TiO 2 was deposited using the solgel based dipcoating method. [START_REF] Grosso | Highly porous TiO2 Anatase optical thin films with cubic mesostructure stabilised at 700°C[END_REF] First, a Ti5E solution was prepared by mixing 19 g of TiCl 4 and 23 g of absolute ethanol. 1mL of this Ti5E solution is then mixed with 5.7 g of absolute ethanol in order to have a concentration of Ti of 0.3 mol.L -1 and deposited onto the CuO/FTO substrate with a withdrawal speed of 2.5 mm/s. A heat treatment at 400°C for 1 min under air is applied between each layer (n = 1, 2 or 3) and the final treatment is 500°C for 1 h in air to convert the hybrid organic-amorphous TiO 2 layer into a porous crystalline TiO 2 anatase. The thickness of the TiO 2 layer is 100 nm for n = 2, evaluated by FESEM analysis.

Characterization of photoelectrodes' microstructure

The morphology of the synthesized films was characterized using a high-resolution scanning electron microscope (Hitachi SU70). XRD patterns were acquired with a Bruker D8 Discover diffractometer in the Bragg-Brentano mode, using Cu K radiation (1.540598Å) and a Ni -filter. Spectra were acquired with a 1D

Lynxeye detector from 2 = 20° 80° at a step width of 0.05° with acquisition time of 1 s and a source slit width of 4 mm. UVvisible absorption spectra were measured using an Agilent Technologies Cary Series spectrophotometer.

Photoelectrochemical characterization

The photoelectrochemical performances of the electrodes were evaluated in a three-electrode configuration under frontside light illumination using a Solartron Ametek 2087A

Potentiostat / Galvanostat / impedancemeter. The electrolyte was an aqueous solution of Na 2 SO 4 (1.0 M) buffered at pH 6 with potassium phosphate (0.1 M). The reference electrode was Ag/AgCl in saturated KCl, and a Pt wire was used as the counter electrode. The photoresponse was measured under chopped irradiation using a 280 W Xe lamp equipped with an ultraviolet filter (Oriel 51272, 400nm) and IR radiation ( > 950 nm) filter.

The light intensity was measured with 1918-R Newport power meter 918D-UVOD3R-UV Silicon Detector (200 -1100 nm) and estimated to 3 suns (240 mW.cm -2 , visible-IR irradiation). [START_REF] Patterson | Under such conditions, the photocurrent is threefold enhanced with regards to solar irradiation[END_REF] The scan rate for the linear sweep voltammetry measurements was 20 mV s -1 . Impedance spectroscopy measurements have been performed using the following parameters: frequency range (200 kHz -0.1 Hz) with a V of 20 mV, at 0 V vs. RHE at pH = 6.

Photocurrent stability tests were carried out by measuring the photocurrent produced under chopped light irradiation (0.1 Hz light/dark cycles frequency) at a fixed potential of 0 V versus RHE.

During linear sweep voltammetry (j-V plots) and chronoamperometry (stability plots), the electrolyte was continuously bubbled with N 2 to remove oxygen and thus eliminate erroneous signals arising from oxygen reduction.

Results and discussion

The crystalline structure of dense CuO films prepared from 1.5M-sol and calcined at various temperatures has been determined and the results are reported in Fig. 1b. All the diffraction peaks could be indexed as the CuO phase (ICDD card No 801916) and no other impurity peaks are found except those relative to FTO substrate.

Additionally, the crystallite size on dense CuO films has been estimated using the Debye-Scherrer formula using the more intense peak and the deconvolution procedure 29 and the results are reported in Fig. SI1. [START_REF] Koshy | Annealing effects on crystallite size and band gap of CuO nanoparticles[END_REF][START_REF] Habibi | Effect of annealing temperature on crystalline phase of copper oxide nanoparticle by coppe acetate precursor and sol-gel method[END_REF] . The crystallite domains slightly increase with the heat treatment from 63 nm to 71 nm for 360°C and 450°C, respectively.

CuO films are crack-free and relatively dense (Fig. 1.a.). They present small particle size, as shown in Fig. 1(a).

Upon thermal annealing, the particle size increases. The cross-section SEM images of the CuO films indicate a thickness of approximatively  500 nm for 5 successive layers deposition (not shown). The photoelectrochemical performances of dense CuO photocathodes were studied at 0 V vs. RHE and pH = 8 for dense CuO films calcined at various temperatures. The best photocurrent, 0.94 mA.cm -2 , was obtained for dense CuO films calcined at 400°C (400-CuO), indicating that these conditions give rise to the best compromise in terms of light absorption and electrode/electrolyte interface area. The performance of these films is comparable to the one reported by Dass et al. with photocurrents of 12 mA.cm -2 for 590 nm-thick CuO/SrTiO 3 photoelectrodes at -0.9 V. vs. SCE (electrolyte, 0.1 M NaOH). Prior to studying the photostability of these CuO photoelectrodes annealed at 400°C, the impact of the nanostructuration onto the photoelectrochemical performances was investigated. To do so, a PIBbPS block copolymer was used during the preparation of the sol in order to induce porosity during the annealing step. Note that for this study, the sol concentration was 0.3 mol.L -1 . The structure and morphology of the mesoporous films annealed at 400 °C for 30 min were studied using Xray diffraction and FE-SEM, respectively (Fig.

360-CuO

400-CuO 450-CuO

SI.2). The

Xray diffraction pattern indicates a pure CuO phase. The surface of the mesoporous film exhibits a network of pores, which is not well organized nor well defined. This poor organization results from the fact that the crystallization of CuO (~ 300°C) occurs after the decomposition of the PIB-b-PS block copolymer (~ 200°C) and causes the fragile porous network to collapse. FESEM crosssection images on these templated CuO films confirm the very low thickness of the CuO films ( 100 nm) after the dip-coating of 5 layers. As the film is very thin, the FTO substrate is accessible to the electrolyte through the pores, decreasing the active surface of the film since the FTO substrate does not participate in the H 2 evolution process. The J-V curves measured at pH = 6 with a scan rate of 20 mV.s -1 for both dense and mesoporous CuO photoelectrodes annealed at 400°C with the same thickness (250 nm) are reported onto Fig. 4. At 0 V vs. RHE, the photocurrent is  0.9 mAcm -2 and  0.7 mA.cm -2 for dense and mesoporous films, respectively. In this specific study, the best result is thus achieved for dense CuO photoelectrodes. This might be explained by i) less electron/hole recombination phenomena because the dense microstructure of the film favours electron/hole transport and ii) reduced light absorption by mesoporous films since their density is lower for similar film thickness. To clarify the origin of the higher photoactivity of dense CuO photoelectrodes, electrochemical impedance spectroscopy (EIS) was employed to investigate the charge transfer rate at the semiconductor/solution interface. The dense CuO photoelectrode has the smallest Rct value both in the dark and under illumination, indicating that dense CuO photoelectrodes possess increased charge transfer across the photocathode/solution interface. [START_REF] Zhang | Highly stable copper oxide composite as an effective photocathode for water splitting via a facile electrochemical synthesis strategy[END_REF] This might explain why dense CuO electrodes exhibit the best PEC performance for HER. Additionally, UV-visible absorption analyses show a difference in absorption intensity between these two electrodes (Fig. SI3) due to the modification of the film density between dense and mesoporous films.

Altogether, these results highlight that dense CuO films exhibit better electrochemical behavior. This is surprising given the fact that we expect mesoporous films to have better electrochemical behavior because a higher specific surface area. Indeed, a higher electrolyte/semiconductor interface would impact the number of reactive sites and the charge transfer. This indicates that the microstructure of the mesoporous film is not optimized (thickness, porosity network) and that further work is required to optimize the architecture of the mesoporous film for photoelectrochemical properties.

Current density (mA.cm -2 ) E vs. RHE a.

10 μm 10 μm

Dense Mesoporous

Light ON Dense Mesoporous A well-known issue of CuO is that it is unstable in aqueous electrolytes. [START_REF] Ma | Cu2O photoelectrodes for solar water splitting: Tuning photoelectrochemical performance by controlled faceting[END_REF] To investigate this, a study of the photocurrent stability of the CuO photocathode was carried out by measuring the current over time at a constant potential of 0 V vs. RHE. Over a timescale of 600 s, the drop in photocurrent was about 10% of the initial photocurrent, which suggests that CuO tends to degrade. This is also confirmed by the change of the electrode color occurring during the chronoamperometry test, going from dark grey to orange. Post-mortem XRD analysis of the photoelectrode (Fig. SI4) confirms the formation of metallic copper, due to reductive decomposition of CuO as shown in the following equation:

CuO + 2e -+ H2O  Cu° + 2OH -
In order to enhance the stability of CuO photoelectrodes, a thin layer of TiO 2 was deposited on top of 400CuO films. [START_REF] Siripala | A Cu2O/TiO2 heterojunction thin film cathode for photoelectrocatalysis[END_REF][START_REF] Rokhmat | Performance Improvement of TiO2/CuO Solar Cell by Growing Copper Particle Using Fix Current Electroplating Method[END_REF][START_REF] Zainun | New approach for generating Cu2O/TiO2 composite films for solar cell applications[END_REF][START_REF] Wang | Introducing a protective interlayer of TiO2 in Cu2O-CuO heterojunction thin film as a highly stable visible light photocathode[END_REF] This strategy is similar to that reported in our previous work., which showed that substantial improvement of CuO/Cu 2 O/FTO photocurrent can be achieved using an anatase TiO 2 layer of 100 nm. [START_REF] Toupin | Engineering n-p junction for photo-electrochemical hydrogen production[END_REF] The TiO 2 film, deposited using the sol-gel dip-coating approach and annealed at 500°C, is crystalline, which is confirmed with small peaks in the XRD pattern corresponding to the anatase structure (Fig. SI5,a). [START_REF] Grosso | Highly porous TiO2 Anatase optical thin films with cubic mesostructure stabilised at 700°C[END_REF] The intensity of the peaks is low because the thickness of the TiO 2 layer that is very small compared to the CuO photoelectrode. This is confirmed by the SEM images of the bilayer CuO/TiO 2 photoelectrode, where a thickness of 100 nm for TiO 2 layer is measured (note that this value is achieved for 2 layers of TiO 2 and is 10 times lower than the value observed for CuO). The absorbance of CuO, TiO We observed a higher resistance for the CuO/TiO 2 , accompanied by the higher arc radius of the EIS spectra. This is attributed to the presence of trap states at the CuO/TiO 2 interfaces, hindering photoelectrons.

If after 300 s, the average photocurrent density does not vary for both the protected and the unprotected photoelectrodes, the difference in stability occurs after 600s where the bi-layers TiO 2 /CuO retain 100% of initial photocurrent, compared to the unprotected CuO electrode, which suffers a 10% loss. The 100 nm-TiO 2 layer appears to be sufficient to encapsulate the copper oxide photocathodes (Table SI1) and protect them against photoreductive corrosion. This result highlights that the protection of CuO photoelectrode by TiO 2 conduces to a decrease of the photocurrent by a factor 2. But, the durability of the electrode is higher for the TiO 2 /CuO photoelectrode as 100% of the photocurrent is maintained after 600 s (10 min).

Conclusion

Pure unprotected protected unprotected protected achieving stable photocurrent of  0.5 mA.cm -2 at 0 V vs RHE and pH = 7. Moreover, this work highlights that the strategy of using the sol-gel dip-coating processing followed by a heattreatment provides an easy, low-cost, and scalable approach to fabricate stable copper-oxide-based materials for hydrogen production. Further work is needed to develop mesoporous films with higher performances for photoelectrochemical application. For example, a systematic study where the thickness of the film is optimized as a function of the porosity will be necessary.

Notes

Electronic supplementary material: The online version of this article (https://doi.org/10.1007/s10971-017-4519-4) contains supplementary material, which is available to authorized users.

Fig. 1 .

 1 Fig. 1. Characterization of dense CuO photoelectrodes. The films were prepared from the 1.5 M-sol. They were calcined at 360°C for 20 min (360-CuO), at 400°C for 20 min (400-CuO) and 450°C for 20 min (450-CuO): (a) SEM images, (b) X-ray diffractograms. (* indicates SnO 2 from the underlying FTO substrate).

Fig. 2 .

 2 Fig. 2. Optical characterization of dense CuO photoelectrodes. (a) UV-visible diffuse reflectance spectra and (b) Tauc plots (αEphoton) 2 vs. Ephoton.

35

 35 photoelectrodes at -0.9 V. vs. SCE (electrolyte, 0.1 M NaOH).[START_REF] Surbhi Choudhary | Sahab Dass Nanostructured CuO/SrTiO3 bilayered thin films for photoelectrochemical water splitting[END_REF]

Fig. 3 .

 3 Fig. 3. Photoelectrochemical performances of dense CuO photoelectrodes. Chronoamperometry measurements at 0 V vs. RHE and pH = 8 for dense CuO photoelectrodes under chopped visiblelight irradiation (3 suns).

Fig. 4 .

 4 Fig. 4. Impact of the nanostructuration onto PEC properties. j-V curves for dense (blue) and mesoporous (orange) CuO films annealed at 400°C for 20 min, obtained from 0.3 M-sol at pH = 6 with a scan rate of 20 mV.s -1 . Inset. SEM-FE images of the dense and mesoporous films.

Fig. 5 Fig. 5 ,

 55 Fig. 5 shows the Nyquist plots of the two photocathodes at a potential of 0 V vs. RHE measured in the dark (plain mark) and under illumination (open mark). The semicircle features the charge transfer across the semiconductor/electrolyte interface and its diameter represents the charge transfer resistance (Rct). As shown in Fig. 5, Rct decreases upon illumination for the two photocathodes, indicating that illumination accelerates the charge transfer reaction at copper oxide/solution interface due to the photoinduced increase of the carrier density.

Fig. 5 .

 5 Fig. 5. Nyquist plots measured for the dense and mesoporous CuO photoelectrodes annealed at 400°C (0.3 Msol) at constant potential of 0 V vs. RHE in the dark and under illumination at pH = 6 and 0 V vs. RHE. Measurements from 200 kHz to 0.1 Hz.

  Fig.6aalso shows the variation of the photocurrent density of unprotected CuO electrode and the bi-layers TiO 2 /CuO electrode at 0 V vs. RHE at pH = 7. The photocurrent decreases from -1 mA.cm -2 for the bare electrode to -0.5 mA.cm 2 for the protected CuO photoelectrode at pH = 7 (Fig.6(a)). Concomitantly, the observed onset potential is also impacted (Fig.6(b)) and evolves from 0.4 V vs. RHE to 0.2 V. vs. RHE for CuO and CuO/TiO 2 photoelectrode respectively. The presence of TiO 2 layer ontop of the CuO photoelectrode conduces to a decrease of the photocurrent that might be linked to the presence of defects at the CuO/TiO 2 interface, the change of band bending due to the presence of TiO 2 layer, then favoring the e -/h recombination.EIS was then performed at the CuO/TiO 2 interface to examine the charge transfer resistance and separation efficiency between the photogenerated electrons and holes. Fig SI.7 shows the typical EIS Nyquist plots under light. We observed a higher resistance for the CuO/TiO 2 , accompanied by the higher arc radius of the EIS

Fig. 6 .

 6 Fig. 6. Unprotected 400-CuO photoelectrode (orange) and TiO 2-protected 400-CuO photoelectrode (blue) (a) Chronoamperometry at 0 V vs. RHE, (b) j-V curves with a scan rate of 20 mV.s -1 . Both experiments were performed at pH = 7 with light chopped at a frequency of 0.1 Hz.

  photoelectrode as 100 % of initial photocurrent was kept over 600 s. This architecture allows

2.1. CuO photoelectrode synthesis

	The CuO films were synthesized by combining dip-coating and sol-gel chemistry, inspired by Ray works 26 . To
	prepare the copper sol, CuCl 2 .2H 2 O at 1.5 mol.L -1 in methanol was obtained by mixing 1.75 g of CuCl 2 .2H 2 O in
	7 mL of methanol (1.5 M-sol). Dense and mesoporous CuO photoelectrodes with a copper sol at 0.3 mol.L -1
	have also been investigated (0.3 M-sol). For the synthesis of mesoporous photoelectrode, 90 mg of Poly-iso-
	butylene-b-polyethylene oxide (Polymer Sources, PIB-b-PEO, Mn=7000-b-8500) block copolymer dissolved in
	ethanol/water mixture was first prepared. To this solution, CuCl 2 .2H 2 O salt was added so that the final
	concentration in Cu is 0.3 mol.L -1 .

Acknowledgements: This work was supported by the ANRT, the Agence Nationale de la Recherche through the LabEx ARCANE programme (ANR-11-LABX-0003-01) and Total-Energie Nouvelle.

Conflict of interest. The authors declare that there is no conflict of interest.