Photoelectrochemical Charge Transfer Imaging at
Nanoscale Catalysts

Changlin Zhou, Bertrand Goudeau, Patrick Garrigue, Frédéric Bedu, Lionel

Santinacci, Zonghua Wang, Neso Sojic, Gabriel Loget, Julie Descamps

► To cite this version:

Changlin Zhou, Bertrand Goudeau, Patrick Garrigue, Frédéric Bedu, Lionel Santinacci, et al., Photo-electrochemical Charge Transfer Imaging at Nanoscale Catalysts. ACS Energy Letters, 2025, 10 (7), pp.3519-3525. 10.1021/acsenergylett.5c01296. http://onlinelibrary.wiley.com/doi/abs/10.1021/acsenergylett.5c01296

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Distributed under a Creative Commons Attribution 4.0 International License
Photoelectrochemical Charge Transfer Imaging
at Nanoscale Catalysts

Changlin Zhou,ª Julie Descamps,ª Bertrand Goudeau,ª Patrick Garrigue,ª Frédéric Bedu,ᵇ

Lionel Santinacci,ᵇ Zonghua Wang,ᶜ Neso Sojic,ª* Gabriel Logetª*

ªUniversity of Bordeaux, Bordeaux INP, ISM, UMR CNRS 5255, 33607 Pessac, France

ᵇAix-Marseille Univ, CNRS, CINaM, Marseille, France

ᶜCharacteristic Laboratory of Advanced Metal Functional Materials and Processing in
Universities of Shandong, School of Mechanical and Electronic Engineering, Qingdao Binhai
University, Qingdao, Shandong, 266555, China

*Email addresses: neso.sojic@enscbp.fr; gabriel.loget@cnrs.fr

**Abstract**

Semiconductor (SC)-based photoelectrodes, employed for the production of solar fuels such as H₂ or small organic molecules, have garnered significant attention and extensive studies because they could play a crucial role in addressing the global energy crisis. While macroscopic photoelectrochemical (PEC) studies of these systems are routinely employed to monitor the overall activity of these systems, the spatiotemporal resolution of local PEC activity down to the nanoscale remains a challenge that could lead to considerable improvement in photoelectrode design and engineering. Here, model Si-based photoanodes precisely coated with water-splitting Ni catalysts of various sizes are studied by photoinduced electrochemiluminescence (PECL) microscopy. Our results demonstrate that this method allows precise imaging of hole-driven photoelectrochemical reactivity down to a scale of ~50 nm, making PECL microscopy a valuable tool for elucidating charge transfer and local interfacial activity of photoelectrodes employed in PEC solar energy conversion processes.
The global energy crisis has underscored the urgent need for sustainable energy conversion solutions, driving significant advances in photoelectrochemistry.¹,² In this field, semiconductor (SC) electrodes serve as critical components for efficiently converting solar energy into storable fuels, offering a promising pathway toward addressing long-term energy demands and mitigating environmental challenges.³⁻⁸ While many SCs can be used to absorb photons and generate photocarriers, most of them are unable to operate for photoelectrochemical (PEC) water splitting due to their instability and require the deposition of protection and catalytic layers to afford efficient solid/liquid charge transfer.⁹ These coatings are crucial for promoting the hydrogen evolution reaction (HER) at the photocathode as well as the oxygen evolution reaction (OER) at the photoanode. Among such photoelectrodes, Si-based photoanodes composed of *n*-type Si (*n*-Si) coated with Ni thin films or nanoparticles (NPs) have recently attracted considerable attention as promising photoanodes for water splitting due to their ease of manufacture and high performance.⁸,¹⁰⁻²¹ PEC characterization of photoelectrodes is always performed by recording the current density response as a function of applied potential or time, allowing the monitoring of the Faradaic charge transfer kinetics at the photoelectrode surface. Although electroanalytical methods are straightforward, they only allow a measurement of the overall charge transfer rate averaged across the entire electrode surface without providing spatially resolved information on
photoelectrochemical activity. However, understanding local activity by monitoring charge transfer heterogeneities is crucial for precise comprehension of photoelectrodes in operation.²²

As mentioned above, water-splitting photoelectrodes are composed of multiple materials that, typically, are: i) a semiconductor absorber, ii) a protective layer and iii) a catalytic coating that can be a thin film or particles spread across the surface. Given the complexity and the inhomogeneity of such systems, the PEC reaction is unlikely to occur uniformly across the surface. Monitoring the local PEC activity is still challenging, although several research groups have explored different techniques,²³ such as laser scanning,²⁴ optical microscopy²⁵,²⁶ and/or scanning electrochemical microscopy.²⁷⁻³¹

On the other hand, electrochemiluminescence (ECL) is an electrochemical process in which a dissolved luminophore undergoes a series of redox reactions at the solid/liquid interface, promoting its transition to a high-energy excited state and the emission of a photon upon relaxing to its ground state.³²,³³ Therefore, ECL provides an optical readout of the electrochemical signal and can be combined with optical microscopy for direct charge transfer visualization.³⁴⁻⁴² Over the last five years, our group has developed the concept of photoinduced electrochemiluminescence (PECL),⁴³,⁴⁴ in which ECL is generated at a SC photoelectrode by the transfer of photogenerated minority carriers. Thus, PECL is a light-conversion process where
both light and electrical bias generate electrochemical luminescence at the semiconductor/liquid interface, enabling tunable emission through the interplay of semiconductor absorption, interfacial charge transfer and luminophore excitation. Recently, PECL microscopy has been used to observe the photogenerated charge transfer at Ir microbands⁴⁵ and Au nanoparticles (AuNPs).⁴⁶,⁴⁷ Although the method is *a priori* limited by the invasiveness (like almost all in situ methods) and optical limitations (which can be overcome by super-resolution methods) inherent in optical microscopy, the method applies to many photoelectrochemical systems in which inhomogeneous solid/liquid charge transfer occurs. Our general hypothesis is that photoelectrochemically active sites on a photoanode can be revealed by PECL. In this work, we propose the use of PECL microscopy as a general method to image photoinduced charge transfer down to the nanoscale. For that, we report proof-of-principle experiments performed with model OER-active photoanodes (*n*-Si modified with NiNPs) operated in conditions relevant to PEC solar water splitting.

As model photoanodes, we employed oxidized *n*-Si (*n*-Si/SiOₓ, see **Figure 1a,d**) partially coated with Ni, referred to here as *n*-Si/NiNPs (see Section *Preparation of the Si surface* in the SI file). This choice is driven by the fact that numerous recent studies have employed similar Ni-coated *n*-Si for solar water splitting.⁸,¹⁰⁻²¹ In particular, it has been recently demonstrated that the use of
inhomogeneous Ni coatings such as dispersed NiNPs on these systems allows reaching better performances<sup>48</sup> than an homogeneous Ni thin film, because of enhanced light absorption (due to the low catalyst coverage)<sup>12</sup> and higher photovoltage (due to the pinch-off effect, see **Figure S1** for more details).<sup>10</sup> In these photoanodes, during OER, photogenerated holes ($h^+$) are transferred to Ni, which acts as a charge collector and an OER catalyst, as shown in **Figure 1a**.

<figure><img src="image_2.png" /><figcaption>**Figure 1.** a) Scheme of a *n*-Si/NiNPs photoanode during water splitting under sunlight illumination. Electrolyte: 0.1 M KOH. *E*<sub>f</sub> and *E*<sub>f,p</sub> represent the Fermi level and the hole quasi-Fermi level, respectively. b) Top-view SEM picture showing the Ni catalyst pattern deposited onto *n*-Si, with NiNPs of diameters ranging from 5 µm to 500 nm (Note that NiNPs with lower diameters are located below but are not visible in this image, as indicated by the right</figcaption></figure>
arrow). c) CVs recorded on n-Si/NiNPs in the dark (black curve) and under simulated sunlight illumination (red curve). Scan rate: 50 mV/s, solution: 0.1 M KOH. d) Scheme of a n-Si/NiNPs photoanode during PECL under NIR irradiation ($\lambda_{exc}$ = 1050 nm). Electrolyte: 0.1 M KOH + 1 mM L-012. e) Scheme of the PECL microscopy setup.

The photoanodes were prepared by a combination of electron beam lithography, thermal evaporation of Ni and a subsequent lift-off process (see Section *Ni Patterning* in the SI file). A scheme of the initially desired Ni pattern is shown in **Figure S2**. Scanning electron microscopy (SEM) measurements of these surfaces after patterning (**Figure 1b**) revealed that our process produced, on purpose, arrays constituted by 9 lines of Ni dots with diameters ranging from 5 µm to 50 nm (as indicated by the arrow in **Figure 1b**). SEM pictures of individual Ni dots of all diameters are shown in **Figure S3**. The height of the Ni coating was ~40-45 nm over most of the Ni dot, as measured by atomic force microscopy (AFM, **Figure S4**). However, the Ni thickness could locally increase to a value as high as ~110 nm at the dot border, as shown in **Figure S5**.

Even if some of these dots have a diameter in the µm-range, for the sake of simplicity, we refer here to them as NiNPs.

As shown in the cyclic voltammograms (CVs) presented in **Figure 1c**, PEC tests were performed in 0.1 M KOH (pH = 13) electrolyte in the dark and under simulated sunlight illumination (AM 1.5G, 100 mW cm⁻²) to assess if these photoanodes were compatible with solar-assisted OER.
The measured CVs revealed the expected behavior, that is: (i) a negligible dark current, (ii) the presence of a characteristic photocurrent response attributed to the Ni<sup>III</sup>/Ni<sup>II</sup> redox couple<sup>49</sup> (see **Figure S6**) followed by a considerable increase in photocurrent around 0.6 V (**Figure 1c**), corresponding to OER (all potentials in this article are referred vs Ag/AgCl, 3 M NaCl). These results confirm that our patterned *n*-Si/NiNPs photoanodes are compatible with solar water splitting at high pH. In the following, these model photoanodes will be employed in the PECL configuration, schematized in **Figure 1d**. In this case, the luminophore 8-amino-5-chloro-2,3-dihydro-7-phenyl-pyrido[3,4-d]pyridazine-1,4-dione (denoted L-012), frequently employed for ECL,<sup>50,51</sup> was added to the electrolyte with a concentration of 1 mM. In these experiments, the electrochemical cell was placed onto an inverted optical microscope equipped with an IR filter and an electron multiplying charge-coupled device (EMCCD) camera with the *n*-Si/NiNPs photoanode facing the objective (experimental details are reported in *Section 1.4* of the SI file). Illumination was performed from the backside of the electrode using a light emitting diode (LED, λ<sub>exc</sub> = 1050 nm), as depicted in **Figure 1e**.
<figure><img src="image_3.png" /><figcaption>**Figure 2.** a,b) CVs (a) and corresponding plots of the PECL intensity as a function of potential (b), recorded at *n*-Si/NiNPs in the dark (black curves) and under NIR illumination (blue curves). Scan rate: 50 mV/s. c-f) PECL images recorded at different potentials 0.2 (c), 0.4 (d), 0.6 (e) and 0.8 V. The colored frames (c-f) correspond to the potentials indicated by the colored crosses in (a,b). g) PECL intensity profile extracted for the NiNPs indicated by the white square in panel (f); inset: magnification of the plot for *y*-coordinates > 100 µm (*y* axis is indicated in Figure 1b). Electrolyte: 0.1 M KOH + 1 mM L-012, exposure time: 1 s, λ<sub>exc</sub> = 1050 nm.</figcaption></figure>

**Figure 2a** presents the CVs obtained in the dark (black curve) and under excitation with λ<sub>exc</sub> = 1050 nm (blue curve) in the PECL electrolyte (0.1 M KOH + 1 mM L-012) with anodic polarization. Again, in the dark, a negligible current was recorded. Conversely, under near-infrared (NIR) illumination, considerable photocurrent was produced, corresponding to the oxidation of L-012 by photogenerated h<sup>+</sup>, as depicted in **Figure 1d**. **Figure 2b** shows that this photocurrent was correlated with PECL emission (blue curve) that started at ~0.3 V and
presented a maximum intensity at 0.9 V. Analysis of the *n*-Si/NiNPs photoanode surface with the microscope during potential scanning allowed us to visualize the local PECL emission during the potential scan. Sequential PECL images of the NiNPs array recorded at 0.2 (red frame), 0.4 (green frame), 0.6 (blue frame) and 0.8 V (yellow frame), are shown in **Figures 2c-f**; the corresponding potentials are indicated in **Figure 2a,b** with colored crosses. Under NIR illumination at an imposed potential of 0.2 V, no light was imaged with our microscopy setup (**Figure 2c**). This control experiment demonstrates that the pictures recorded at more anodic potentials corresponded to PECL emission. As it can be observed in the images recorded at potentials > 0.2 V (**Figure 2d-f**), PECL was generated only at the location of the NiNPs and not onto the uncoated *n*-Si surface, indicating a preferential interfacial charge transfer at the surface of the catalysts under sufficiently high anodic potential. This is in good agreement with the charge transfer mechanisms reported in the literature for OER-active metal NP-coated *n*-Si photoanodes.<sup>8,10–21,52,53</sup> The PECL spots became brighter with the potential increase, revealing the increasing rate for L-012 oxidation as a function of the applied potential and, consequently, the brighter PECL emission. While the larger NiNPs were always clearly visible (even at a low potential of 0.4 V) in these conditions, the smaller NiNPs (with a diameter < 500 nm) were only observed at the higher potentials. **Figure 2g** presents the PECL profile as a function of the
y-coordinate (Figure 1b), extracted from the region of interest (ROI) indicated by the white
dashed square of Figure 2f (representative NiNPs SEM pictures are shown in Figure S3). As
expected, due to the optical diffraction limit, the PECL full-width at half maximum was much
larger than the NiNP for diameters below ~1 µm, as shown by Figure S7a. The PECL profiles
show that the PECL images are not saturated for the largest NiNPs in the 1-to-5 µm diameter
range. Figure 2f also reveals that PECL intensity depended on the diameter of the NiNP, with a
quasi-linear variation, as presented in Figure S7b. In addition, the temporal evolution of the
PECL signal was measured on the whole NiNP array by applying a constant potential of 0.5 V
and analyzing sequential PECL images. Figure S8 shows that PECL was instantaneously
emitted when the NIR light (λ<sub>exc</sub> = 1050 nm) was switched on and that a significant PECL signal
was still generated over 30 min of experiment. To sum up, these results demonstrate that PECL
can be employed to image charge transfer at NiNP catalysts deposited on *n*-Si. The PECL
analysis (Figure 2f,g) reveals that it is possible to observe PECL emission for NiNPs having a
diameter below 1 µm, suggesting that the method is suitable for nanometer-scale imaging of
OER catalysts. This is promising because typical size range of NiNP catalysts that are employed
for OER at inhomogenously coated *n*-Si photoanodes is 500-50 nm.<sup>8,10–21,52,53</sup>
<figure><img src="image_4.png" /><figcaption>**Figure 3.** a,b) Top-view SEM pictures (a) and corresponding PECL images (b) of 50 nm-diameter NiNPs under NIR illumination ($\lambda_{exc}$ = 1050 nm) at 1 V. c) Corresponding PECL intensity profiles extracted for the 6 selected NiNPs in panel (a,b). Electrolyte: 0.1 M KOH + 1 mM L-012, exposure time: 10 s.</figcaption></figure>

Here, correlative SEM/PECL analysis of single NiNPs is discussed. **Figure 3a** presents SEM pictures of 6 selected NiNPs having a diameter of ~50 nm, i.e., the smallest OER catalysts present on our *n*-Si/NiNPs photoanodes. **Figure 3b** shows the corresponding PECL images obtained under illumination ($\lambda_{exc}$ = 1050 nm) and simultaneous application of a potential of 1 V.

**Figure 3c** presents the corresponding PECL profiles as a function of the *x*-coordinate, extracted
from this image. These figures show that the 6 Ni NPs generated a strong PECL signal,
demonstrating that they were all electrically connected to the *n*-Si substrate and all promoted
photoelectrochemical solid/liquid charge transfer. In addition, they all emitted PECL with a
comparable intensity, meaning that the subtle differences in morphology that can be observed by
SEM (Figure 3a) did not influence considerably photogenerated hole transfer. Based on this
observation, we might postulate that the lower PECL intensity observed for the NiNP #5 and #6
(see Figure 3c) was caused by a larger charge transfer resistance at the *n*-Si/NiNP interface.
However, to be verified, this hypothesis would require further single NP electrical analyses with
conductive AFM.⁵⁴

Finally, the PECL emission of single NiNPs was tracked over time during potentiostatic
polarization at 1 V. **Figure 4a** indicates, by colored squares, the 6 NiNPs that were selected for
this analysis, which had diameters of 250 or 50 nm. In good agreement with the previously
discussed potentiodynamic experiments of **Figure 2c-f**, the larger NiNPs produced more intense
PECL emission (~1300 vs ~700 a.u.). In all case, PECL intensity increased during the first 100 s,
which we explain by the oxidation of the NiNP outer layer to NiOOH, known to improve charge
transfer by increasing the band bending around the NiNPs.¹⁰ All studied NiNPs exhibited an
independent stochastic variation in PECL intensity and emitted PECL over 15 min of
polarization. This indicates that they were all electrically connected to *n*-Si and promoted an
efficient photogenerated solid/liquid charge transfer. We can observe that some NiNPs exhibited
a decrease of PECL intensity (pink and orange squares and curves in **Figure 4a,b**). Although
hypothetical, this could be explained by a variation of the NiNP chemical state and/or
morphology, including specific Ni oxidation (as supported by X-ray photoelectron spectroscopy,
see **Figure S9**), the detachment of some Ni parts at the boundary of the particle (see the AFM of
**Figure S4**) or a detrimental evolution of the *n*-Si/Ni contact (caused by local Si oxidation).
<figure><img src="image_5.png" /><figcaption>**Figure 4.** a) PECL image recorded at *n*-Si/NiNPs, showing the emission of NiNPs with diameters of 250, 100 and 50 nm at *t* = 10 s. b,c) Evolution of the PECL intensity of single 250 nm-*(b)* and 50 nm-*(c)* diameter NiNPs, as a function of time. The selected NiNPs are indicated by colored squares in panel a). Exposure time: 10 s, *E* = 1 V, *λ*<sub>exc</sub> = 1050 nm.</figcaption></figure>

To conclude, we prepared photoanodes made of nanoparticulate NiNPs OER catalysts precisely patterned on photoactive *n*-Si as model photoanodes for solar water splitting in alkaline medium. The upconversion PECL of these systems was studied by optical microscopy under
illumination with NIR excitation ($\lambda_{exc}$ = 1050 nm) in an electrolyte whose composition is close to that typically employed for water splitting by PEC, that is, L-012 (1 mM) in an aqueous solution of 0.1 M KOH. Our results show that PECL imaging enables reliable nanoscale observation of photoinduced interfacial hole transfer with a minimum resolution of 50 nm and over a long temporal range. This work provides a new monitoring method for catalysts activity monitoring.

However, to perform PECL imaging experiments, some technical limitations may exist. A specific setup combining optical microscopy and electrochemistry is required. But, it could be developed with simple optical components (LED, lenses, filters, etc.). To go further and achieve high-resolution PECL imaging of the smallest electrodes, a thin-layer PECL cell should be used in order to use a microscope objective with both high numerical aperture and long working distance. In addition, the sensitivity of the PECL is lower compared to ECL, which is almost a background free technology. Indeed, the optical PECL signal emitted by the luminophore should be separated from the excitation wavelength as in fluorescence and phosphorescence. However, by selecting appropriate semiconductor materials with a bandgap enabling NIR excitation and a luminophore system emitting visible light, both wavelengths can be easily discriminated.

Moreover, illumination of the semiconductor electrode could be performed from the backside (as in the present work) decreasing the light background and simplifying the optical setup.
In addition to the mapping of active sites, PECL microscopy could be used, for instance, to investigate activation processes by comparing PECL images recorded before and after activation.

In addition, it should enable the visualization of catalytic “hot spots” and “dead” phases at the surface of photoelectrodes and catalysts. Overall, this method has the potential to lead to considerable improvements in water-splitting solar energy conversion systems.

## Supporting information

Experimental details, materials, methods, description of the photoanodes, additional SEM, AFM, voltammograms, PECL analyses and XPS spectra are provided in supporting information.

## Acknowledgments

This work is part of the VISIBLE project of PEPR LUMA and was supported by the French National Research Agency, as a part of the France 2030 program, under grant ANR-24-EXLU-0014. It was also supported by ANR (LiCORN, ANR-20-CE290006 and 2PhotonECL, ANR-23-CE06-0024-02). C. Z. acknowledges the China Scholarship Council for his PhD scholarship. Philippe Legros and Christine Labrugère from Placamat are acknowledged for SEM and XPS characterization.
Conflicts of interest

There are no conflicts to declare.

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Photoelectrochemical Charge Transfer Imaging
at Nanoscale Catalysts

Changlin Zhou,ª Julie Descamps,ª Bertrand Goudeau,ª Patrick Garrigue,ª Frédéric Bedu,ᵇ
Lionel Santinacci,ᵇ Zonghua Wang,ᶜ Neso Sojic,ª* Gabriel Logetª*

<sup>a</sup>University of Bordeaux, Bordeaux INP, ISM, UMR CNRS 5255, 33607 Pessac, France

<sup>b</sup>Aix-Marseille University, CNRS, CINaM, Marseille, France

©Characteristic Laboratory of Advanced Metal Functional Materials and Processing in
Universities of Shandong, School of Mechanical and Electronic Engineering, Qingdao Binhai
University, Qingdao, Shandong, 266555, China

*Corresponding Author
# 1 Experimental Section

## 1.1 Chemicals

The *n*-type silicon wafers (1-10 Ω cm resistivity, phosphorus-doped, single side polished, 500 ± 25 µm) (100 mm) were purchased from University Wafers. Acetone (VLSI electronic grade, from VWR Chemicals) and ethanol absolute (VLSI electronic grade, from Avantor Science) were used without further purification. L-012 was purchased at FUJIFILM Wako Pure Chemical Corporation. Concentrated sulfuric acid (H₂SO₄ 96%, VLSI) and hydrogen peroxide (H₂O₂ 30%, VLSI) were purchased from Sigma-Aldrich. The ultrapure water (18.2 MΩ cm) was provided by Millipore Water Purification System.

## 1.2 Preparation of electrodes

This part includes the preparation of Si surface, photolithography, sputtering and the electrode fabrication. Most of these methods are similar to our previous work,¹ except the electrode fabrication. Here, we used a corner of the electrode to make the ohmic connection to the copper tape so that it does not cover all the backside and allows LED illumination from the back.

### Preparation of the Si surface

Silicon surface preparation was conducted using Teflon vessels and tweezers, which were pre-cleaned by immersion in Piranha solution at 105 °C for 30 minutes, followed by thorough
rinsing with ultrapure water. The *n*-type silicon wafers (1-10 Ω cm resistivity, phosphorus-doped, single side polished, 500 ± 25 µm) (100 mm) were subjected to ultrasonic cleaning for 10 minutes in sequential baths of acetone, alcohol, and ultrapure water. Subsequently, the wafers were heated to 105 °C in Piranha solution for 30 minutes, rinsed again with ultrapure water, and dried under an Ar flow.

## Ni Patterning

Arrays of Ni nanodots were grown on the *n*-Si wafers by an e-beam lithographic process. After degreasing the wafers in acetone and isopropanol (IPA) and cleaning under a plasma O₂, a 170 nm-thick layer of polymethyl metacrylate (PMMA, ARP 679.03 from Allresist, Germany) was spin-coated onto the Si surface and baked at 170°C for 10 min. The resist was then patterned by electron-beam lithography (EBL) using a Pioneer station (Raith GmbH, Germany). The electron beam was scanned over the sample with a 5 nm step, with an acceleration voltage of 20 kV and a current of 0.03 nA, resulting in an electron dose of 120 µC·cm⁻². The Ni array consisted of circular dots distributed in parallel lines with constant diameter and a pitch of 20 µm. The diameter was progressively ranged for each line: 50 nm, 100 nm, 250 nm, 500 nm, 1 µm, 2 µm, 3 µm, 4 µm, and 5 µm. The exposed area was then removed by successively dipping the sample for 1 min in AR-600-55 developer (from Allresist, Germany) and IPA. A ~40
nm-thick layer of Ni was then uniformly evaporated using an AUTO 306 thermal evaporator (Edwards BOC). The final Ni dots were achieved by lift-off in acetone.

## Electrode processing

The Si pieces were processed into electrodes following a defined protocol. First, the wafers were cut into 12.5 mm × 12.5 mm pieces. To establish an ohmic contact on the wafer's backside, a corner of the surface was scratched using a diamond glass cutter, ensuring the contact area did not obstruct subsequent LED irradiation. A droplet of InGa eutectic (Sigma Aldrich, 99.99%, metals basis) was applied to the scratched region, followed by the placement of Cu tape. A thin layer of silver paste (Electron Microscopy Sciences) was then applied to cover the InGa eutectic contact and part of the copper tape. After allowing the paste to dry, Kapton tape was used to shield the backside, protecting the ohmic contact.

## 1.3 Surface characterization

The surface of the photoanode samples were cleaned by ethanol and ultrapure water before characterization, followed by drying under an Ar flow. Scanning electron microscope (SEM) was performed on a SU7000 (Hitachi) under 1.00 kV with both UD and MD mode. AFM measurements were conducted using a Multimode 8 platform (Veeco, now Bruker) equipped
with a silicon tip (ScanAsyst-Air, resonance frequency: 70 kHz, spring constant: 0.4 N/m) in air
under Interleave mode.

**1.4 Electrochemistry and microscopy experiments**

The electrochemical experiments were performed in a 3-electrode system adapted on a P. type
a-clipse chamber from the Idylle company using a PalmSens4 as a potentiostat. The working
electrode was the photoanode with a fixed area of 0.5 cm², the reference electrode was an
Ag/AgCl (3 M NaCl) electrode and the counter electrode was a Pt wire. The Si surface electrode
was sealed with an O-ring on the backside of a homemade cell. A transparent glass window on
the frontside allowed for the transmission of photons. The electrolyte was composed of 1 mM
L-012 dissolved in 0.1 M KOH. The Si electrode was illuminated from the backside by a 1050
nm LED (Thorlabs M1050L4). The PECL emitted from the Si electrode was transmitted through
a shortpass filter with a cut-off wavelength at 750 nm (Thorlabs, FESH0750) in order to remove
the IR component from the incident light and was imaged with an inverted microscope LEICA
connected to an EMCCD camera (Hamamatsu). A 10x Objective (NPlan, NA = 0.25) and a 40x
Objective (APO, NA = 0.8, water objective) were used for low magnification and high
magnification, respectively.
## 1.5 Water splitting

The photoanode was connected to a copper wire using the method described in the electrode fabrication section. The wafer was then shielded with epoxy resin, leaving only the Ni nanoparticle pattern exposed. The exposed surface area of the working electrode was approximately 0.14 cm². The counter and reference electrode were the same as described before, the electrolyte was 0.1 M KOH. Illumination was provided by a solar simulator (LS0106, LOT Quantum Design) equipped with an AM 1.5G filter. During operation, the photoanodes were frontside-illuminated through the electrolyte.

## 1.6 Image processing

The images in this study were processed using ImageJ software (version 1.54f, National Institutes of Health, Bethesda, MD, USA). The processing steps included brightness and contrast adjustments to optimize image visibility, rotation and cropping to align and extract regions of interest, and color modifications to enhance the visualization of target structures.
## 2 Supporting Figures

<figure><img src="image_6.png" /></figure>

**Figure S1.** Scheme of the conduction band energy as a function of the position inside n-Si in the pinch-off regime (Reproduced from Ref.²: Oh et al., Nanoscale 2021, 13 (3), 1997) with permission from the Royal Society of Chemistry, adapted from ref.³), where $q\phi_m$ is the low barrier height at the metal contact, $q\phi_h$ is the high barrier height, and $q\phi_{eff}$ is the effective barrier height.

*Note related to Figure S1:* The photoanodes employed in this study are currently described as inhomogeneous junctions operating in the pinch-off regime. Herein, a mixed contact is established between n-Si and i) a low barrier height ($q\phi_m$) contact at the n-Si/Ni interface and ii) a high barrier height ($q\phi_h$) contact, which is supposed to originate from the n-Si/NiOOH region.⁴ The pinch-off effect is extremely beneficial because it takes advantage of the $q\phi_h$ region to generate a high photovoltage and the n-Si/M region to create a low resistance pathway for photogenerated minority carriers, which are readily injected at the catalytic surface for the electrochemical reaction. In addition, the n-Si surface that is not covered by NiNP is expected to be inactive as covered by an insulating $SiO_x$ layer.⁵
<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>**Figure S2.** Top: Scheme showing the map of a Ni NPs pattern. Bottom: A piece of silicon wafer has 4 motives; each pattern has an “L” mark on the top to facilitate microscope localization.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Figure S3.** SEM pictures of single NiNPs of different size, the diameter is indicated in the top left corner of each panel.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure S4.** AFM images (top) and corresponding 3D images (bottom) of representative single NiNPs with a diameter of 3 µm, 1 µm and 500 nm.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Figure S5.** Height profiles of single NiNPs with a diameter of 3 µm, 1 µm and 500 nm extracted from the AFM pictures of Figure S2. The corresponding lines where the profiles were extracted are displayed in the right column.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Figure S6.** CVs recorded on *n*-Si/NiNPs and under simulated sunlight illumination. Scan rate: 50 mV/s, solution: 0.1 M KOH.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>**Figure S7.** Plot of the a) PECL peak FWHM, b) maximum PECL intensity as a function of the NiNP diameter. Black discs: experimental datapoints, red lines: linear fits of the experimental data.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>**Figure S8.** PECL intensity as a function of time recorded on a *n*-Si/NiNPs photoanode (employed multiple times in previous PECL experiments) biased at 0.5 V. The red arrow indicates when the NIR light is switched ON. b,c) Corresponding PECL images acquired at a x10 objective. Exposure time: 1 s.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>**Figure S9.** XPS survey and Ni 2p (inset) spectra measured on a *n*-Si/NiNPs photoanode employed for a 30 min-long PECL imaging run at 1 V.</figcaption></figure>

*Note related to Figure S9:* The survey spectrum presents the expected elements and indicates that no pollution occurs on the surface, even after prolonged use. Note that the Ni 2p signal, shown in the inset of this figure, is relatively weak due to the low amount of Ni on the surface. The peaks located at 855.7 and 874 eV, indicative of Ni 2p<sub>3/2</sub> and Ni 2p<sub>1/2</sub> reveal the presence of Ni(OH)<sub>2</sub>. No peak is present at 852 eV, showing that most of the measurable Ni is in an oxidized state, indicating the prevalence of Ni(OH)<sub>2</sub>. These results show that in our operating condition, the Ni<sup>0</sup> is converted into Ni(OH)<sub>2</sub> and possibly NiOOH in situ, as suggested by the CV of Figure S6.
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