# In situ photo-electrochemical investigation using surface X-ray diffraction: from ultra-high vacuum to solid/electrolyte interfaces

Axel Wilson, I M Nadeem, H Hussain, X Torrelles, J Imran, D Garai, Robert Lindsay, Gregory Cabailh, J Zegenhagen, G Thornton, et al.

► To cite this version:

Axel Wilson, I M Nadeem, H Hussain, X Torrelles, J Imran, et al.. In situ photo-electrochemical investigation using surface X-ray diffraction: from ultra-high vacuum to solid/electrolyte interfaces. Journal of Physics: Conference Series, 2025, 10.1088/1742-6596/3010/1/012153. [hal-05099661]

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# In situ photo-electrochemical investigation using surface X-ray diffraction: from ultra-high vacuum to solid/electrolyte interfaces

A. Wilson¹,⁸*, I. M. Nadeem¹,², H. Hussain¹, X. Torrelles³, J. Imran², D. Garai¹,⁴, Robert Lindsay⁵,⁶, Gregory Cabailh⁷, J. Zegenhagen¹, G. Thornton² and C. Nicklin¹

1 Diamond Light Source Ltd, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, United Kingdom

2 London Centre for Nanotechnology and Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ, United Kingdom

3 Institut de Ciència de Materials de Barcelona (CSIC), Campus UAB, 08193 Bellaterra, Spain

4 Amity Centre for Spintronic Materials, Amity Institute of Applied Sciences, Amity University, Noida 201313, Uttar Pradesh, India

5 Corrosion@Manchester, Department of Materials, The University of Manchester, Manchester M13 9PL, United Kingdom

6 Photon Science Institute, The University of Manchester, Manchester, M13 9PL, United Kingdom

7 Sorbonne Université, CNRS, UMR 7588, Institut des NanoSciences de Paris, 4 Place Jussieu, F-75005 Paris, France

8 Sorbonne Université, CNRS, Laboratoire de Réactivité de Surface (LRS), F-75005 Paris, France

* Corresponding author: axel.wilson@sorbonne-universite.fr

**Abstract.** A Photo-ElectroChemical DrOplet Cell (PEC-DOC) has been developed for the I07 surface diffraction beamline of Diamond Light Source to investigate *solid-liquid* interfaces using Surface X-Ray Diffraction (SXRD). PEC-DOC is ultrahigh vacuum (UHV) compatible, and so can be directly attached to a UHV instrument, such as that available in the Surface and Interface Laboratory at Diamond Light Source. Hence, samples can be prepared using standard surface preparation methods, such as sputtering/annealing, and then be characterised using X-ray photoemission spectroscopy (XPS) and scanning tunnelling microscopy (STM). Subsequently, a sample can be transferred in UHV in PEC-DOC to I07, where the structure of the solid-liquid interface can be investigated.

## 1. Introduction

Solid-liquid interfaces are central to numerous scientific and technological processes, including catalysis,¹,² electrochemical energy conversion,³,⁴ and environmental remediation. Understanding the atomic and molecular structure of these interfaces is essential for optimizing devices such as fuel cells,⁵ batteries,⁶,⁷ and photoelectrochemical cells.⁸,⁹ However, many conventional surface science techniques, typically conducted under ultrahigh vacuum (UHV), face significant challenges when applied to solid-liquid interfaces due to the complexity and dynamic nature of liquids. To address this limitation, a novel Photo-ElectroChemical DrOplet Cell (PEC-DOC) has been developed for the I07 surface diffraction beamline at Diamond Light Source, enabling detailed investigations of solid-liquid interfaces through Surface X-ray Diffraction (SXRD).

Several technical solutions have been proposed to study surface/liquid interfaces under electrochemical control. Concerning X-ray diffraction/scattering studies, these interfaces have been extensively studied using mylar-window cells,¹⁰ which are most appropriate when UHV preparation of the sample is not required. For systems requiring the latter, the cell of Renner *et al.* has been one of the most successful to date, as it is compatible with the constraints of UHV preparation, and allows solution flow during measurement at grazing X-ray incidence.¹¹ PEC-DOC improves on this design and implements key changes, including a sustainable construction, enabling more complex experiments.

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Content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
PEC-DOC has been designed to fulfil a demanding list of specifications for utilisation on beamline I07 at the Diamond Light Source. Firstly, it should facilitate the preparation of a sample and its characterisation under UHV in an off-line instrument. The *as-prepared* sample should then be transferrable without leaving UHV to the diffractometer of I07 beamline to allow initial measurements with SXRD in vacuum. Following acquisition of these data, one must be able to vent PEC-DOC with an inert gas to atmospheric pressure to enable introduction of a flowing solution to form the *solid/liquid* interface of interest; this interface needs to be stable for numbers of hours to allow data acquisition. In addition, the cell must incorporate working, counter and reference electrodes to perform electrochemistry. Finally, line-of-sight to the interface is required to allow illumination during acquisition, for instance with UV light.

This article presents the technical solutions that were developed to meet the requirements outlined above. It also details the operating procedure for conducting a typical experiment, and showcases applications carried out using the photoelectrochemical cell.

## 2. PhotoElectroCatalytic DrOplet Cell features and design

### 2.1 *Sample preparation, characterisation and transfer*

Most of the UHV work carried out during an experiment is performed in the Surface and Interface Laboratory (SIL), a support lab independent of the I07 beamline. It is accessible to users upon request, especially for beamtime preparation. SIL may be booked a week in advance of beamtime to calibrate the sample preparation procedure and take advantage of the numerous characterisation techniques available.

The SIL set-up, presented in figure 1a, is composed of three UHV chambers, preparation, transfer and analysis, along with a docking gate valve on which PEC-DOC is mounted. The *preparation chamber* incorporates a sputter gun and a PBN heater for sample preparation up to 1200°C. It is also equipped with a quartz microbalance and a number of gate valves to allow small equipment, such as molecule dosers or UHV effusion cells, on linear drives to be introduced without having to vent/bakeout the entire system. The transfer chamber comprises a small volume load lock, a 6-samplekarousel and a transverse transfer arm leading to the docking gate valve, which is utilised to transfer samples into PEC-DOC; once PEC-DOC is decoupled, it has a small footprint and can easily be mounted on the diffractometer of the I07 beamline. The *analysis chamber* houses an Omicron UPS-XPS, monochromated Cu Kα X-ray source, and a flood gun. It is also equipped with an Omicron variable temperature STM-AFM comprising line of sight viewports for dosing materials onto a sample whilst scanning. In addition, it houses an Omicron LEED, filament heater, and sputter gun for sample preparation.
<figure><img src="image_3.png" /><figcaption>**Figure 1.** a) The UHV instrument in SIL. b) PEC-DOC mounted on the I07 beamline diffractometer. In both pictures PEC-DOC is outlined in red.</figcaption></figure>

For experiments involving PEC-DOC, it is mounted on the SIL UHV instrument as shown in figure 1a and baked at 130°C for 36h using PEC-DOC's 80 l/s turbopump to reach low 10⁻⁹ mbar. To ensure compatibility, samples must be mounted on Omicron-type elevated Mo sample plates. These sample plates have a rectangle, that protrudes 2 mm from the base, on which samples are mounted with Ta metal strips. The purpose of this geometry is to allow access to grazing incidence and exit angles on I07 diffractometer. Once the sample preparation and characterisation are complete, the plate is translated into the PEC-DOC sample holder using a linear drive. The PEC-DOC gate valve is then closed, and the chamber dismounted from the SIL UHV instrument. PEC-DOC is subsequently moved on a trolley to the I07 diffractometer (see figure 1b), where SXRD, GIXRD and GI-SAXS data can be acquired under UHV to complement the XPS, LEED, and STM characterisation.

## 2.2 Drop formation, electrode configuration and sample illumination

The solid/liquid interface is formed under 1 bar of inert gas. To this end, PEC-DOC is vented through the turbopump vent valve. Once vented, the gate valve located on top of PEC-DOC, visible in figures 1 and 2a, is opened, and the liquid delivery device shown in figure 2 is promptly introduced, screwed in position and sealed using a DN 40 CF Viton gasket. A metallic port aligner located above the top gate valve is utilised to align the glass capillary of the liquid delivery device with the sample and set the sample-capillary distance. An initial coarse alignment is done visually, followed by fine alignment using the X-ray beam by scanning across sample and capillary using the x and y motions of the diffractometer's hexapod for two perpendicular directions.
<figure><img src="image_4.png" /><figcaption>**Figure 2.** a) Schematic representation of PEC-DOC with liquid delivery device in place. b) Projection of the liquid delivery device, G2MR, sample and UV light. c) image of the surface/liquid interface captured through the endoscope in experimental lighting conditions. d) Schematic of the liquid delivery device fitted with electrodes for electrochemistry.</figcaption></figure>

The liquid delivery device illustrated in figure 2 is composed of two concentric capillaries. The inner capillary is a 1/16" OD/1/32" ID PTFE tube ending approximately 5 mm away from the sample. The outer capillary seen in figure 2c is a 4 mm OD/3 mm ID borosilicate tube measuring approximately 200 mm. A Microlab 600C dual syringe pump (Hamilton, USA) is utilised to simultaneously pump liquid onto the sample via the inner capillary and remove excess liquid via the outer capillary. Actuating both syringes allows a continuous flow of liquid over the sample interface, while maintaining the shape and volume of the drop. To ensure that this process is functioning as expected, it is essential to visualise the shape of the droplet without delay and at a sufficient framerate. This is achieved using a rigid 45° endoscope equipped with a camera (Basler ACE, Germany).

Concerning electrochemical measurements, the PEC-DOC sample holder is isolated from ground and has a connection to allow the sample to be a working electrode. A 0.1 mm Pt wire (Goodfellow Cambridge Ltd, UK) is inserted inside the inner capillary to serve as a counter electrode. A refillable Ag/AgCl miniature reference electrode (eDAQ Europe) is inserted in the cross piece above the glass capillary. This three-electrode system can be connected to a CompactStat.h potentiostat (Ivium Technologies, The Netherlands) available at DLS for electrochemistry applications. Finally, the surface/liquid interface can be illuminated using the custom *glass-to-metal reducer* (G2MR) visible in Figures 2a and b. The G2MR consists of a stainless-steel sleeve with a DN 40 CF on top welded to a DN 63 CF borosilicate viewport. The assembly is reinforced by three aluminium brackets ensuring the load mounted on top is not borne by the glass/metal brazing. This provides a doughnut shape borosilicate ring through which visible light, to improve the endoscope camera performance, or UV-light can be shone onto the interface. A prototype UV-illumination device was developed (Figure 2b, labelled UV light) using an array of 16, 365 nm
LEDs (ILS, UK), providing up to 10 mW/cm² on the interface. This prototype clamps to the aluminium bracket using a 3D printed support for easy orientation. At the time of writing, this specific illumination prototype has not been utilised in situ to carry out SXRD measurement of solid/liquid interfaces.

### 3. Applications

#### 3.1 Reduced Anatase TiO₂ (101) / water interface

PEC-DOC has been used to study the structure of the interface formed between anatase TiO₂ (101) – (A₁₀₁) and H₂O.¹² Titanium dioxide has been investigated for hydrogen fuel production¹³ due to its photo-assisted water-splitting properties.¹⁴ Several studies have focused on understanding the structure under UHV conditions.¹⁵⁻¹⁹ A₁₀₁ is thought to have reduced reactivity to water compared to rutile TiO₂ (110) – R₁₁₀ – because of the absence of surface oxygen vacancies.²⁰ These vacancies, which promote dissociation on R₁₁₀,²¹ are absent on A₁₀₁, as they are more stable in subsurface sites.²⁰ However, it has been predicted that a liquid interface could trap excess electrons at bound hydroxyl complexes.²² In this study, a 2 mm thick natural single crystal of anatase, with a surface area of 4 × 4 mm², was prepared under UHV in the SIL. The process involved cycles of Ar sputtering (PAr = 6.10⁻⁶ mbar, 1.5 kV, 10 min) and annealing (T < 750°C, 10 min). Pre-characterization of the sample was carried out using LEED and XPS before being mounted under UHV in PEC-DOC. The PEC-DOC was subsequently transferred according to the method described in section 2.1. A drop of ultrapure water was applied to create the interface, following the procedure outlined in section 2.2.

<figure><img src="image_5.png" /><figcaption>**Figure 3.** Comparison of experimental CTRs for as-prepared A₁₀₁ in UHV (red), and for the A₁₀₁ interface with an ultrathin water film (blue) and bulk water (gray). CTRs are offset for clarity. Ball and stick model of the proposed d) A₁₀₁ interface with an ultrathin water film and e) A₁₀₁ interface with bulk water. The black arrows represent the relative magnitude and direction of atom displacements with respect to bulk lattice positions. Hydrogen atoms were not included in the experimental fitting procedure due to their small X-ray scattering strength, and so are only displayed for illustrative purposes.</figcaption></figure>

A subset of three experimental crystal truncation rods (CTRs) from the full dataset (ref. ¹² SI) and the best fits for A₁₀₁ in UHV, covered with a 10 ± 2 monolayer water film and bulk water, are
shown in Figure 3 in red, blue, and grey, respectively. Atomic displacements associated with interface formation are indicated by black arrows on figure 3, showing mixed associative and dissociative water adsorption. SXRD models for the A₁₀₁ interface with the ultrathin water film suggest ordering in the contact layer only, with complete coverage of adsorbed H₂O/OH species on five-coordinate titanium atoms (Ti5c, marked 1 on figure 3d). Two distinct Ti-OH₂/OH species with 25% and 75% surface coverage and differing bond lengths were fitted. The SXRD model for the interface with bulk water shows an additional ordered layer above the contact layer. Like the ultrathin case, the contact layer contains two distinct Ti5c-OH₂/OH species, with 25% and 75% surface coverage and bond lengths of 2.18 ± 0.03 Å and 1.95 ± 0.01 Å, respectively (Figure 3e). The second layer appears to consist of H₂O molecules that are hydrogen-bonded to the molecules in the contact layer, as indicated by the bond distances. The surface atomic displacements after the formation of both aqueous interfaces are close to zero, suggesting that the expansion of the surface observed in UHV is reversed upon the formation of the interfaces. This behaviour has also been observed at the R₁₁₀/H₂O interface.²³ We find that both the ultrathin water film and bulk water induce a similar contact layer on A₁₀₁, with the primary difference being the presence of an ordered second monolayer at the A₁₀₁/bulk water interface. This suggests that the ultrathin water film is too thin to behave as bulk water.

This study underscores the importance of the substrate environment in determining reactivity. For A₁₀₁, the aqueous interface is reactive, whereas the UHV substrate remains inert at room temperature. Since the aqueous interface is more relevant for photocatalysis, this also emphasizes the significance of studying systems in realistic environments.

## 3.2 Reduced Rutile TiO₂ (110) / 0.1 M KCl electrified interface

To further advance the understanding of electrolyte interfaces with R₁₁₀, its interface with KCl was studied at pH 6, creating a neutral environment that enables electrochemical control with a reduced risk of substrate corrosion.²⁴⁻²⁶

A R₁₁₀ single crystal was prepared using a wet chemical preparation method which yields a UHV-like termination of R₁₁₀ without UHV sample preparation.²⁷,²⁸ The substrate was pre-reduced to provide electrical conductivity using cycles of sputter/annealing. The 0.1M KCl electrolyte was prepared with high-purity KCl dissolved in deionized water, degassed with nitrogen gas. A droplet of the electrolyte was formed on the R₁₁₀ surface to create the interface. For this R₁₁₀/KCl interface,²⁹ the CTR data for the 0.1M KCl(aq)/R₁₁₀ interface consisted of 2577 reflections from 9 rods, modelled by considering displacements in a surface slab of 2.5 R₁₁₀ unit cells. The structural parameters included 33 atomic displacements, 16 Debye-Waller (DW) factors, a scaling factor, and occupancies. A subset of three CTRs ([1, 0 1], [1, 1 1], [1, 2 1]) was chosen for further electrochemical control using chronoamperometry. The KCl/R₁₁₀ interface structure derived from the best SXRD data fit is presented in Figure 4. This structure shows similarities to HCl/R₁₁₀ and NaOH/R₁₁₀ models,³⁰ particularly in the coordination of Cl⁻ to the Ti5c surface site. However, a distinct elongation in the Ti-Cl bond is observed for the 0.1 M KCl interface due to the presence of the K⁺ cation. An ordered O layer is found above the K⁺ contact layer likely represents water hydrogen bonded to K.
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Ball and stick illustration depicting a side view of the best-fit model for the 0.1M KCl(aq) / R110 interface, with the directions of substrate atom displacements indicated.</figcaption></figure>

Modelling indicates a significantly raised Debye Waller (DW) parameter for the cation, possibly indicating dynamic exchange between K⁺ and H₃O⁺ ions in solution. The reported DW for O above the K⁺ layer aligns with previously reported values for anion/cation SXRD studies.³¹ However, our model does not indicate the presence of O between the bound Cl⁻ at the Ti5c site and K⁺ at the O site found on metal substrates.³²,³³ The K-Cl distance in our model is consistent with known K/Cl/Ti complexes,³⁴ and the model is used as a basis for interpreting the following electrochemical CTRs under a voltage bias of +0.3 V.

<figure><img src="image_7.png" /><figcaption>**Figure 5.** Cyclic voltammogram (CV) of the TiO₂ (110)/0.1M KCl(aq) performed offline using an adaptation of the wet chemical preparation. Three sequential scans (a-c) were performed from -1.0 V to +2.5 V, with an E step of 10 mV and a scan rate of 50 mV/s. An Ivium CompactStat three-electrode potentiostat was employed using PEC-DOC with R110 as the working electrode, Pt wire at the counter electrode and Ag/AgCl as the reference electrode.</figcaption></figure>
Cyclic voltammetry (CV) for KCl/R₁₁₀ identified +0.3 V as a suitable potential for
chronoamperometry, with minimal electrochemical activity and low risk of corrosion. The first
scan of the CV acquired with the PEC-DOC tabletop (Figure 5) exhibited an electrochemical event,
suggesting the electrochemical annealing of R₁₁₀ and removal of residual aliphatic carbon not fully
eliminated by UV treatment alone. In a near ambient pressure X-ray photoelectron spectroscopy
study,³⁵ which employed a similar variation of the wet chemical preparation of R₁₁₀, we can
confirm most of the C 1s signal from the 'as loaded' sample is assigned to C-C bonds.
Electrooxidation of carboxylates adsorbed on metal surfaces reported a potential range of 1.0-1.6
V, with certain aromatic C compounds reporting electrooxidation in ~1.9 V.³⁶⁻³⁸ The lack of a
corresponding feature in the CV in Figure 5 suggests the absence of carboxylates at the interface.
Previous work has shown that field and non-covalent effects influence K⁺ interfacial behaviour. In
particular, negative potentials at the working electrode have been found to induce widening of the
K⁺ layer (field effect), while adsorbed OH at Ag(111) stabilises K⁺ closer to the substrate (non-
covalent effect).³⁹ ⁴⁰ These phenomena explain changes in Cl-K layer spacing, such as those seen
on Cu(111).³²

<figure><img src="image_8.png" /><figcaption>**Figure 6.** Crystal truncation rods of the electrochemical interface of 0.1 M KCl(aq) & R₁₁₀. The plots for the experimental error bars of the KCl/R₁₁₀ in purple and the respective electrochemical error bars in red. Each CTR is presented with the equivalent [h, k, l] rods at +0.3 V.</figcaption></figure>

Electrochemical CTRs, presented on a linear scale in Figure 6, show that changes in the anti-Bragg regions, especially for [1, 0 l] and [1, 2 l] rods, are prominent at lower l values. Tentative modelling suggests that a +0.3 V bias leads to the removal of the cationic layer, contrary to previous reports on the field effect on metal substrates. The complexity of the electrochemical [1, 1 l] rod, which resembles the UHV model, indicates that not all electrochemical changes can be fully interpreted due to the limited dataset. Nevertheless, these findings provide insights into the dynamics at the electrified KCl/R₁₁₀ interface.

**4. Conclusion**

The Photo-ElectroChemical DrOplet Cell addresses key challenges in solid-liquid interface research by enabling direct, real-time observations of interface dynamics under controlled electrochemical and photoelectrochemical conditions. It also facilitates detailed structural studies of surfaces and interfaces using techniques such as Grazing Incidence X-ray Diffraction (GI-XRD), Grazing Incidence Small Angle X-ray Scattering (GI-SAXS), and rod acquisition for SXRD modelling. The integration of UHV-based surface preparation with liquid-phase experimentation opens up new opportunities to explore catalytic reactions, electrode processes, and other interfacial phenomena at a fundamental level. By allowing the preparation, transfer, and characterization of samples in a seamless workflow, the PEC-DOC at the I07 beamline represents a powerful tool for the scientific community. It provides new insights into the fundamental
mechanisms governing solid-liquid interfaces, with applications ranging from energy storage and conversion to environmental catalysis and material degradation. This novel approach overcomes long-standing challenges in the field, enabling the investigation of interfaces under conditions that closely mimic real-world applications, while maintaining the precise control and fundamental structure of single-crystal surfaces.

A potential improvement for this project would be the redesign of the liquid delivery system. Currently, the reference electrode is positioned too far from the working/counter electrode junction. A more efficient design could involve a compact, 3D-printed counter/reference electrode assembly that is small enough to be introduced through the top gate valve and positioned just a few centimetres away from the interface.

## Acknowledgements

This work was supported by the European Research Council Advanced Grant ENERGYSURF to GT, EPSRC (EP/L015862/1), grant PID2021-1232760B-100 funded by MCIN/AEI/10.13039/501100011033 and the Severo Ochoa Programme for Centres of Excellence in R&D (CEX2023-001263-S) from the Spanish Research Agency. Axel Wilson has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie Grant No. 665593 awarded to the UK Science and Technology Facilities Council. Debi Garai was supported by DST/INSPIRE Fellowship/[IF160579] awarded by the Government of India Ministry of Science and Technology.

## Data availability

The data presented in figures 5 and 6 are openly available in Zenodo at http://doi.org/10.5281/zenodo.14645899.

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