# Advanced CIGS-mesoporous TiO2 hybrid photocathode functionalized with cobalt quaterpyridine for solar-driven CO reduction

Debashrita Sarkar, Hichem Ichou, Léo Choubrac, Stéphane Diring, Nathanaelle Schneider, Julien Bonin, Nicolas Barreau, Fabrice Odobel, Marc Robert

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Debashrita Sarkar, Hichem Ichou, Léo Choubrac, Stéphane Diring, Nathanaelle Schneider, et al. Advanced CIGS-mesoporous TiO2 hybrid photocathode functionalized with cobalt quaterpyridine for solar-driven CO reduction. Journal of Materials Chemistry A, 2025, 10.1039/D5TA06206C. hal-05345664

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Advanced CIGS-mesoporous TiO₂ hybrid photocathode functionalized with cobalt quaterpyridine for solar-driven CO₂ reduction

Debashrita Sarkar<sup>1,2#</sup>, Hichem Ichou<sup>3#</sup>, Léo Choubrac<sup>4</sup>, Stéphane Diring<sup>3</sup>, Nathanaelle Schneider<sup>5</sup>, Julien Bonin<sup>1</sup>, Nicolas Barreau*<sup>4</sup>, Fabrice Odobel*<sup>3</sup> and Marc Robert*<sup>1,6</sup>

<sup>1</sup>Sorbonne Université, CNRS, Institut Parisien de Chimie Moléculaire, IPCM, F-75005 Paris, France; e-mail address: [marc.robert@sorbonne-universite.fr](mailto:marc.robert@sorbonne-universite.fr)

Present address :<sup>2</sup>Dutch Institute of Fundamental Energy Research, 5612 AJ Eindhoven, The Netherlands

<sup>3</sup>Nantes Université, CNRS, CEISAM, UMR 6230, F-44000 Nantes, France; e-mail address: [fabrice.odobel@univ-nantes.fr](mailto:fabrice.odobel@univ-nantes.fr)

<sup>4</sup>Nantes Université, CNRS, Institut des Matériaux Jean Rouxel – UMR 6502, F-44000, Nantes, France; e-mail address: [nicolas.barreau@cnrs-imn.fr](mailto:nicolas.barreau@cnrs-imn.fr)

<sup>5</sup>Institut Photovoltaïque d'Île-de-France (IPVF), UMR 9006, CNRS, Ecole Polytechnique - IP Paris, Chimie Paristech – PSL, F-91120, Palaiseau, France

<sup>6</sup>Institut Universitaire de France (IUF), F-75005 Paris, France

# These authors have contributed equally.

Abstract

Hybrid photocathodes that integrate inorganic semiconductors with molecular catalysts offer a promising strategy for photoelectrochemical CO₂ reduction into value-added products. In this work, we present the design and characterization of a high-performance photocathode based on copper indium gallium sulfide (CIGSu), functionalized with a cobalt quaterpyridine (CoQPy) molecular catalyst. The device features a thin (5 nm) TiO₂ protective layer deposited by atomic layer deposition (ALD) on CIGSu/CdS, followed by a mesoporous TiO₂ layer formed under mild conditions using UV curing and low-temperature annealing (150°C). The mesoporous structure enables high CoQPy loading through chemisorption *via* phosphonic acid anchoring groups. Under simulated sunlight, the optimized photocathode delivers a photocurrent density of *ca.* 2 mA/cm² with 95% CO selectivity in carbonate buffer, double the
performance of systems using low-porosity TiO₂. This work marks progress toward efficient, molecularly functionalized photocathodes for aqueous CO₂ reduction.

## Keywords

*Photocathode, CO₂ reduction, mesoporous TiO₂, CuInGaS₂, cobalt quaterpyridine, artificial photosynthesis*

## Introduction

The conversion of CO₂ and sunlight into fuels and chemical commodities, an approach inspired by natural photosynthesis and often termed artificial photosynthesis, offers a promising strategy for advancing a circular economy by utilizing CO₂ as a renewable carbon source. An important milestone toward this goal is the solar-driven reduction of CO₂ to CO under aqueous conditions.¹ The development of hybrid photoelectrodes, combining an inorganic semiconductor (SC) with a molecular catalyst, has emerged as a promising strategy to simultaneously leverage the high selectivity and catalytic efficiency of molecular systems with the strong light-harvesting and the charge transport properties of semiconducting materials.²⁻⁴ Among p-type semiconductors (SCs) used in photoelectrode fabrication, chalcopyrites compounds, with the general formula Cu(In,Ga)(S,Se)₂, stand out as particularly promising materials. Their tunable bandgap, controllable *via* the In/Ga ratio, combined with a high visible-light absorption coefficient (ε = 10⁷-10⁸ m⁻¹) and their ability to generate strongly reducing electrons (up to -2 V vs. SCE), make them ideal candidates for efficient photoelectrochemical (PEC) systems.⁵⁻¹⁰ Although bare CIGSSe has been shown to reduce CO₂ without the aid of a catalyst,¹¹, ¹² semiconductors typically exhibit limited efficiency, modest selectivity, and moderate stability. Surface functionalization with a molecular catalyst can significantly enhance selectivity.¹³, ¹⁴ For instance, the cobalt quaterpyridine complex is a highly selective and efficient catalyst for CO₂-to-CO conversion and offers the additional benefit of being transparent to visible light, making it well-suited for integration into PEC systems.¹⁵⁻¹⁸

Since the 1980s,¹⁹ numerous research groups have developed hybrid photoelectrodes for CO₂ reduction, incorporating molecular complexes with a variety of SCs, including p-type silicon (p-Si),²⁰, ²¹ InP,²² GaP,²³ perovskites,²⁴⁻²⁷ and others.²⁸, ²⁹ Among these, crystalline silicon remains the most extensively studied SC material. For example, Reisner *et al.* immobilized a
cobalt bis-terpyridine³⁰ or a cobalt phthalocyanine³¹ CO₂ reduction catalyst *via* phosphonic acid anchoring groups on a TiO₂ mesoporous layer coated over silicon photocathode. The selectivity for CO production was approximately 60%, with a photocurrent density of less than 1 mA/cm². Noblat and Fabre reported the selective reduction of CO₂ to CO in an organic medium using a manganese bipyridine-based catalyst polymerized onto silicon nanowires.³² More recently, Cahoon *et al.* investigated p-type silicon-based photocathodes for CO₂ reduction catalysis. However, applied potentials were highly cathodic (exceeding -1.5 V vs. SCE), and the product selectivity remained modest, reaching a maximum of 80%.³³⁻³⁵ The deposition of graphene oxide onto silicon-based photocathodes has been shown to enable the immobilization of cobalt porphyrin³⁶ or a cobalt phthalocyanine³⁷, leading to stable photocurrents for the solar-driven production of CO and methanol, respectively, in aqueous electrolytes. Li *et al.* introduced a highly absorbing catalytic layer-comprising catalyst-loaded multiwalled carbon nanotubes (MWCNTs) on a TiO₂-protected silicon photocathode, with precision-drilled holes to allow light to reach the silicon absorber.³⁸ This configuration delivered a stable photocurrent density of ca. 1.5 mA/cm² over 2 h in neutral aqueous solution, achieving both 100% faradaic efficiency and selectivity for CO production at a low bias of 0.11 V vs. RHE. We previously demonstrated that p-type Cu(In,Ga)Se₂-based solar cells, combined with a transparent ZnO/AZO window and a dense TiO₂ protection layer, which also serves as a platform for catalyst immobilization, can achieve photocurrent densities around 1 mA/cm² and 97% selectivity for CO₂-to-CO conversion at near-neutral pH in aqueous conditions.¹⁷ Additionally, we showed that the molecular catalyst can be encapsulated within one of the zinc oxide-based window layers, enabling precise control over the catalyst loading within the solar cell.³⁹ However, employing ZnO as a transparent top window layer for catalyst incorporation limits the system's applicability in aqueous environments. Replacing this layer with a transparent, porous material, stable in water, could significantly improve both stability and overall performance. More recently, we proposed an innovative strategy using CIGSu photocathodes with back-side illumination, thereby eliminating the need for transparent catalyst and protective layers on the front side.⁴⁰

Although significant progress has been made in recent years-especially with silicon-based photocathodes-overall performance remains limited, particularly in terms of operational stability. Consequently, developing hybrid photoelectrodes capable of delivering high solar fuel production rates at low applied potentials with long-term stability in aqueous environments remains a challenge.
In this study, we integrated Cu(In,Ga)S₂ (hereafter denoted as CIGSu) with a cobalt quaterpyridine molecular catalyst bearing two phosphonic acid anchoring groups (denoted as CoQPy-P, **Figure S7a**) to develop an efficient hybrid half-PEC system. A mesoporous TiO₂ (mes-TiO₂) layer was employed as the top layer, replacing the previously used dense TiO₂, to increase the electrode/electrolyte interface and to enable higher catalyst loading. To further improve both stability and performance, a thin, nanometer-scale dense TiO₂ layer deposited by atomic layer deposition (ALD)⁴¹, ⁴² was introduced as a protective window on the SC. Collectively, these modifications led to a marked improvement in performance compared to our earlier CIGS-based photoelectrode system.¹⁵

## Results and Discussion

The fabrication of the CIGSu-based photocathode builds on a modified version of our previously reported procedure (see the Supporting Information for details). The complete device stack consists of SLG/Mo/CIGSu/CdS, where SLG refers to soda lime glass serving as the substrate, Mo (molybdenum) functions as the back contact, and CIGSu (copper indium gallium sulfide) acts as the p-type light-absorbing SC layer. A thin n-type cadmium sulfide (CdS) layer is deposited on top of CIGSu to form a p-n junction, enabling efficient collection of photogenerated electrons upon illumination (**Figure S1a**). The opaque back contact, deposited by DC sputtering of molybdenum (Mo) and commonly referred to as the back contact, is used to collect photogenerated holes ($h^+$), while electrons ($e^-$) are directed toward the CIGSu/CdS interface. The resulting solar cell exhibits an open-circuit voltage ($V_{oc}$) of 770 mV, a short-circuit current density ($J_{sc}$) of 18 mA/cm², and a fill factor of 65%, corresponding to a power conversion efficiency (PCE) of 9%, indicating good photovoltaic performance. (**Figure S1b**).

### 1. Fabrication of a mesoporous TiO₂ layer

To facilitate the loading of a substantial amount of the molecular catalyst (CoQPy-P) onto the CIGSu photoelectrode, a mesoporous TiO₂ layer was deposited, effectively increasing the surface area at the photoelectrode/electrolyte interface. This approach, central to dye-sensitized solar cell (DSSC) design,⁴³ is traditionally used to enhance light absorption by increasing dye coverage on the photoanode. More recently, it has also been adapted for PEC systems employing inorganic semiconductors other than CIGS.²⁹⁻³¹, ⁴⁴⁻⁴⁶ To achieve this objective, a thin layer of TiO₂ nanoparticles (NPs) was initially deposited over the CdS layer forming the p-n
junction, using a screen-printing technique. The resulting film was then sintered to remove
residual organic additives, such as polymers and terpineol from the TiO₂ ink, facilitating
interparticle neck formation and improving electron diffusion within the TiO₂ network. This
process, commonly employed in dye-sensitized solar cell fabrication,⁴³,⁴⁷ traditionally involves
high-temperature oven treatment (450°C for 30 min). However, CIGSu cells cannot tolerate
temperatures above 200°C without undergoing degradation. Grätzel *et al.* previously reported
the formation of a mesoporous TiO₂ layer on a Cu₂O photocathode using UV irradiation at
room temperature for sintering.²⁹ Taking inspiration from this approach, we refined the method
by applying mild heating (below 200°C) combined with time-controlled UV irradiation to
sinter TiO₂ NPs deposited on CIGSu (**Figure S2**). Control experiments were performed to
optimize the process and verify its effectiveness. The first control involved monitoring the
removal of organic additives *via* attenuated total reflectance-infrared spectroscopy (IR-ATR)
(**Figure S3**). Subsequently, a dye-sensitized solar cell (DSSC) incorporating a well-established
benchmark dye (D35) was produced to ensure that the device performance was comparable to
the one achieved with conventional high-temperature sintering (450°C for 30 min, **Figure S4**
and **Table S1**). Finally, the electrocatalytic activity of CoQPy-P was assessed after
immobilization on a mesoporous TiO₂ film supported on FTO (FTO/mes-TiO₂/CoQPy-P,
**Figure S5**). These experiments confirmed that the current density obtained with CoQPy-P on
TiO₂ films sintered under mild conditions (UV irradiation plus 4 h of heating at 150°C) matched
the one observed with conventional, more severe sintering (450°C for 30 min).

<figure><img src="image_2.png" /><figcaption>**Figure 1.** X-ray photoelectron spectroscopy (XPS) analysis of SLG/Mo/CIGSu/CdS/mes-TiO₂-CoQPy-P with the binding energies of (a) N 1s, (b) P 2p, and (c) Co 2p; (d) SEM image of the cross section of the SLG/Mo/CIGSu/CdS/mes-TiO₂-CoQPy-P displaying the thickness of each component.</figcaption></figure>
Overall, these control experiments confirm the efficacy of this new, mild conditions approach for preparing mesoporous TiO₂ films suitable for PEC systems employing CIGSu (see the Supporting Information for further details). Lastly, catalyst immobilization was achieved through a simple self-assembly process, in which the film was immersed in a CoQPy-P solution to facilitate its chemisorption onto the mesoporous TiO₂ surface (Figure S6). The resulting hybrid photocathode is designated as SLG/Mo/CIGSu/CdS/mes-TiO₂-CoQPy-P.

## 2. Characterization of the grafted catalyst on the photoelectrode

The resulting hybrid photocathode was analysed using a range of complementary characterization techniques. First, the visible absorption spectrum of CoQPy-P anchored onto mesoporous TiO₂ deposited on FTO was measured to evaluate the catalyst's light absorption within the visible range. Only a slight increase in optical density from 300 to 720 nm was observed, indicating that the molecular catalyst is largely transparent between 400 and 800 nm (Figure S7b). Next, the successful grafting of CoQPy-P onto CIGSu was verified by analysing the infrared spectrum with attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR), as shown in **Figures S7c-d**. The integrity of the catalyst structure was demonstrated by the presence of characteristic peaks between 1600 and 1450 cm⁻¹, corresponding to C–C stretching vibrations of the aromatic ring. Notably, distinct peaks at 1000 cm⁻¹ (attributable to P–OH bonds) present in the free CoQPy-P molecule (Figure S7c) disappeared after grafting, which is consistent with bidentate binding of the phosphonic acid groups *via* the P–OH functionalities, while the P=O stretching vibration (1232 cm⁻¹) remained largely unchanged (Figure S7d).¹⁷, ⁴⁸, ⁴⁹ Third, XPS measurements revealed peaks at 399 eV and 133 eV, corresponding to the N 1s orbital of the pyridinic group and the P 2p orbital of the phosphonic acid functional moiety in the modified quaterpyridine, respectively (Figure 1a-c).⁵⁰ The Co 2p₁/₂ and 2p₃/₂ are observed at 796 eV and 780 eV and are representative of Co²⁺ with their satellite peaks at 802 eV and 783 eV.⁵¹ The thicknesses of the individual layers in the hybrid catalyst were determined by cross-sectional scanning electron microscopy (SEM), revealing values of *ca.* 4 µm for the mesoporous TiO₂, 40 nm for CdS, 1.5 µm for CIGSu, and 0.5 µm for the Mo back contact layer (Figure 1d).

Finally, the catalyst loading was quantified by inductively coupled plasma spectrometry-optical emission spectroscopy (ICP-OES) and led to a value of 25 ± 3 nmol/cm² (Table S2). As expected, the loading is significantly higher than our previous work on dense TiO₂ (*ca.* 2 nmol/cm²),¹⁷ which may lead to higher catalytic performances.
### 3. *Photoelectrocatalytic CO₂ reduction*

The photocatalytic activity of the SLG/Mo/CIGSu/CdS/mes-TiO₂/CoQPy-P hybrid photoelectrode was first investigated by chopped light linear sweep voltammetry (LSV, **Figure 2a**). PEC CO₂ reduction experiments were conducted in a four-neck glass electrochemical cell fitted with a quartz window, using a 0.1 M KHCO₃ solution (pH 6.8) as the electrolyte. A saturated calomel electrode (SCE) served as the reference electrode, while a platinum mesh housed in a ceramic frit was employed as the counter electrode (**Figure S8**). A high photocurrent density up to -2.4 mA/cm² was recorded at -0.4 V vs. RHE in LSV (**Figure 2a**).

<figure><img src="image_3.png" /></figure>

**Figure 2.** (a) LSV under chopped light conditions for SLG/Mo/CIGSu/CdS/mes-TiO₂-CoQPy-P under CO₂ and Ar atmospheres; (b) LSV under chopped light conditions of SLG/Mo/CIGSu/CdS/ALD-TiO₂/mes-TiO₂-CoQPy-P with different thickness of ALD-TiO₂ protective layer under CO₂ atmosphere; (c) 1 h chronoamperometric measurement for SLG/Mo/CIGSu/CdS/ALD-TiO₂(5nm)/mes-TiO₂-CoQPy-P at E = -0.06 V vs. RHE; (d) Effect of selectivity and partial current density for CO₂-to-CO PEC reaction. Catalytic conditions: 0.1 M KHCO₃, pH 6.8, solar simulator 100 mW/cm², AM 1.5G, irradiated surface 0.25 cm², scan rate 5 mV/s; electrolysis time -1h.
An extended electrolysis was performed for 1 h at an applied potential of -0.06 V vs. RHE (Figure S9 and Table S3). In these conditions, an average photocurrent density of -0.3 mA/cm² was obtained with a selectivity of 92% for CO, and 8% for H₂, the only by-product, thus confirming the remarkable performance of the SLG/CIGSu/CdS/mes-TiO₂/CoQPy-P. In the absence of the catalyst, the current density remained very low during 30 min of photoelectrolysis, with the hydrogen as the main product (Selectivity 100%, H₂) (Figure S10b).

The appearance of current spikes upon illumination is characteristic of charge recombination, suggesting that electron extraction by the catalyst is not fully efficient. Given that CoQPy-P possesses a high turnover frequency,¹³ these spikes are unlikely to result from slow catalytic rates. It is well established that trap states commonly found at the surfaces of covalent semiconductors are responsible for charge recombination or potential pinning of charge carriers.², ⁵² Beard *et al*. demonstrated that incorporating an ultrathin TiO₂ layer onto p-type GaInP₂ semiconductors can reduce charge recombination.⁵³ Moreover, since CdS is prone to photocorrosion, a conformal TiO₂ layer can protect it from the aqueous electrolyte.⁵⁴, ⁵⁵ Ultimately, the adhesion of TiO₂ nanoparticles to the photoelectrode would be significantly improved in the presence of an oxide layer, such as an underlying compact protective TiO₂ layer, compared to CdS.

### 4. Introduction of a protective layer

Accordingly, a 3 or 5 nm TiO₂ compact layer, deposited *via* atomic layer deposition (ALD), was introduced as a protective coating on the CdS layer of the photocathode assembly. This was followed by the deposition of a mesoporous TiO₂ layer using the mild procedure described above. The resulting hybrid photocathode architecture is designated as SLG/Mo/CIGSu/CdS/ALD-TiO₂/mes-TiO₂/CoQPy-P (Scheme 1). Simultaneously, LSV under chopped light irradiation show a clear increase in photocurrent, indicating enhanced catalytic performance and aligning with previous literature reports (Figure 2b).⁵³

Constant potential electrolysis (CPE) experiment under continuous light illumination of the hybrid photoelectrode SLG/Mo/CIGSu/CdS/ALD-TiO₂/mes-TiO₂/CoQPy-P at E = -0.06 V vs. RHE was then carried out to assess the system stability. The photocathode delivers an average -0.95 mA/cm² over 1 h operation, illustrating the higher stability of the hybrid electrode as compared to non-protected photocathode (Figure 2c). The comparison of 3nm and 5nm ALD TiO₂ as a protective layer on towards selectivity for CO and partial current density is displayed
in **Figure 2d**. Figure 2d shows a clear improvement in partial photocurrent density with increasing ALD TiO₂ thickness, from -0.3 mA/cm² for the sample without TiO₂ to -0.4 mA/cm² for the 3 nm layer, and up to -0.9 mA/cm² for the 5 nm layer. The CO selectivity does decrease slightly with ALD deposition, but remains high (>90%) across all samples. These results suggest that the dense ALD TiO₂ layer has a beneficial effect. However, a thickness layer of 10 nm or thicker leads to detrimental impact on performances, likely due to a decreased efficiency of electron transport. The photocurrent density of -0.2 mA/cm² and a lower selectivity of 60% towards CO formation (**Figure S11, Table S4**).

The stability of the hybrid photoelectrode components was evaluated by ICP-OES analysis of the electrolyte, monitoring Cu and Cd concentrations after 1 h electrolysis (**Table 1**). Significantly lower amounts of Cd (0.005 ppm vs. 0.01 ppm) and Cu (0.002 ppm vs. 0.06 ppm) were detected in the presence of the TiO₂ protective layer, confirming the effective corrosion resistance provided by the 5 nm ALD TiO₂ coating. Overall, the 5 nm ALD TiO₂ layer delivered optimal PEC performance and stability, and was therefore selected for subsequent experiments.

<table><caption>**Table 1.** Cu and Cd concentrations in the electrolyte from ICP-OES measurements, with and without ALD TiO₂ protective layer after 1 h of CPE at E = -0.06 V vs. RHE.</caption><thead><tr><th></th><th>without ALD-TiO<sub>2</sub></th><th colspan="2">with ALD-TiO<sub>2</sub></th></tr><tr><th>ALD layer thickness</th><th></th><th>3 nm</th><th>5 nm</th></tr></thead><tbody><tr><th>Cu (ppm)</th><td>0.06</td><td>0.011</td><td>0.002</td></tr><tr><th>Cd (ppm)</th><td>0.011</td><td>0.007</td><td>0.005</td></tr></tbody></table>

### 5. Role of the chemical bath catalyst concentration

The initial CoQPy-P grafting was performed with a concentration of 0.2 mM in the soaking bath. The substrate of mes-TiO₂ being mesoporous, it was expected to incorporate a higher amount of molecular catalyst than a dense TiO₂. The catalyst concentration in the soaking bath was increased to 0.5 mM, and the catalyst loading was quantified by ICP-OES and led to a value of 49 ± 2 nmol/cm² (**Table S2**), double the amount compared to the soaking in 0.2 mM. The PEC performance of SLG/Mo/CIGSu/CdS/ALD-TiO₂/mes-TiO₂/CoQPy-P-0.5 was investigated using chopped light LSV (**Figure S12a**), yielding the best results. Further increasing the catalyst concentration to 1 mM did not enhance performance, likely due to catalyst aggregation or partial detachment over time. At -0.4 V vs. RHE, the electrode exhibited high current density of 5.5 mA/cm² (**Figure 3a**) and minimal recombination current, indicating
efficient electron transfer to the surface. During electrolysis at -0.06 V vs. RHE, initial current densities of -2 to -2.5 mA/cm² (across three electrodes) decreased to approximately -1.65 mA/cm² after 30 min, gradually declining thereafter (Figure 3b, S12b-d), but demonstrating clear improvement over electrodes with lower catalyst loadings. Figure 3c compares total

<figure><img src="image_4.png" /></figure>

**Figure 3:** (a) Chopped light LSVs for SLG/Mo/CIGSu/CdS/ALD-TiO₂-5nm/mes-TiO₂-CoQPy-P-0.5. (b) 1 h chronoamperometric measurement for SLG/Mo/CIGSu/CdS/ALD-TiO₂-5nm/mes-TiO₂/CoQPy-P-0.5 at E = -0.06 V vs. RHE (apparent spikes on the current trace correspond to successive short time (few seconds) interruption of light irradiation so as to ensure the photoelectrode remain stable during the course of the experiment). (c) Effect the catalyst bath concentration of on Faradaic efficiency and total current density for SLG/Mo/CIGSu/CdS/ALD-TiO₂/mes-TiO₂-CoQPy-P under constant potential electrolysis for 0.2 mM and 0.5 mM catalyst bath concentration. (d) Products formed during electrolysis at E = -0.06 V vs. RHE of CO₂ at SLG/Mo/CIGSu/CdS/ALD-TiO₂(5nm)/mes-TiO₂-CoQPy-P-0.5 photoelectrode. Conditions: 0.1 M KHCO₃, pH 6.8, RE: saturated calomel electrode, CE: Pt mesh, solar simulator 100 mW/cm², AM 1.5G, irradiated surface 0.25 cm², scan rate during LSV 5 mV/s.
current densities ($j_{total}$) for different hybrid photoelectrodes after 1800 s at -0.06 V vs. RHE, with results reproducible across multiple electrodes (Figure S12). Selectivity for CO remained exceptionally high-over 95% during the first hour. (Figure 3d; Table S5). Although the current density decreased notably within the first hour, these values still represent a substantial enhancement over previous systems operating in aqueous media at similar (near 0 V) bias, which is about 50 mV more positive than $E^0(CO_2/CO)$, while maintaining 95% selectivity for CO production (Table S5). The morphological and compositional stability of the SLG/Mo/CIGSu/CdS/ALD-TiO$_2$ (5 nm)/mesoporous-TiO$_2$-CoQPy-P-0.5 photocathode was assessed upon pre- and post-catalysis characterization. SEM and EDS analyses were conducted on a fresh electrode and after controlled potential electrolysis (CPE) at -0.06 V for 2 h. As shown in Figure S14, the mesoporous TiO$_2$ layer retains its particle-like, porous morphology, and the elemental mapping confirms a homogeneous distribution of Co, N, P, and O from the CoQPy catalyst, along with C, Ti, and O signals attributed to the underlying TiO$_2$ framework. These results show that both the morphology and catalyst loading remain largely unaltered after catalysis, validating the stability of the hybrid photocathode.

## Conclusion and perspectives

In this study, we present a novel fabrication approach for depositing a mesoporous TiO$_2$ layer onto a CIGSu/CdS photocathode, and investigate its effect on photoelectrocatalytic CO$_2$ reduction under aqueous conditions. To enable this, we developed a gentle sintering process, combining mild heating at 150°C with UV irradiation, which is fully compatible with the stability of the underlying CIGSu/CdS layers. This method produces a mesoporous TiO$_2$ top layer with significantly increased surface area, allowing for approximately tenfold higher catalyst loading compared to flat surfaces and yielding greater photocurrent density. Introducing a thin, dense intermediate TiO$_2$ layer *via* ALD between the CIGSu/CdS and mesoporous TiO$_2$ further enhanced current density and electrode stability-likely due to improved electron extraction by the catalyst and better protection of the underlying layers. By optimizing catalyst loading, we achieved average photocurrent densities of 1-2 mA/cm$^2$ over an hour with CO selectivity of 95% in carbonate buffer, twice the performance of our previous, low-porosity TiO$_2$-based systems. This study constitutes a step towards the development of efficient photocathodes for CO$_2$ reduction working in aqueous electrolyte. We hope these innovations will contribute to the development of better performing PEC systems for artificial photosynthesis.
**Author contributions**

The project was conceived by N.B, F.O. and M.R. D.S., H.I., L.C performed experiments.
Results analysis, draft preparation and editing were performed by all authors.

**Conflicts of interest**

There are no conflicts to declare

**Acknowledgments**

This work received financial support from the French National Agency for Research (ANR)
for the project PECALO (ANR-20-CE05-0019-01) and the EUR LUMOMAT project
(Investments for the Future program ANR-18-EURE-0012). Partial financial support to M.R.
from the Institut Universitaire de France (IUF) is also thanked. Damien Coutancier is warmly
thanked for the help in ALD experiments.

**References**

1. S. Y. Chae, S. Y. Lee, S. G. Han, H. Kim, J. Ko, S. Park, O.-S. Joo, D. Kim, Y. Kang, U. Lee, Y. J. Hwang and B. K. Min, *Sustain. Energ. Fuels*, 2020, **4**, 199-212.

2. J. L. White, M. F. Baruch, J. E. Pander, Y. Hu, I. C. Fortmeyer, J. E. Park, T. Zhang, K. Liao, J. Gu, Y. Yan, T. W. Shaw, E. Abelev and A. B. Bocarsly, *Chem. Rev.*, 2015, **115**, 12888-12935.

3. L. K. Putri, B.-J. Ng, W.-J. Ong, S.-P. Chai and A. R. Mohamed, *Adv. Energy Mater.*, 2022, **12**, 2201093.

4. D. A. Garcia Osorio, G. Neri and A. J. Cowan, *ChemPhotoChem*, 2021, **5**, 595-610.

5. K. Wang, Y. Ma, Y. Liu, W. Qiu, Q. Wang, X. Yang, M. Liu, X. Qiu, W. Li and J. Li, *Green Chem.*, 2021, **23**, 3207-3240.

6. Y. Chen, X. Feng, M. Liu, J. Su and S. Shen, *Nanophotonics*, 2016, **5**, 524-547.

7. M. Asaduzzaman, M. Hasan and A. N. Bahar, SpringerPlus, 2016, **5**, 578.

8. J. J. Frick, R. J. Cava and A. B. Bocarsly, *Chem. Mater.*, 2018, **30**, 4422-4431.

9. N. Gaillard, D. Prasher, M. Chong, A. Deangelis, K. Horsley, H. A. Ishii, J. P. Bradley, J. Varley and T. Ogitsu, *ACS Appl. Energy Mater.*, 2019, **2**, 5515-5524.

10. Y. Liu, M. Xia, D. Ren, S. Nussbaum, J.-H. Yum, M. Grätzel, N. Guijarro and K. Sivula, *ACS Energy Lett.*, 2023, **8**, 1645-1651.

11. Z. Hu, J. Gong, Z. Ye, Y. Liu, X. Xiao and J. C. Yu, *J. Catal.*, 2020, **384**, 88-95.

12. R. R. Prabhakar, S. Shukla, H. Li, R. S. Kim, W. Chen, J. Beaudelot, J. D'Haeen, D. R. Santos, P. M. Vereecken, G.-M. Rignanese, E. J. Crumlin, J. Yano, B. Vermang and J. W. Ager, *EES Catal.*, 2025, **3**, 327-336.

13. E. Boutin, L. Merakeb, B. Ma, B. Boudy, M. Wang, J. Bonin, E. Anxolabéhère-Mallart and M. Robert, *Chem. Soc. Rev.*, 2020, **49**, 5772-5809.

14. K. E. Dalle, J. Warnan, J. J. Leung, B. Reuillard, I. S. Karmel and E. Reisner, *Chem. Rev.*, 2019, **119**, 2752-2875.

15. K.-M. Lam, *J. Chem. Soc., Dalton Trans.*, 1995.

16. Z. Guo, S. Cheng, C. Cometto, E. Anxolabéhère-Mallart, S.-M. Ng, C.-C. Ko, G. Liu, L. Chen, M. Robert and T.-C. Lau, *J. Am. Chem. Soc.*, 2016, **138**, 9413-9416.

17. P. B. Pati, R. Wang, E. Boutin, S. Diring, S. Jobic, N. Barreau, F. Odobel and M. Robert, *Nat. Commun.*, 2020, **11**, 3499.
18. B. Ma, M. Blanco, L. Calvillo, L. Chen, G. Chen, T.-C. Lau, G. Dražić, J. Bonin, M. Robert and G. Granozzi, *J. Am. Chem. Soc.*, 2021, **143**, 8414-8425.

19. J. P. Collin and J. P. Sauvage, *Coord. Chem. Rev.*, 1989, **93**, 245-268.

20. D. Zhang, J. Shi, W. Zi, P. Wang and S. Liu, *ChemSusChem*, 2017, **10**, 4324-4341.

21. K. Sun, S. Shen, Y. Liang, P. E. Burrows, S. S. Mao and D. Wang, *Chem. Rev.*, 2014, **114**, 8662-8719.

22. T. Arai, S. Sato, T. Kajino and T. Morikawa, *Energy Environ. Sci.*, 2013, **6**, 1274-1282.

23. M. Halmann, *Nature*, 1978, **275**, 115-116.

24. E. Edwardes Moore, V. Andrei, A. R. Oliveira, A. M. Coito, I. A. C. Pereira and E. Reisner, *Angew. Chem., Int. Ed.*, 2021, **60**, 26303-26307.

25. V. Andrei, G. M. Ucoski, C. Pornrungroj, C. Uswachoke, Q. Wang, D. S. Achilleos, H. Kasap, K. P. Sokol, R. A. Jagt, H. Lu, T. Lawson, A. Wagner, S. D. Pike, D. S. Wright, R. L. Z. Hoye, J. L. MacManus-Driscoll, H. J. Joyce, R. H. Friend and E. Reisner, *Nature*, 2022, **608**, 518-522.

26. V. Andrei, B. Reuillard and E. Reisner, *Nat. Mat.*, 2020, **19**, 189-194.

27. K. Bienkowski, R. Solarska, L. Trinh, J. Widera-Kalinowska, B. Al-Anesi, M. Liu, G. K. Grandhi, P. Vivo, B. Oral, B. Yılmaz and R. Yıldırım, *ACS Catal.*, 2024, **14**, 6603-6622.

28. K. Sekizawa, S. Sato, T. Arai and T. Morikawa, *ACS Catal.*, 2018, **8**, 1405-1416.

29. M. Schreier, J. Luo, P. Gao, T. Moehl, M. T. Mayer and M. Grätzel, *J. Am. Chem. Soc.*, 2016, **138**, 1938-1946.

30. J. J. Leung, J. Warnan, K. H. Ly, N. Heidary, D. H. Nam, M. F. Kuehnel and E. Reisner, *Nat. Catal.*, 2019, **2**, 354-365.

31. S. Roy, M. Miller, J. Warnan, J. J. Leung, C. D. Sahm and E. Reisner, *ACS Catal.*, 2021, **11**, 1868-1876.

32. E. Torralba-Peñalver, Y. Luo, J.-D. Compain, S. Chardon-Noblat and B. Fabre, *ACS Catal.*, 2015, **5**, 6138-6147.

33. S. He, S. R. Bottum, J. C. Dickenson, H. R. M. Margavio, N. D. Keller, O. A. Oyetade, R. J. Gentile, T. S. Teitsworth, S. J. Shin, J. L. Dempsey, A. J. M. Miller, R. N. Sampaio, S. J. Tereniak, C. L. Donley, M. R. Lockett, G. N. Parsons, G. J. Meyer and J. F. Cahoon, *J. Am. Chem. Soc.*, 2025, **147**, 11145-11151.

34. O. A. Oyetade, Y. Wang, S. He, H. R. M. Margavio, S. R. Bottum, C. L. Rooney, H. Wang, C. L. Donley, G. N. Parsons, T. Cohen-Karni and J. F. Cahoon, *ACS Appl. Mater. Inter.*, 2024, **16**, 37885-37895.

35. X. Jia, E. Stewart-Jones, J. L. Alvarez-Hernandez, G. P. Bein, J. L. Dempsey, C. L. Donley, N. Hazari, M. N. Houck, M. Li, J. M. Mayer, H. S. Nedzbala and R. E. Powers, *J. Am. Chem. Soc.*, 2024, **146**, 7998-8004.

36. Z. Wei, Y. Su, W. Pan, J. Shen, R. Fan, W. Yang, Z. Deng, M. Shen and Y. Peng, *Angew. Chem., Int. Ed.*, 2023, **62**, e202305558.

37. B. Shang, C. L. Rooney, D. J. Gallagher, B. T. Wang, A. Krayev, H. Shema, O. Leitner, N. J. Harmon, L. Xiao, C. Sheehan, S. R. Bottum, E. Gross, J. F. Cahoon, T. E. Mallouk and H. Wang, *Angew. Chem., Int. Ed.*, 2023, **62**, e202215213.

38. Z. Wen, S. Xu, Y. Zhu, G. Liu, H. Gao, L. Sun and F. Li, *Angew. Chem., Int. Ed.*, 2022, **61**, e202201086.

39. J. Guerrero, E. Bajard, N. Schneider, F. Dumoulin, D. Lincot, U. Isci, M. Robert and N. Naghavi, *ACS Energy Lett.*, 2023, **8**, 3488-3493.

40. H. Gao, G. Liu, Y. Zhu, Z. Wen, X. Liu, G. Wang and F. Li, *Green Chem. Eng.*, 2023, **4**, 433-438.

41. L. Pan, J. H. Kim, M. T. Mayer, M.-K. Son, A. Ummadisingu, J. S. Lee, A. Hagfeldt, J. Luo and M. Grätzel, *Nat. Catal.*, 2018, **1**, 412-420.

42. M. Schreier, F. Héroguel, L. Steier, S. Ahmad, J. S. Luterbacher, M. T. Mayer, J. Luo and M. Grätzel, *Nat. Energy*, 2017, **2**, 17087.

43. A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson, *Chem. Rev.*, 2010, **110**, 6595-6663.

44. L. Gong, P. Zhang, G. Liu, Y. Shan and M. Wang, *J. Mater. Chem. A*, 2021, **9**, 12140-12151.
45. J. J. Leung, J. Warnan, D. H. Nam, J. Z. Zhang, J. Willkomm and E. Reisner, *Chem. Sci.*, **2017**, *8*, 5172-5180.

46. S. Chandrasekaran, N. Kaeffer, L. Cagnon, D. Aldakov, J. Fize, G. Nonglaton, F. Baleras, P. Mailley and V. Artero, *Chem. Sci.*, **2019**, *10*, 4469-4475.

47. E. C. Muniz, M. S. Góes, J. J. Silva, J. A. Varela, E. Joanni, R. Parra and P. R. Bueno, *Ceram. Int.*, **2011**, *37*, 1017-1024.

48. S. A. Paniagua, A. J. Giordano, O. N. L. Smith, S. Barlow, H. Li, N. R. Armstrong, J. E. Pemberton, J.-L. Brédas, D. Ginger and S. R. Marder, *Chem. Rev.*, **2016**, *116*, 7117-7158.

49. R. Luschtinetz, G. Seifert, E. Jaehne and H.-J. P. Adler, *Macromol. Symp.*, **2007**, *254*, 248-253.

50. B. Peng, Y. Xu, K. Liu, X. Wang and F. M. Mulder, *ChemElectroChem*, **2017**, *4*, 2140-2144.

51. D. Sarkar, S. Ganguli, A. Mondal and V. Mahalingam, *ChemElectroChem*, **2022**, *9*, e202101140.

52. G. Yi, Q. Wang, J. Arbiol and P. Tang, *Mater. Today Chem.*, **2023**, *34*, 101795.

53. Y. Yang, J. Gu, J. L. Young, E. M. Miller, J. A. Turner, N. R. Neale and M. C. Beard, *Science*, **2015**, *350*, 1061-1065.

54. J. A. Nasir, Z. U. Rehman, S. N. A. Shah, A. Khan, I. S. Butler and C. R. A. Catlow, *J. Mater. Chem. A*, **2020**, *8*, 20752-20780.

55. Y. Lai, N. B. Watkins, C. Muzzillo, M. Richter, K. Kan, L. Zhou, J. A. Haber, A. Zakutayev, J. C. Peters, T. Agapie and J. M. Gregoire, *ACS Energy Lett.*, **2022**, *7*, 1195-1201.
## Graphic TOC

<figure><img src="image_5.png" /></figure>

A Cu(In,Ga)S₂-based hybrid photocathode covered by a protective layer and functionalized by a Co molecular complexes reduces CO₂ in water with high rate and selectivity.