In-situ electrosynthetic fabrication of highly crystalline, aqua-compatible palladium halide perovskite films for photoelectrocatalysis

In-situ electrosynthetic fabrication of highly crystalline, aqua-compatible palladium halide perovskite films for photoelectrocatalysis | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Physical Sciences - Article In-situ electrosynthetic fabrication of highly crystalline, aqua-compatible palladium halide perovskite films for photoelectrocatalysis Pabitra K. Nayak, Amogh Ravi, Anku Guha, Vaishali Arunachalam, and 11 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9106903/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Semiconductors provide the material foundation for solar energy conversion, enabling the direct transformation of sunlight into electrical or chemical energy. Among emerging semiconductor classes, metal halide perovskites (HaPs) have attracted intense attention owing to their exceptional optoelectronic properties; however, their instability in aqueous media severely limits their application in photoelectrochemical energy conversion. We report electrosynthetic perovskite in-situ crystallization and film fabrication (EPIC-Fab), a scalable aqueous electrosynthesis and direct deposition strategy for palladium-based vacancy-ordered double perovskites (VODPs), specifically Cs2PdCl6, Cs2PdBr6, and Cs2PdI6. This method enables direct growth of highly crystalline films on a wide range of conductive substrates, including uneven tubular geometries, using water as the sole solvent and requiring no post-deposition annealing. Film thickness and morphology can be tuned through deposition parameters and duration. Unlike conventional lead halide perovskites, the resulting VODP films exhibit intrinsic high thermal and water stability. We demonstrate that these films function as efficient photoelectrocatalysts for hydrogen evolution (Faradic efficiency >94%) without additional cocatalysts or protective layers. Notably, a light-assisted self-repair process enables maintaining operational stability during continuous photoelectrochemical hydrogen evolution for > 100 hours. This work establishes an aqueous, surface-agnostic synthetic paradigm for vacancy-ordered double perovskites for optoelectronic and energy conversion applications. Physical sciences/Materials science/Materials for energy and catalysis/Photocatalysis Physical sciences/Materials science/Materials for devices/Electronic devices Physical sciences/Materials science/Condensed-matter physics/Semiconductors Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction Semiconductors play a crucial role in energy conversion technologies, enabling the transformation of sunlight into electricity and chemical fuel. Their capacity to absorb light and produce charge carriers underpins their applications in photovoltaics and photocatalysis. For efficient photoelectrochemical hydrogen generation, an emerging technology with huge commercial interest, semiconductor materials must exhibit optimal light absorption and long-term stability under aqueous conditions and illumination. Tandem architecture-based electrodes, such as GaInP/GaInAs with Rh catalyst and GaInP/GaInAs/GaAs protected by crystalline TiO 2 and Rh nanoparticles, have achieved solar-to-hydrogen (STH) efficiencies of > 10 % in acidic conditions. However, they exhibit poor operational stability with lifetimes of 40 hours and 3 hours, respectively 1,2 . The monolithic photoelectrode, InGaP/GaAs/Ni, shows stability of 150 hours under alkaline conditions 3 . Emerging chalcogenide systems such as Cu 3 BiS 3 films and their tandem integration with BiVO 4 have not crossed the operational stability of 60 hours 4 . Nanowire-structured p-Si photocathodes, molecularly modified and protected by TiO 2 , particulate CTGS (Cu 2 Sn x Ge 1-x S 3 ) photocathodes and triple-junction amorphous silicon solar cells integrated with CoWO and CoWS catalysts have demonstrated promising activity but with poor stabilities of < 10 hours 5–7 . Molecular photoelectrodes leveraging π–π interactions between PEDOT and polycarbazole networks, combined with NiS 2 catalysts, have shown enhanced charge transport, high external quantum efficiency (EQE), and stable operation exceeding 120 hours 8 . III–V semiconductor-based devices, including p-doped GaInP 2 nanopillar photocathodes and GaN nanowire-modified Si, have set benchmarks for STH efficiency (over 20%) and long-term stability, with the latter operating for 3000 hours 9,10 . Advanced protective strategies, such as graded MoS x /TiO 2 11 layers and cryogelated polyacrylamide overlayers 12,13 , have further improved the durability of Sb 2 Se 3 and GaInP 2 photocathodes to more than 100 hours. Photoelectrochemical devices with operational stabilities greater than 100 hours and good efficiencies require architectures of multiple layers, often consisting of a photo-absorber layer, a protection layer and a catalyst layer. Hence, large-scale photoelectrocatalytic hydrogen production is still in its infancy because of the lack of a single material with high STH efficiency 9,10 . Among the emerging classes of semiconductors, metal halide perovskites (HaPs) have attracted significant attention owing to their exceptional optoelectronic properties, including strong visible-light absorption 14,15 , high charge-carrier mobility 16,17 , and long diffusion lengths 18–21 . These materials have revolutionized the field of solar photovoltaics, achieving power conversion efficiencies exceeding 27% 22–24 . However, achieving durability in operational conditions remains a challenge for halide perovskite materials. The chemical instability of halide perovskites in humid, thermal, and illuminated environments limits their practical implementation in direct light-to-chemical energy conversion, particularly in photoelectrochemical hydrogen generation, where direct contact with electrolyte solutions accelerates the degradation of HaPs. 25–27 . However, despite their good light-absorbing properties, ABX 3 type HaPs need protection and catalyst layers to be useful in photoelectrochemical reactions in aqueous media 28–33 . Even with protective layers, most HaP-based photocatalytic systems have poor stability under operational conditions 27–32 . In this context, vacancy-ordered double perovskites (VODPs), with the general formula A 2 BX 6 , are a promising class of materials for photoelectrochemical reactions in aqueous conditions 34 . Here, the ABX 3 HaP structure is modified by systematically removing the alternate B-site cation to create isolated [BX 6 ] octahedra in an antifluorite-type lattice 35 . This structure imparts enhanced chemical stability compared to that of ABX 3 variants. VODPs can retain some of the key features of ABX 3 perovskites, such as highly crystalline thin films, while offering a platform for lead-free compositions and improved stability in aqueous media. Moreover, they can host a far wider range of B-site cations than ABX 3 perovskites, and this compositional freedom enables high-entropy compositions incorporating five or six different metal centres in a single stable phase 36 . Despite the structural isolation of [BX 6 ] octahedra, electronic coupling via the halide sublattice enables dispersive bands and supports good charge transport, as evidenced by low electron effective masses in compounds such as Cs 2 SnI 6 and Cs 2 TeI 6 35,37,38 . Thin films of HaP are generally prepared through solution-based or vacuum thermal deposition methods. Though solution-based methods such as spin coating 39,40 , drop coating 41 and spray coating 42 are easy and cost-effective, but the low solubility of certain precursors in conventional solvents such as DMSO and DMF poses an obstacle in the formation of thick and continuous films. Moreover, solution-based deposition methods are not suitable for conformal coating on protruding substrates. Even in vapour-phase deposition, shadowing remains a challenge on non-planar substrates. There is a need to develop a new, scalable, environmentally friendly method for the synthesis and surface-agnostic deposition of water-stable halide perovskite films with controlled morphology for photoelectrochemical applications. Here, we report a generic method for in-situ electrosynthesis and deposition of palladium-based VODP films: Cs 2 PdCl 6 , Cs 2 PdBr 6 , and Cs 2 PdI 6 , enabling conformal coating onto a wide range of conductive substrates, including fluorinated tin oxide and indium tin oxide coated glass, glassy carbon electrode, carbon paper, carbon cloth and various metals. In contrast to the generally employed environmentally hazardous solvents, this electrosynthetic perovskite in-situ crystallization and film fabrication (EPIC-Fab) method uses water as the sole solvent during deposition. The deposited films show high crystallinity, and the material can be conformally deposited onto tubular substrates. The deposited films show high thermal and water stability. We demonstrate photo-electrocatalytic applications of these materials without any external load of catalysis or protection layer. These materials demonstrate >100 hours of operational stability without any noticeable drop in efficiency under continuous operation. We unravel the mechanism underlying the films' exceptional stability, driven by a light-mediated self-repairing process. The surface-agonistic deposition and scalability pave the way for the further development of stable, self-repairable VODP materials with different compositions for energy conversion and optoelectronic applications. Results and Discussion Perovskite films preparation by electrosynthesis: We show the schematic for VODP film fabrication in Figure 1a . We used electrosynthesis and deposition in H 2 O as the solvent system (pH~ 0 (0.5M H 2 SO 4 )), having CsX and PdX 2 salts, employing a three-electrode setup consisting of a conductive substrate as working electrode, saturated calomel electrode (SCE) as reference, and graphite rod as counter electrode. (See the methods section). In aqueous salt solutions, Pd exists in the 2 + oxidation state, forming compounds like Cs 2 PdX 4 that readily dissociate in water 43 . However, when Pd is in its 4 + oxidation state, it forms compounds such as Cs 2 PdBr 6 , which remain insoluble and stable in water 43 . This prompted us to utilize this difference in stability of Pd compounds to make thin films of the VODPs by changing the oxidation state of Pd in-situ. Pd 2+ ions in the solution diffuse from the bulk to the surface of the working electrode, adsorb onto it, and undergo a two-electron oxidation to form Pd 4+ ions (as schematized in Figure 1a ). The aqueous medium also contained an optimized concentration of Cs + and halide ions (Cl - , Br - , I - ). The formation of Pd 4+ , along with the availability of Cs + and halide ions, leads to the nucleation of vacancy-ordered double perovskite (VODP) on the anode surface. Under constant bias, these nuclei grow and coalesce over time, resulting in continuous film formation ( Figure 1a ). Prior to the deposition of VODP thin films, we determined the potentials required for Pd 2+ oxidation and subsequent perovskite deposition using cyclic voltammetry ( Supplementary Figure S1). We found that potentials above 1.0 V and 1.2 V (vs saturated calomel electrode, SCE) are required for the deposition of Cs 2 PdBr 6 and Cs 2 PdCl 6 , respectively. We did not notice any films forming below these thresholds. Although deposition of Cs 2 PdI 6 can occur above 1.0 V (vs SCE), the film quality is poor due to the low aqueous solubility of PdI 2 . To overcome that issue, we added an optimised amount of KI, where excess iodide ions form soluble Pd-iodide complexes. We found that pre-polarization of the electrode to negative potentials before the application of the positive deposition potential improves the film uniformity for Cs 2 PdI 6 deposition. Characterization of the deposited films: In Figure 1(b-d), we show the simulated XRD traces derived from single-crystal data for Cs 2 PdCl 6 (ref. 44 ) Cs 2 PdBr 6 (ref. 43 ) and Cs 2 PdI 6 (ref. 45 ), alongside the experimentally observed XRD traces of the corresponding thin films. The agreement between experimental and simulated traces confirms the phase purity and crystallinity of the films prepared by the EPIC-Fab method. We observed the expected systematic changes in the XRD peak position as the halide ion changes from chloride to iodide. The systematic change in the XRD peak positions is due to variations in lattice parameters arising from differences in the ionic radii of the halides. Figures 2 (a-c) show scanning electron microscopy (SEM) images of the deposited films. Cs 2 PdCl 6 and Cs 2 PdBr 6 films exhibit well-defined crystalline features extending over several tens of micrometres, and Cs 2 PdI 6 film shows micrometre-scale crystalline domains. We note that the films were deposited at room temperature, and no post-deposition annealing or processing was required to obtain highly crystalline, continuous films. Cross-sectional SEM images ( Supplementary Figure S2 ) show that the deposition method produces a several-micrometre-thick and compact film with controlled thickness. The optical image of the VODP film deposited on a large-area (15 cm 2 ) FTO-coated glass ( Supplementary Figure S3 ) shows the scalability potential of the EPIC-Fab method. The area of film deposition can be further increased by using a larger solvent bath. We present the energy-dispersive spectroscopy (EDS) data of these films in Supplementary Figure S4 . The EDS analysis showed elemental ratios of 2:1:6 (Cs:Pd:X, where X = Cl, Br, or I) for the films deposited from the respective halide precursors, thereby confirming the stoichiometry of the in-situ synthesized compounds. We show the Raman spectra of these films in the Supplementary Figure S5 . The Raman spectra of the three types of VODP films indicate a gradual shift of peaks to the lower wavenumbers from Cs 2 PdCl 6 to Cs 2 PdI 6 , which are consistent with the increasing Pd-X (X=Cl, Br, I) bond lengths as the halogen changes from Cl to I. We also checked for any PdO formation during deposition using Raman spectroscopy. Any PdO deposition would give a characteristic B 1g peak at 647 cm -1 (ref. 46 ). We did not observe any peak around 647 cm -1 in the deposited films (Supplementary Figure S5) , indicating the absence of PdO formation during the film deposition. The absence of XRD peaks of PdO in the XRD traces also corroborates the absence of PdO. We then extended this deposition method to deposit VODPs on carbon paper, which has networks of tubular carbon rods. The porous, mesh-like open structure provides a high surface area for material deposition, facilitating efficient mass transport in electrochemical applications. We provide the detailed deposition method in the method sections. By adjusting the deposition parameters, we synthesized and deposited Cs 2 PdCl 6 and Cs 2 PdBr 6 conformally on the carbon papers. We show the SEM images of the pristine carbon paper and Cs 2 PdCl 6 and Cs 2 PdBr 6 deposited on carbon paper in Figures 2(d-f) . We observed compact VODP film deposition on carbon fibers. The powder XRD of films deposited on carbon paper confirmed their phase purity, indicating successful synthesis and conformal coating on this substrate ( Supplementary Figure S6 ). We note here that making compact and thick Pd-based VODP films has been challenging due to the poor solubility of these compounds in solvents such as DMSO, DMF, and NMP which are commonly used to make Pb halide perovskite films. For example, patchy films are obtained by spin coating methods ( Supplementary Figure S7) when we used chemically synthesized Cs 2 PdBr 6 43 . We then measured the UV-Visible absorption of the crystallite flakes obtained from the deposited films, which we present in the Supplementary Figure S8 . We found that all the materials have absorbance in the visible range. However, it was not possible to determine a clear band edge from the absorbance measurement. This could be due to the fact that in several micrometre-thick films, the substantial sub-bandgap absorption obscures the clear bandgap transition. We used X-ray Photoelectron Spectroscopy (XPS) valence-band spectra to determine the lower limit of the bandgap. Unlike UV-Vis absorption spectroscopy, which is influenced by thick samples, the probe thickness of XPS measurement is self-limiting, which is 2-5 nm. We present the valence band (VB) edge positions of the electrosynthesized Cs 2 PdX 6 (X = Cl, Br, I) films determined using X-ray Photoelectron Spectroscopy (XPS) valence band spectra in Supplementary Figure S9 . We measured VB edge positions to be at 2.3 eV for Cs 2 PdCl 6 , 1.5 eV for Cs 2 PdBr 6 , and 1.3 eV for Cs 2 PdI 6 below the Fermi level. Surface photovoltage (SPV) measurements ( Supplementary Figure S10 ) indicate that these films are n-type semiconductors and hence the Fermi level is close to the conduction band edge. As a result, the measured energy difference between the VB edge and the Fermi level is close to the bandgap of the materials. This trend indicates that as the halide changes from Cl to Br to I, the band gap narrows. We then performed quantum chemical calculations (see Methods) to have insight into the opto-electronic properties of the VODPs. Hybrid functional (HSE06) calculations with spin-orbit coupling (SOC) reveal that Cs 2 PdI 6 has a bandgap of 0.865 eV, while Cs 2 PdBr 6 and Cs 2 PdCl 6 have band gaps of 1.498 eV and 2.689 eV, respectively. The valence band edges are composed of halide atoms, while the conduction band edges involve both Pd and halide contributions ( Supplementary Figure S11 ). The valence bands are flatter with less dispersion. Overall, the quantum chemical calculations of bandgap values closely align with the experimental observations, indicating that Cs 2 PdCl 6 should have the widest bandgap and Cs 2 PdI 6 the narrowest, consistent with the expected electronic structure trends for halide perovskites. Stability of the VODP films and their application electrochemical hydrogen evolution: We found Cs 2 PdX 6 (X = Cl, Br, I) perovskite films to be stable in water for more than 1000 hours (see Supplementary Figures S12 and S13 ). The films are also stable after incubating the films at 120 °C for more than 300 hours ( Supplementary Figure S12 and Supplementary Figure S13 ), indicating the films are stable as reported for the chemically synthesized powders 43 . From our UV-Vis absorbance measurements, XPS valence band-edge measurements and quantum chemical calculations, we deduced that these materials can effectively absorb photons in the visible range. The VODP films also exhibit high intrinsic electronic conductivity (see Supplementary Figure S14 ). This also supports the fact that we can grow several micrometer-thick perovskites without changing the applied voltage, owing to the good conductivity of the deposited films. The high conductivity, thermal and water stability of the VODP films prompted us to examine their use in photoelectrochemical hydrogen evolution reaction. The EPIC-Fab method also enabled the production of highly crystalline, continuous films on high-surface-area substrates such as carbon paper, making them suitable electrodes for the hydrogen evolution reaction (HER). The linear sweep voltammetry (LSV) curves for Cs 2 PdCl 6 and Cs 2 PdBr 6 on carbon paper and for Cs 2 PdI 6 on FTO ( Supplementary Figure S15 ) show that the perovskite films significantly outperform the bare substrates, confirming their role as HER electrocatalysts. Cs 2 PdCl 6 and Cs 2 PdBr 6 exhibit high HER current densities of ~150 mA/cm 2 at -0.26 V vs RHE. Cs 2 PdI 6 on FTO exhibits a lower current density due to its reduced effective surface area. We verified the evolution of hydrogen by Gas Chromatography and Mass Spectrometric measurements conducted in and media (see Supplementary Schematic S1, Supplementary Note S1 and Supplementary Figure S16 ). Although gas chromatography confirms the electrochemical generation of hydrogen gas, employing D 2 O based mass spectrometry provides definitive evidence that the evolved hydrogen is derived from water. This isotopic labelling approach eliminates ambiguity about the gas produced during electrolysis. We determined the faradic efficiency of the electrochemical process using gas chromatography ( Supplementary Figure S17 ) and found it to be ~ 94.4% (see Supplementary Note S2 ) Figures 3(a-c) show the photo-response of the Cs 2 PdX 6 films over three on and off cycles with a photocurrent density of ~7.1 mAcm - 2 , ~7.0 mAcm - 2 and ~8.3 mAcm - 2 for Cs 2 PdCl 6 , Cs 2 PdBr 6 and Cs 2 PdI 6 respectively, at a light illumination power density of 00 mW.cm -2 and at -0.26 V(w.r.t RHE) bias. With a faradaic efficiency of ~94.4 %, we calculated corresponding light to hydrogen conversion efficiencies of ~8.2%, ~8.1%, and ~9.6% for Cs 2 PdCl 6 , Cs 2 PdBr 6 and Cs 2 PdI 6, respectively (see Supplementary Note 3 ). We performed a light-power dependent efficiency study to understand how the photocurrent density and light to hydrogen conversion efficiency of the Cs 2 PdX 6 films respond to varying illumination intensities ( Supplementary Figure S18 ). We found that the current generation deviates from linear behaviour at higher light power densities, indicating that the number of available catalytic sites on the surface is a limiting factor. By mapping the relationship between light power density, photocurrent density and light to hydrogen conversion efficiency, we can identify optimal illumination conditions to maximize hydrogen production without incurring losses due to surface site limitations. We obtained a maximum efficiency of ~14 % for Cs 2 PdI 6 at an illumination power density of 10 mW.cm -2 ( Supplementary Figure S18 ) and a maximum photocurrent density of 14 mA.cm -2 at a light power density of 400 mW.cm -2 ( Supplementary Figure S18 ). Figures 3(d-f) show uninterrupted hydrogen production by all three types of VODPs for more than 100 hours at 0.26 V(w.r.t RHE) bias and 100mW.cm -2 light intensity. We found that a stable current density of 150 mA cm -2 for Cs 2 PdCl 6 and Cs 2 PdBr 6 films. In the absence of light, the current decays to less than half of the initial current value within 7-8 hours of continuous operation ( Supplementary Figure S19 ). Light-assisted self-healing: We observed that the VODP films exhibit very high stability during photoelectrochemical hydrogen generation. XRD spectra of the perovskite before and after HER under illuminated conditions ( Supplementary Figure S20 ) confirm that the light plays a crucial role in its stability during the reaction. This could be due to continuous material regeneration during light assisted hydrogen evolution reaction. We hypothesised that during prolonged electrochemical hydrogen evolution, a very small portion of the Cs 2 PdX 6 material undergoes reduction under a continuous negative bias. During this process, some of the Pd 4+ ions are converted into Pd 2+ . However, because Pd 2+ complexes such as Cs 2 PdX 4 are water-soluble, they may leach into solution. To test our hypothesis, we analysed Pd ions content in the solution used during the HER for 10 hours in the dark. Though Cs 2 PdCl 6 is water-insoluble, we still detected Pd ions in the Inductively Coupled Plasma Optical Emission Spectroscopic (ICP-OES) analysis (see Supplementary Table S1 ). The cyclic voltammogram under dark conditions shows a Faradaic process at around 0.4 V vs RHE that corresponds to the oxidation of Pd 2+ to Pd 4+ ( Supplementary Figure S21 ) confirming the formation of Pd 2+ during the HER in the dark. Absence of any palladium underpotential deposition peaks characteristic of Pd 0 rules out the possibility of Pd 0 at the operating negative potential. The Faradaic peak in cyclic voltammogram disappears when the HER is done under illumination ( Supplementary Figure S21 ). The cyclic voltammetry suggests that no detectable Pd 2+ formation occurs during HER under light ( Supplementary Figure S21) . The ICP-OES ( Supplementary Table S1 ) of the solvent used for 100 hours of continuous operation did not show any detectable amount of Pd ion in the solution. We explain this stability in the schematic diagram shown in Figure 4(c) and (d) . During photo-generation of carriers, the electrons are used to reduce protons to hydrogen, which happens at the surface of the film. The photo-generated holes produced in film travel through the film and external circuit to the other counter electrode to complete the complementary reaction at that electrode. However, if any of the Pd 4+ is converted to Pd 2+ due to electrons injected in the dark or photo-generated electrons, the holes revert it back to Pd 2+ while still maintaining charge neutrality. The light-assisted self-repair mechanism prevents material degradation and is responsible for its ultra-high stability under operational conditions. These materials pave the way for durable, single-component photoelectrocatalysts for hydrogen production. By leveraging their intrinsic water and thermal stability, catalytic activity and light assisted regeneration ability, future research can focus on scaling up Cs 2 PdX 6 films for industrial water-splitting applications, reducing the need for complex protective layers. Additionally, the versatility of our electrosynthesis approach enables exploration of compositional tuning with the Cs 2 PdX 6 family and other VODPs to further optimize the PEC HER performance. Conclusion The EPIC-Fab method enables scalable fabrication of highly crystalline palladium-based VODP films, specifically Cs 2 PdCl 6 , Cs 2 PdBr 6 and Cs 2 PdI 6 , directly on conducting substrates using water as the only solvent. Highly crystalline films can be prepared at room temperature without requiring any post-processing or annealing, making it a single-step method for material synthesis cum film deposition. The morphology and the thickness of the films can be fine-tuned by varying the electrochemical deposition parameters and duration. This surface-agnostic method overcomes the limitations of typical solution- or vapour-phase deposition methods, enabling conformal film deposition even on tubular substrates for photo-electrochemical energy conversion applications. The wastage of precursor material is lower than that in spin-coating or vapour-phase deposition, which are commonly used to prepare continuous films. The resulting VODP films exhibit intrinsic resistance to thermal and aqueous degradation and sustain photoelectrochemical hydrogen evolution with light-to-hydrogen conversion efficiencies approaching 14%. The light-assisted self-repair mechanism ensures continuous performance without degradation, achieving operational stability exceeding 100 hours. This concept of electrosynthesis and deposition of VODPS opens up new possibilities for durable, lead-free perovskite materials in sustainable optoelectronics materials development. Methods and materials Materials: Palladium (II) chloride (ReagentPlus ® , 99%), cesium chloride (ReagentPlus ® , 99.9%), palladium (II) bromide (99%), cesium bromide (99.9% trace metal basis), %), palladium (II) iodide (99.98 trace metal basis%), cesium iodide (99.9% trace metal basis) and potassium iodide (ACS reagent, ≥ 99.0%) were procured from Sigma-Aldrich. Sulfuric acid (about 98%) was procured from EMPARTA ® , Merck Life Science Private Limited. Deuterium oxide was purchased from Sigma-Aldrich. FTO coated glasses (resistivity < 10 ohms/sq) were purchased from Dyesol. Methods: Thin-film preparation: Biologic SP-300 potentiostat was used for the deposition of the halide perovskites on the FTO substrates. We prepared Cs 2 PdCl 6 films by dissolving 0.1 M PdCl 2 , 0.2 M CsCl, and 0.5 M H 2 SO 4 in DI water, and deposited them on FTO or carbon paper by applying 1.5 V (vs SCE) for 5 minutes; film thickness was controlled by deposition time. For Cs 2 PdBr 6 , we used a solution of 0.1 M PdBr 2 , 0.2 M CsBr, and 0.5 M H 2 SO 4 in DI water, and deposited films on similar substrates at 1.0 V for 5 minutes, with thickness again tuned via deposition time. To prepare Cs 2 PdI 6 , we dissolved 253 mg PdI 2 , 350 mg CsI, and 5 g KI in 5 mL DI water, and deposited the films on FTO by prepolarizing at –0.5 V for 2 minutes, followed by 1.0 V for 2 minutes; the thickness was adjusted by varying the duration of the positive bias. Characterization techniques: Raman spectroscopic investigation of the electrodeposited films was done using a Renishaw inVia TM confocal Raman microscope with a 532 nm laser. X-ray diffraction (XRD) was done using a Rigaku SmartLab ® X-ray diffractometer. Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) were done using JEOL JSM 7200F FE-SEM. Gas chromatography was done using a Thermo Scientific TRACE 1110 Gas Chromatograph. Mass spectrometric measurements were done using Stanford Research Systems (SRS) Residual Gas Analyzer (RGA) 200. Surface photovoltage measurements were done using a KP Technology Single-Point Kelvin Probe system (KP020). The photoelectrochemical measurements were performed using a SOLIS-525C High-Power LED (525nm, green, 2.4 W). ICP-OES was done using a Perkin-Elmer Avio 220 Max ICP-OES instrument. Absorbance measurements: For performing absorption measurements, crystals (typical size ) were extracted by scratching the halide perovskite films grown on FTO and transferred to a thick c-cut sapphire substrate. Optical absorption ( ) spectra were recorded at room temperature ( ) in the wavelength region . A homebuilt setup 47 was used which could perform reflectance ( ) and transmittance ( ) spectroscopy on the same sample spot with a spatial resolution of . A tungsten halogen lamp acted as the light source for these measurements, while a focal length spectrometer coupled with an EMCCD camera was used to record the spectra. Absorption spectra were deduced using the equation . Scattering losses were not accounted for in our measurements. XPS valence band measurement: High resolution XPS valence band measurements were performed using Al K a ( h n = 1486.6 eV ) radiation (energy) and a SCIENTA R4000 electron energy analyzer. The base pressure of the instrument was > 10 -10 mbar. As prepared samples were mounted on Cu sample holder and measured at room temperature with total instrumental resolution of ~ 400 meV. Computational: We have employed Density Functional Theory (DFT) embedded 48,49 in Vienna Ab-initio Simulation Package (VASP 50 ) for computing structural and electronic properties. The exchange correlation function is of the Perdew- Burke- Erzehenhorf (PBE 51 ) type generalized gradient approximation. Projector Augmented Wave (PAW 52 ) pseudopotentials for Cs, Pd, Br, Cl, and I are 6s 1 , 4s 1 4d 9 , 4s 2 4p 5 , 3s 2 3p 5 , and 5s 2 5p 5 , respectively. The Brillouin zone has been sampled using 5 x 5 x 5 Monkhorst pack k-mesh. The cutoff energy is taken to be 500 eV throughout the calculations with plane wave basis set describing the valence electrons. The Hellmann-Feynman forces are taken to be 0.001 eV/ to get the optimised structure with minimum energy configuration. The Bandstructure have been simulated using Hybrid functional of the type HSE06 for more accurate xc energy and spin orbit coupling (SOC) has been taken into account to consider the relativistic effect of heavy elements in the system. Declarations Data Availability Statement: The data that support the findings of this study are available from the corresponding author, [PN], upon reasonable request. Acknowledgement: Funding: P.K.N., TNN., AKR, and PRS acknowledge support from the Department of Atomic Energy (DAE), India, under project RTI 4007. P.K.N. also acknowledges support from the Department of Science and Technology (DST), India, via the Swarna Jayanti Fellowship. AA acknowledges financial support from the following projects funded by the Government of India: NM-ICPS of the DST through the I-HUB Quantum Technology Foundation (Pune, India), Project No. CRG/2022/007008 of SERB, MoE-STARS project No. MoE-STARS/STARS-2/2023-0912, CEFIPRA CSRP Project No. 7104-2, VAIBHAV fellowship number INAE/DST-VF/2024/I/03, DST National Quantum Mission project No. DST/QTC/NQM/QMD/2024/4 (G) and DST-CEFIPRA CSRP Project No. 7104-2. N.B. acknowledge the CSIR India, for financial support through Awards No. 09/1020(0177)/2019EMR-I. The work was funded by the DAE, Govt. of India, through institutional funding to HRI. JK, PKP and SC acknowledge HRI for providing access to the infrastructure and high-performance computing facility. Author contributions: PKN, TNN and AKR conceived the idea. AKR performed all the synthesis, characterization and electrochemical studies under the supervision of PKN and TNN. AKR and PKN wrote the first draft of the manuscript and subsequently revised it based on input from all authors. AG contributed to the development of electrochemical deposition methods. VA contributed to data interpretation and visualisation. XPS measurements were performed by NB under the supervision of RSS. The quantum chemical calculations were performed by JK and PK under the supervision of SC. The microscopic Uv-Vis measurement was performed by AS and RMK under the supervision of AA. Mass spectrometry was performed by SKS and AKR under the supervision of PRS. PKN coordinated among the co-authors and supervised the project. Competing interests: A patent based on this work has been granted by the Indian Patent Office (Application No /202541033941) List of Supplementary Materials: Supplementary Figures. S1-21 Supplementary Note S1-3 Supplementary Table S1 Supplementary Scheme1 References Cheng, W.-H. et al. Monolithic Photoelectrochemical Device for Direct Water Splitting with 19% Efficiency. ACS Energy Lett. 3 , 1795–1800 (2018). May, M. M., Lewerenz, H.-J., Lackner, D., Dimroth, F. & Hannappel, T. Efficient direct solar-to-hydrogen conversion by in situ interface transformation of a tandem structure. Nat. Commun. 6 , 8286 (2015). Varadhan, P., Fu, H.-C., Kao, Y.-C., Horng, R.-H. & He, J.-H. An efficient and stable photoelectrochemical system with 9% solar-to-hydrogen conversion efficiency via InGaP/GaAs double junction. Nat. Commun. 10 , 5282 (2019). Huang, D. et al. 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Cs 2 PdCl 6 Crystal Structure: Datasheet from ‘PAULING FILE Multinaries Edition – 2022’ in SpringerMaterials (https://materials.springer.com/isp/crystallographic/docs/sd_1925231). Preprint at https://materials.springer.com/isp/crystallographic/docs/sd_1925231. Schu, B., Heines, P., Savin, A. & Keller, H.-L. Crystal Structures and Pressure-Induced Redox Reaction of Cs 2 PdI 4 .I 2 to Cs 2 PdI 6 . Inorganic Chemistry, 39 , 732-735 (2000) Otto, K., Hubbard, C. P., Weber, W. H. & Graham, G. W. Raman spectroscopy of palladium oxide on γ-alumina applicable to automotive catalysts. Appl. Catal. B 1 , 317–327 (1992). Chauhan, B. et al. Universal Thickness-Dependent Absorption in Solids at the Nanoscale. http://arxiv.org/abs/2510.21354 (2025). Hohenberg, P. & Kohn, W. Inhomogeneous Electron Gas. Physical Review 136 , B864 (1964). Kohn, W. & Sham, L. J. Self-Consistent Equations Including Exchange and Correlation Effects. Physical Review 140 , A1133 (1965). Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B 50 , 17953 (1994). Perdew, J. P., Burke, K. & Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 77 , 3865 (1996). Kresse, G. & Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 54 , 11169 (1996). Additional Declarations Yes there is potential Competing Interest. A patent based on this work has been granted by the Indian Patent Office (Application No /202541033941) Supplementary Files SIVODP260312final.docx In-situ electrosynthetic fabrication of highly crystalline, aqua-compatible palladium halide perovskite films for photoelectrocatalysis Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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(b-d)\u003cstrong\u003e \u003c/strong\u003eSimulated XRD traces derived from single-crystal data for (b) Cs\u003csub\u003e2\u003c/sub\u003ePdCl\u003csub\u003e6\u0026nbsp; \u003c/sub\u003e(c) Cs\u003csub\u003e2\u003c/sub\u003ePdBr\u003csub\u003e6\u003c/sub\u003e and (d) Cs\u003csub\u003e2\u003c/sub\u003ePdI\u003csub\u003e6\u003c/sub\u003e alongside the experimentally measured XRD traces of the corresponding thin films.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-9106903/v1/b62877338d3c5cfdf5c7ab69.png"},{"id":104871321,"identity":"6128a583-0173-43cf-8ae2-1ef0cf636635","added_by":"auto","created_at":"2026-03-18 08:06:58","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":3533749,"visible":true,"origin":"","legend":"\u003cp\u003eScanning electron microscopy (SEM) images of the VODP films. (a) Cs\u003csub\u003e2\u003c/sub\u003ePdCl\u003csub\u003e6\u003c/sub\u003e and (b) Cs\u003csub\u003e2\u003c/sub\u003ePdBr\u003csub\u003e6\u003c/sub\u003e and (c) Cs\u003csub\u003e2\u003c/sub\u003ePdCl\u003csub\u003e6\u003c/sub\u003e films grown on FTO, showing well-defined crystalline features extending over several micrometers. (d) Carbon paper (e) Cs\u003csub\u003e2\u003c/sub\u003ePdCl\u003csub\u003e6\u003c/sub\u003e films on carbon paper (f) Cs\u003csub\u003e2\u003c/sub\u003ePdBr\u003csub\u003e6\u003c/sub\u003e on carbon paper.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-9106903/v1/46f918e00b5d8830e6275a08.png"},{"id":104871320,"identity":"88b972f7-74e4-4fba-894f-eb9fb2339cc1","added_by":"auto","created_at":"2026-03-18 08:06:58","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1577499,"visible":true,"origin":"","legend":"\u003cp\u003e(a–c) Chronoamperometric plots showing the photocurrent response of Cs\u003csub\u003e2\u003c/sub\u003ePdCl\u003csub\u003e6\u003c/sub\u003e, Cs\u003csub\u003e2\u003c/sub\u003ePdBr\u003csub\u003e6\u003c/sub\u003e, and Cs\u003csub\u003e2\u003c/sub\u003ePdI\u003csub\u003e6\u003c/sub\u003e electrodes under light (532 nm, 100 mW/cm\u003csup\u003e2\u003c/sup\u003e) on/off cycles, demonstrating their reproducible photoresponse. (d–f) Long-term chronoamperometric measurements under continuous illumination, showing stable photoelectrochemical hydrogen generation over 100 hours for each perovskite film. Cs\u003csub\u003e2\u003c/sub\u003ePdCl\u003csub\u003e6\u003c/sub\u003e and Cs\u003csub\u003e2\u003c/sub\u003ePdBr\u003csub\u003e6\u003c/sub\u003e were on carbon paper, while Cs\u003csub\u003e2\u003c/sub\u003ePdI\u003csub\u003e6\u003c/sub\u003e was on FTO-coated glass.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-9106903/v1/987f218c3b229ff78365c819.png"},{"id":104871322,"identity":"d2f64931-f37f-4739-938f-2152b7f9f4cb","added_by":"auto","created_at":"2026-03-18 08:06:58","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":5739037,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic illustration of the electrochemical, photo-electrical hydrogen evolution and light‑assisted self‑healing behavior of Cs\u003csub\u003e2\u003c/sub\u003ePdX\u003csub\u003e6\u003c/sub\u003e (X = Cl, Br, I) perovskite electrodes. (a) HER under dark conditions, (b) Partial material degradation in the dark due to Pd\u003csup\u003e4+\u003c/sup\u003e → Pd\u003csup\u003e2+\u003c/sup\u003e reduction and subsequent leaching of water‑soluble Cs\u003csub\u003e2\u003c/sub\u003ePdX\u003csub\u003e4\u003c/sub\u003e species. (c) Light‑assisted regeneration of the perovskite lattice, where photogenerated holes re‑oxidize Pd\u003csup\u003e2+\u003c/sup\u003e to Pd\u003csup\u003e4+\u003c/sup\u003e, preventing dissolution. (d) Stable photoelectrochemical HER under illumination, enabled by continuous self‑repair and preserved structural integrity.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-9106903/v1/21a159b09ac3920485406b7c.png"},{"id":106092876,"identity":"7684b2cd-b831-4887-b540-15dc9d001c1e","added_by":"auto","created_at":"2026-04-03 11:28:57","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":15147448,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9106903/v1/0340e815-1723-46b7-aca4-77650f5ce88e.pdf"},{"id":104871324,"identity":"48114ef0-c6c0-4d8e-b201-a21a7175f50b","added_by":"auto","created_at":"2026-03-18 08:06:58","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":61196062,"visible":true,"origin":"","legend":"In-situ electrosynthetic fabrication of highly crystalline, aqua-compatible palladium halide perovskite films for photoelectrocatalysis","description":"","filename":"SIVODP260312final.docx","url":"https://assets-eu.researchsquare.com/files/rs-9106903/v1/554051b4b1aa830eba48afb9.docx"}],"financialInterests":"\u003cb\u003eYes\u003c/b\u003e there is potential Competing Interest.\nA patent based on this work has been granted by the Indian Patent Office (Application No /202541033941)","formattedTitle":"In-situ electrosynthetic fabrication of highly crystalline, aqua-compatible palladium halide perovskite films for photoelectrocatalysis","fulltext":[{"header":"Introduction","content":"\u003cp\u003eSemiconductors play a crucial role in energy conversion technologies, enabling the transformation of sunlight into electricity and chemical fuel. Their capacity to absorb light and produce charge carriers underpins their applications in photovoltaics and photocatalysis. For efficient photoelectrochemical hydrogen generation, an emerging technology with huge commercial interest, semiconductor materials must exhibit optimal light absorption and long-term stability under aqueous conditions and illumination. Tandem architecture-based electrodes, such as GaInP/GaInAs with Rh catalyst and GaInP/GaInAs/GaAs protected by crystalline TiO\u003csub\u003e2\u003c/sub\u003e and Rh nanoparticles, have achieved solar-to-hydrogen (STH) efficiencies of \u0026gt; 10 % in acidic conditions. However, they exhibit poor operational stability with lifetimes of 40 hours and 3 hours, respectively\u003csup\u003e\u0026nbsp;1,2\u003c/sup\u003e. The monolithic photoelectrode, InGaP/GaAs/Ni, shows stability of 150 hours under alkaline conditions\u003csup\u003e3\u003c/sup\u003e. Emerging chalcogenide systems such as Cu\u003csub\u003e3\u003c/sub\u003eBiS\u003csub\u003e3\u003c/sub\u003e films and their tandem integration with BiVO\u003csub\u003e4\u003c/sub\u003e have not crossed the operational stability of 60 hours\u003csup\u003e4\u003c/sup\u003e. Nanowire-structured p-Si photocathodes, molecularly modified and protected by TiO\u003csub\u003e2\u003c/sub\u003e, particulate CTGS (Cu\u003csub\u003e2\u003c/sub\u003eSn\u003csub\u003ex\u003c/sub\u003eGe\u003csub\u003e1-x\u003c/sub\u003eS\u003csub\u003e3\u003c/sub\u003e) photocathodes and triple-junction amorphous silicon solar cells integrated with CoWO and CoWS catalysts have demonstrated promising activity but with poor stabilities of \u0026lt; 10 hours\u003csup\u003e5\u0026ndash;7\u003c/sup\u003e. Molecular photoelectrodes leveraging \u0026pi;\u0026ndash;\u0026pi; interactions between PEDOT and polycarbazole networks, combined with NiS\u003csub\u003e2\u003c/sub\u003e catalysts, have shown enhanced charge transport, high external quantum efficiency (EQE), and stable operation exceeding 120 hours\u003csup\u003e8\u003c/sup\u003e. III\u0026ndash;V semiconductor-based devices, including p-doped GaInP\u003csub\u003e2\u003c/sub\u003e nanopillar photocathodes and GaN nanowire-modified Si, have set benchmarks for STH efficiency (over 20%) and long-term stability, with the latter operating for 3000 hours\u003csup\u003e9,10\u003c/sup\u003e. Advanced protective strategies, such as graded MoS\u003csub\u003ex\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e11\u003c/sup\u003e layers and cryogelated polyacrylamide overlayers\u003csup\u003e12,13\u003c/sup\u003e, have further improved the durability of \u0026nbsp;Sb\u003csub\u003e2\u003c/sub\u003eSe\u003csub\u003e3\u003c/sub\u003e and GaInP\u003csub\u003e2\u003c/sub\u003e photocathodes to more than 100 hours. \u0026nbsp;Photoelectrochemical devices with operational stabilities greater than 100 hours and good efficiencies require architectures of multiple layers, often consisting of a photo-absorber layer, a protection layer and a catalyst layer. Hence, large-scale photoelectrocatalytic hydrogen production is still in its infancy because of the lack of a single material with high STH efficiency\u003csup\u003e\u0026nbsp;9,10\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eAmong the emerging classes of semiconductors, metal halide perovskites (HaPs) have attracted significant attention owing to their exceptional optoelectronic properties, including strong visible-light absorption\u003csup\u003e14,15\u003c/sup\u003e, high charge-carrier mobility\u003csup\u003e16,17\u003c/sup\u003e, and long diffusion lengths\u003csup\u003e18\u0026ndash;21\u003c/sup\u003e. These materials have revolutionized the field of solar photovoltaics, achieving power conversion efficiencies exceeding 27%\u003csup\u003e22\u0026ndash;24\u003c/sup\u003e. However, achieving durability in operational conditions remains a challenge for halide perovskite materials. The chemical instability of halide perovskites in humid, thermal, and illuminated environments limits their practical implementation in direct light-to-chemical energy conversion, particularly in photoelectrochemical hydrogen generation, where direct contact with electrolyte solutions accelerates the degradation of HaPs. \u003csup\u003e25\u0026ndash;27\u003c/sup\u003e. \u0026nbsp; However, despite their good light-absorbing properties, ABX\u003csub\u003e3\u003c/sub\u003e type HaPs need protection and catalyst layers to be useful in photoelectrochemical reactions in aqueous media\u003csup\u003e28\u0026ndash;33\u003c/sup\u003e . Even with protective layers, most HaP-based photocatalytic systems have poor stability under operational conditions \u003csup\u003e27\u0026ndash;32\u003c/sup\u003e. In this context, vacancy-ordered double perovskites (VODPs), with the general formula A\u003csub\u003e2\u003c/sub\u003eBX\u003csub\u003e6\u003c/sub\u003e, are a promising class of materials for photoelectrochemical reactions in aqueous conditions\u003csup\u003e34\u003c/sup\u003e. \u0026nbsp; Here, the ABX\u003csub\u003e3\u003c/sub\u003e HaP structure is modified by systematically removing the alternate B-site cation to create isolated [BX\u003csub\u003e6\u003c/sub\u003e] octahedra in an antifluorite-type lattice\u003csup\u003e35\u003c/sup\u003e. This structure imparts enhanced chemical stability compared to that of ABX\u003csub\u003e3\u003c/sub\u003e variants. VODPs can retain some of the key features of ABX\u003csub\u003e3\u003c/sub\u003e perovskites, such as highly crystalline thin films, while offering a platform for lead-free compositions and improved stability in aqueous media. Moreover, they can host a far wider range of B-site cations than ABX\u003csub\u003e3\u003c/sub\u003e perovskites, and this compositional freedom enables high-entropy compositions incorporating five or six different metal centres in a single stable phase\u003csup\u003e36\u003c/sup\u003e. Despite the structural isolation of [BX\u003csub\u003e6\u003c/sub\u003e] octahedra, electronic coupling via the halide sublattice enables dispersive bands and supports good charge transport, as evidenced by low electron effective masses in compounds such as Cs\u003csub\u003e2\u003c/sub\u003eSnI\u003csub\u003e6\u003c/sub\u003e and Cs\u003csub\u003e2\u003c/sub\u003eTeI\u003csub\u003e6\u003c/sub\u003e \u003csup\u003e35,37,38\u003c/sup\u003e. Thin films of HaP are generally prepared through solution-based or vacuum thermal deposition methods. \u0026nbsp;Though solution-based methods such as spin coating\u003csup\u003e39,40\u003c/sup\u003e, drop coating\u003csup\u003e41\u003c/sup\u003e and spray coating\u003csup\u003e42\u003c/sup\u003e are easy and cost-effective, but the low solubility of certain precursors in conventional solvents such as DMSO and DMF poses an obstacle in the formation of thick and continuous films. Moreover, solution-based deposition methods are not suitable for conformal coating on protruding substrates. Even in vapour-phase deposition, shadowing remains a challenge on non-planar substrates. There is a need to develop a new, scalable, environmentally friendly method for the synthesis and surface-agnostic deposition of water-stable halide perovskite films with controlled morphology for photoelectrochemical applications.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eHere, we report a generic method for in-situ electrosynthesis and deposition of palladium-based VODP films: Cs\u003csub\u003e2\u003c/sub\u003ePdCl\u003csub\u003e6\u003c/sub\u003e, Cs\u003csub\u003e2\u003c/sub\u003ePdBr\u003csub\u003e6\u003c/sub\u003e, and Cs\u003csub\u003e2\u003c/sub\u003ePdI\u003csub\u003e6\u003c/sub\u003e, enabling conformal coating onto a wide range of conductive substrates, including fluorinated tin oxide and indium tin oxide coated glass, glassy carbon electrode, carbon paper, carbon cloth and various metals. In contrast to the generally employed environmentally hazardous solvents, this\u0026nbsp;\u003cem\u003eelectrosynthetic perovskite\u003c/em\u003e \u003cem\u003ein-situ crystallization and film fabrication\u003c/em\u003e (EPIC-Fab) method uses water as the sole solvent during deposition. The deposited films show high crystallinity, and the material can be conformally deposited onto tubular substrates. The deposited films show high thermal and water stability. We demonstrate photo-electrocatalytic applications of these materials without any external load of catalysis or protection layer. These materials demonstrate \u0026gt;100 hours of operational stability without any noticeable drop in efficiency under continuous operation. We unravel the mechanism underlying the films\u0026apos; exceptional stability, driven by a light-mediated self-repairing process. The surface-agonistic deposition and scalability pave the way for the further development of stable, self-repairable VODP materials with different compositions for energy conversion and optoelectronic applications.\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cp\u003ePerovskite films preparation by electrosynthesis:\u003c/p\u003e\n\u003cp\u003eWe show the schematic for VODP film fabrication in \u003cstrong\u003eFigure 1a\u003c/strong\u003e. We used electrosynthesis and deposition in H\u003csub\u003e2\u003c/sub\u003eO as the solvent system (pH~ 0 (0.5M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e)), having CsX and PdX\u003csub\u003e2\u003c/sub\u003e salts, employing a three-electrode setup consisting of a conductive substrate as working electrode, saturated calomel electrode (SCE) as reference, and graphite rod as counter electrode. (See the methods section). \u0026nbsp;In aqueous salt solutions, Pd exists in the 2\u003csup\u003e+\u003c/sup\u003e oxidation state, forming compounds like Cs\u003csub\u003e2\u003c/sub\u003ePdX\u003csub\u003e4\u003c/sub\u003e that readily dissociate in water\u003csup\u003e\u0026nbsp;43\u003c/sup\u003e. However, when Pd is in its 4\u003csup\u003e+\u003c/sup\u003e oxidation state, it forms compounds such as Cs\u003csub\u003e2\u003c/sub\u003ePdBr\u003csub\u003e6\u003c/sub\u003e, which remain insoluble and stable in water\u003csup\u003e\u0026nbsp;43\u003c/sup\u003e. This prompted us to utilize this difference in stability of Pd compounds to make thin films of the VODPs by changing the oxidation state of Pd in-situ. Pd\u003csup\u003e2+\u003c/sup\u003e ions in the solution diffuse from the bulk to the surface of the working electrode, adsorb onto it, and undergo a two-electron oxidation to form Pd\u003csup\u003e4+\u003c/sup\u003e ions (as schematized in \u003cstrong\u003eFigure 1a\u003c/strong\u003e). The aqueous medium also\u0026nbsp;contained an optimized concentration of Cs\u003csup\u003e+\u0026nbsp;\u003c/sup\u003eand halide ions (Cl\u003csup\u003e-\u003c/sup\u003e, Br\u003csup\u003e-\u003c/sup\u003e, I\u003csup\u003e-\u003c/sup\u003e). The formation of Pd\u003csup\u003e4+\u003c/sup\u003e, along with the availability of Cs\u003csup\u003e+\u0026nbsp;\u003c/sup\u003eand halide ions, leads to the nucleation of vacancy-ordered double perovskite (VODP) on the anode surface. Under constant bias, these nuclei grow and coalesce over time, resulting in continuous film formation (\u003cstrong\u003eFigure 1a\u003c/strong\u003e).\u003c/p\u003e\n\u003cp\u003ePrior to the deposition of VODP thin films, we determined the potentials required for Pd\u003csup\u003e2+\u003c/sup\u003e oxidation and subsequent perovskite deposition using cyclic voltammetry (\u003cstrong\u003eSupplementary Figure S1).\u003c/strong\u003e We found that potentials above 1.0 V and 1.2 V (vs saturated calomel electrode, SCE) are required for the deposition of Cs\u003csub\u003e2\u003c/sub\u003ePdBr\u003csub\u003e6\u003c/sub\u003e and Cs\u003csub\u003e2\u003c/sub\u003ePdCl\u003csub\u003e6\u003c/sub\u003e, respectively. We did not notice any films forming below these thresholds. Although deposition of Cs\u003csub\u003e2\u003c/sub\u003ePdI\u003csub\u003e6\u003c/sub\u003e can occur above 1.0 V (vs SCE), the film quality is poor due to the low aqueous solubility of PdI\u003csub\u003e2\u003c/sub\u003e. To overcome that issue, we added an optimised amount of KI, where excess iodide ions form soluble Pd-iodide complexes. We found that pre-polarization of the electrode to negative potentials before the application of the positive deposition potential improves the film uniformity for Cs\u003csub\u003e2\u003c/sub\u003ePdI\u003csub\u003e6\u003c/sub\u003e deposition.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCharacterization of the deposited films:\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eIn \u003cstrong\u003eFigure 1(b-d),\u003c/strong\u003e we show the simulated XRD traces derived from single-crystal data for Cs\u003csub\u003e2\u003c/sub\u003ePdCl\u003csub\u003e6\u003c/sub\u003e (ref.\u003csup\u003e44\u003c/sup\u003e) Cs\u003csub\u003e2\u003c/sub\u003ePdBr\u003csub\u003e6\u0026nbsp;\u003c/sub\u003e(ref.\u003csup\u003e43\u003c/sup\u003e)\u0026nbsp;and Cs\u003csub\u003e2\u003c/sub\u003ePdI\u003csub\u003e6\u0026nbsp;\u003c/sub\u003e(ref. \u003csup\u003e45\u003c/sup\u003e), alongside the experimentally observed XRD traces of the corresponding thin films. The agreement between experimental and simulated traces confirms the phase purity and crystallinity of the films prepared by the EPIC-Fab method. We observed the expected systematic changes in the XRD peak position as the halide ion changes from chloride to iodide. The systematic change in the XRD peak positions is due to variations in lattice parameters arising from differences in the ionic radii of the halides. \u003cstrong\u003eFigures 2 (a-c)\u003c/strong\u003e show scanning electron microscopy (SEM) images of the deposited films. Cs\u003csub\u003e2\u003c/sub\u003ePdCl\u003csub\u003e6\u003c/sub\u003e and Cs\u003csub\u003e2\u003c/sub\u003ePdBr\u003csub\u003e6\u003c/sub\u003e films exhibit well-defined crystalline features extending over several tens of micrometres, and Cs\u003csub\u003e2\u003c/sub\u003ePdI\u003csub\u003e6\u003c/sub\u003e film shows micrometre-scale crystalline domains. We note that the films were deposited at room temperature, and no post-deposition annealing or processing was required to obtain highly crystalline, continuous films. Cross-sectional SEM images (\u003cstrong\u003eSupplementary Figure S2\u003c/strong\u003e) show that the deposition method produces a several-micrometre-thick and compact film with controlled thickness. The optical image of the VODP film deposited on a large-area (15 cm\u003csup\u003e2\u003c/sup\u003e) FTO-coated glass (\u003cstrong\u003eSupplementary Figure S3\u003c/strong\u003e) shows the scalability potential of the EPIC-Fab method. The area of film deposition can be further increased by using a larger solvent bath. We present the energy-dispersive spectroscopy (EDS) data of these films in \u003cstrong\u003eSupplementary Figure S4\u003c/strong\u003e. The EDS analysis showed elemental ratios of 2:1:6 (Cs:Pd:X, where X = Cl, Br, or I) for the films deposited from the respective halide precursors, thereby confirming the stoichiometry of the in-situ synthesized compounds. We show the Raman spectra of these films in the \u003cstrong\u003eSupplementary Figure S5\u003c/strong\u003e. The Raman spectra of the three types of VODP films indicate a gradual shift of peaks to the lower wavenumbers from Cs\u003csub\u003e2\u003c/sub\u003ePdCl\u003csub\u003e6\u003c/sub\u003e to Cs\u003csub\u003e2\u003c/sub\u003ePdI\u003csub\u003e6\u003c/sub\u003e, which are\u0026nbsp;consistent with the increasing\u0026nbsp;Pd-X (X=Cl, Br, I)\u0026nbsp;bond lengths as the halogen changes from Cl to I. We also checked for any PdO formation during deposition using Raman spectroscopy. Any PdO deposition would give a characteristic B\u003csub\u003e1g\u003c/sub\u003e peak at 647 cm\u003csup\u003e-1\u0026nbsp;\u003c/sup\u003e(ref.\u003csup\u003e46\u003c/sup\u003e). We did not observe any peak around 647 cm\u003csup\u003e-1\u003c/sup\u003e in the deposited films \u003cstrong\u003e(Supplementary Figure S5)\u003c/strong\u003e, indicating the absence of PdO formation during the film deposition. The absence of XRD peaks of PdO in the XRD traces also corroborates the absence of PdO. We then extended this deposition method to deposit VODPs on carbon paper, which has networks of tubular carbon rods. The porous, mesh-like open structure provides a high surface area for material deposition, facilitating efficient mass transport in electrochemical applications. We provide the detailed deposition method in the method sections. By adjusting the deposition parameters, we synthesized and deposited Cs\u003csub\u003e2\u003c/sub\u003ePdCl\u003csub\u003e6\u003c/sub\u003e and Cs\u003csub\u003e2\u003c/sub\u003ePdBr\u003csub\u003e6\u003c/sub\u003e conformally on the carbon papers. We show the SEM images of the pristine carbon paper and Cs\u003csub\u003e2\u003c/sub\u003ePdCl\u003csub\u003e6\u003c/sub\u003e and Cs\u003csub\u003e2\u003c/sub\u003ePdBr\u003csub\u003e6\u003c/sub\u003e deposited on carbon paper in \u003cstrong\u003eFigures 2(d-f)\u003c/strong\u003e. We observed compact VODP film deposition on carbon fibers. The powder XRD of films deposited on carbon paper confirmed their phase purity, indicating successful synthesis and conformal coating on this substrate (\u003cstrong\u003eSupplementary Figure S6\u003c/strong\u003e). We note here that making compact and thick Pd-based VODP films has been challenging due to the poor solubility of these compounds in solvents such as DMSO, DMF, and NMP which are commonly used to make Pb halide perovskite films. For example, patchy films are obtained by spin coating methods (\u003cstrong\u003eSupplementary Figure S7)\u003c/strong\u003e when we used chemically synthesized Cs\u003csub\u003e2\u003c/sub\u003ePdBr\u003csub\u003e6\u003c/sub\u003e\u003csup\u003e43\u003c/sup\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eWe then measured the UV-Visible absorption of the crystallite flakes obtained from the deposited films, which we present in the \u003cstrong\u003eSupplementary Figure S8\u003c/strong\u003e. We found that all the materials have absorbance in the visible range. However, it was not possible to determine a clear band edge from the absorbance measurement. This could be due to the fact that in several micrometre-thick films, the substantial sub-bandgap absorption obscures the clear bandgap transition. We used X-ray Photoelectron Spectroscopy (XPS) valence-band spectra to determine the lower limit of the bandgap. Unlike UV-Vis absorption spectroscopy, which is influenced by thick samples, the probe thickness of XPS measurement is self-limiting, which is 2-5 nm. \u0026nbsp;We present the valence band (VB) edge positions of the electrosynthesized Cs\u003csub\u003e2\u003c/sub\u003ePdX\u003csub\u003e6\u003c/sub\u003e (X = Cl, Br, I) films determined using X-ray Photoelectron Spectroscopy (XPS) valence band spectra in \u003cstrong\u003eSupplementary Figure S9\u003c/strong\u003e. We measured VB edge positions to be at 2.3 eV for Cs\u003csub\u003e2\u003c/sub\u003ePdCl\u003csub\u003e6\u003c/sub\u003e, 1.5 eV for Cs\u003csub\u003e2\u003c/sub\u003ePdBr\u003csub\u003e6\u003c/sub\u003e, and 1.3 eV for Cs\u003csub\u003e2\u003c/sub\u003ePdI\u003csub\u003e6\u003c/sub\u003e below the Fermi level. Surface photovoltage (SPV) measurements (\u003cstrong\u003eSupplementary Figure S10\u003c/strong\u003e) indicate that these films are n-type semiconductors and hence the Fermi level is close to the conduction band edge. As a result, the measured energy difference between the VB edge and the Fermi level is close to the bandgap of the materials. This trend indicates that as the halide changes from Cl to Br to I, the band gap narrows.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;We then performed quantum chemical calculations (see Methods) to have insight into the opto-electronic properties of the VODPs. Hybrid functional (HSE06) calculations with spin-orbit coupling (SOC) reveal that Cs\u003csub\u003e2\u003c/sub\u003ePdI\u003csub\u003e6\u003c/sub\u003e has a bandgap of 0.865 eV, while Cs\u003csub\u003e2\u003c/sub\u003ePdBr\u003csub\u003e6\u003c/sub\u003e and Cs\u003csub\u003e2\u003c/sub\u003ePdCl\u003csub\u003e6\u0026nbsp;\u003c/sub\u003ehave band gaps of 1.498 eV and 2.689 eV,\u0026nbsp;respectively. The valence band edges are composed of halide atoms, while the conduction band edges involve both Pd and halide contributions (\u003cstrong\u003eSupplementary Figure S11\u003c/strong\u003e). The valence bands are flatter with less dispersion. Overall, the quantum chemical calculations of bandgap values closely align with the experimental observations, indicating that Cs\u003csub\u003e2\u003c/sub\u003ePdCl\u003csub\u003e6\u003c/sub\u003e should have the widest bandgap and Cs\u003csub\u003e2\u003c/sub\u003ePdI\u003csub\u003e6\u003c/sub\u003e the narrowest, consistent with the expected electronic structure trends for halide perovskites.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;Stability of the VODP films and their application electrochemical hydrogen evolution:\u003c/p\u003e\n\u003cp\u003eWe found Cs\u003csub\u003e2\u003c/sub\u003ePdX\u003csub\u003e6\u003c/sub\u003e (X = Cl, Br, I) perovskite films to be stable in water for more than 1000 hours (see \u003cstrong\u003eSupplementary Figures S12\u003c/strong\u003e and \u003cstrong\u003eS13\u003c/strong\u003e). The films are also stable after incubating the films at 120 \u0026deg;C for more than 300 hours (\u003cstrong\u003eSupplementary Figure S12\u003c/strong\u003e and \u003cstrong\u003eSupplementary Figure S13\u003c/strong\u003e), indicating the films are stable as reported for the chemically synthesized powders\u003csup\u003e43\u003c/sup\u003e. From our UV-Vis absorbance measurements, XPS valence band-edge measurements and quantum chemical calculations, we deduced that these materials can effectively absorb photons in the visible range. The VODP films also exhibit high intrinsic electronic conductivity (see \u003cstrong\u003eSupplementary Figure S14\u003c/strong\u003e). This also supports the fact that we can grow several micrometer-thick perovskites without changing the applied voltage, owing to the good conductivity of the deposited films. The high conductivity, thermal and water stability of the VODP films prompted us to examine their use in photoelectrochemical hydrogen evolution reaction.\u003c/p\u003e\n\u003cp\u003eThe EPIC-Fab method also enabled the production of highly crystalline, continuous films on high-surface-area substrates such as carbon paper, making them suitable electrodes for the hydrogen evolution reaction (HER). The linear sweep voltammetry (LSV) curves for Cs\u003csub\u003e2\u003c/sub\u003ePdCl\u003csub\u003e6\u003c/sub\u003e and Cs\u003csub\u003e2\u003c/sub\u003ePdBr\u003csub\u003e6\u003c/sub\u003e on carbon paper and for Cs\u003csub\u003e2\u003c/sub\u003ePdI\u003csub\u003e6\u003c/sub\u003e on FTO (\u003cstrong\u003eSupplementary Figure S15\u003c/strong\u003e) show that the perovskite films significantly outperform the bare substrates, confirming their role as HER electrocatalysts. Cs\u003csub\u003e2\u003c/sub\u003ePdCl\u003csub\u003e6\u003c/sub\u003e and Cs\u003csub\u003e2\u003c/sub\u003ePdBr\u003csub\u003e6\u003c/sub\u003e exhibit high HER current densities of ~150 mA/cm\u003csup\u003e2\u003c/sup\u003e at -0.26 V vs RHE. \u0026nbsp;Cs\u003csub\u003e2\u003c/sub\u003ePdI\u003csub\u003e6\u003c/sub\u003e on FTO exhibits a lower current density due to its reduced effective surface area. We verified the evolution of hydrogen by Gas Chromatography and Mass Spectrometric measurements conducted in \u0026nbsp;and \u0026nbsp;media (see \u003cstrong\u003eSupplementary Schematic S1,\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eSupplementary Note S1\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eand Supplementary Figure S16\u003c/strong\u003e). \u0026nbsp;Although gas chromatography confirms the electrochemical generation of hydrogen gas, employing D\u003csub\u003e2\u003c/sub\u003eO based mass spectrometry provides definitive evidence that the evolved hydrogen is derived from water. This isotopic labelling approach eliminates ambiguity about the gas produced during electrolysis.\u0026nbsp;We determined the faradic efficiency of the electrochemical process using gas chromatography (\u003cstrong\u003eSupplementary Figure S17\u003c/strong\u003e) and found it\u0026nbsp;to be ~ 94.4% (see\u003cstrong\u003e\u0026nbsp;Supplementary Note S2\u003c/strong\u003e)\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eFigures 3(a-c)\u003c/strong\u003e show the photo-response of the Cs\u003csub\u003e2\u003c/sub\u003ePdX\u003csub\u003e6\u003c/sub\u003e films over three on and off cycles with a photocurrent density of ~7.1 mAcm\u003csup\u003e-\u003c/sup\u003e\u003csup\u003e2\u003c/sup\u003e, ~7.0 mAcm\u003csup\u003e-\u003c/sup\u003e\u003csup\u003e2\u003c/sup\u003e and ~8.3 mAcm\u003csup\u003e-\u003c/sup\u003e\u003csup\u003e2\u003c/sup\u003e for Cs\u003csub\u003e2\u003c/sub\u003ePdCl\u003csub\u003e6\u003c/sub\u003e, Cs\u003csub\u003e2\u003c/sub\u003ePdBr\u003csub\u003e6\u003c/sub\u003e and Cs\u003csub\u003e2\u003c/sub\u003ePdI\u003csub\u003e6\u003c/sub\u003e respectively, at a light illumination power density of \u0026nbsp;00 mW.cm\u003csup\u003e-2\u003c/sup\u003e and at -0.26 V(w.r.t RHE) bias. With a faradaic efficiency of ~94.4 %, we calculated corresponding light to hydrogen conversion efficiencies of ~8.2%, ~8.1%, and ~9.6% for Cs\u003csub\u003e2\u003c/sub\u003ePdCl\u003csub\u003e6\u003c/sub\u003e, Cs\u003csub\u003e2\u003c/sub\u003ePdBr\u003csub\u003e6\u003c/sub\u003e and Cs\u003csub\u003e2\u003c/sub\u003ePdI\u003csub\u003e6,\u003c/sub\u003e respectively (see\u003cstrong\u003e\u0026nbsp;Supplementary Note 3\u003c/strong\u003e). We performed a light-power dependent efficiency study to understand how the photocurrent density and light to hydrogen conversion efficiency of the Cs\u003csub\u003e2\u003c/sub\u003ePdX\u003csub\u003e6\u003c/sub\u003e films respond to varying illumination intensities (\u003cstrong\u003eSupplementary Figure S18\u003c/strong\u003e). We found that the current generation deviates from linear behaviour at higher light power densities, indicating that the number of available catalytic sites on the surface is a limiting factor. By mapping the relationship between light power density, photocurrent density and light to hydrogen conversion efficiency, we can identify optimal illumination conditions to maximize hydrogen production without incurring losses due to surface site limitations. We obtained a maximum efficiency of ~14 % for Cs\u003csub\u003e2\u003c/sub\u003ePdI\u003csub\u003e6\u003c/sub\u003e at an illumination power density of 10 mW.cm\u003csup\u003e-2\u003c/sup\u003e (\u003cstrong\u003eSupplementary Figure S18\u003c/strong\u003e) and a maximum photocurrent density of 14 mA.cm\u003csup\u003e-2\u003c/sup\u003e at a light power density of\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e400 mW.cm\u003csup\u003e-2\u003c/sup\u003e (\u003cstrong\u003eSupplementary Figure S18\u003c/strong\u003e).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFigures 3(d-f)\u003c/strong\u003e show uninterrupted hydrogen production by all three types of VODPs for more than 100 hours at 0.26 V(w.r.t RHE) bias and 100mW.cm\u003csup\u003e-2\u003c/sup\u003e light intensity. \u0026nbsp;We found that a stable current density of 150 mA cm\u003csup\u003e-2\u003c/sup\u003e for Cs\u003csub\u003e2\u003c/sub\u003ePdCl\u003csub\u003e6\u003c/sub\u003e and Cs\u003csub\u003e2\u003c/sub\u003ePdBr\u003csub\u003e6\u003c/sub\u003e\u0026nbsp; films. \u0026nbsp; In the absence of light, the current decays to less than half of the initial current value within \u0026nbsp;7-8 hours of continuous operation (\u003cstrong\u003eSupplementary Figure S19\u003c/strong\u003e).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;Light-assisted self-healing:\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eWe observed that the VODP films exhibit very high stability during photoelectrochemical hydrogen generation. XRD spectra of the perovskite before and after HER under illuminated conditions (\u003cstrong\u003eSupplementary Figure S20\u003c/strong\u003e) confirm that the light plays a crucial role in its stability during the reaction. This could be due to continuous material regeneration during light assisted hydrogen evolution reaction. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eWe hypothesised that during prolonged electrochemical hydrogen evolution, a very small portion of the Cs\u003csub\u003e2\u003c/sub\u003ePdX\u003csub\u003e6\u003c/sub\u003e material undergoes reduction under a continuous negative bias. During this process, some of the Pd\u003csup\u003e4+\u003c/sup\u003e ions are converted into Pd\u003csup\u003e2+\u003c/sup\u003e. However, because Pd\u003csup\u003e2+\u003c/sup\u003ecomplexes such as Cs\u003csub\u003e2\u003c/sub\u003ePdX\u003csub\u003e4\u003c/sub\u003e are water-soluble, they may leach into solution. To test our hypothesis, we analysed Pd ions content in the solution used during the HER for 10 hours in the dark. Though Cs\u003csub\u003e2\u003c/sub\u003ePdCl\u003csub\u003e6\u003c/sub\u003e is water-insoluble, we still detected Pd ions in the Inductively Coupled Plasma Optical Emission Spectroscopic (ICP-OES) analysis (see \u003cstrong\u003eSupplementary Table S1\u003c/strong\u003e). \u0026nbsp;The cyclic voltammogram under dark conditions shows a Faradaic process at around 0.4 V vs RHE that corresponds to the oxidation of Pd\u003csup\u003e2+\u003c/sup\u003e to Pd\u003csup\u003e4+\u003c/sup\u003e (\u003cstrong\u003eSupplementary Figure S21\u003c/strong\u003e\u003cu\u003e)\u0026nbsp;\u003c/u\u003e confirming the formation of Pd\u003csup\u003e2+\u003c/sup\u003e\u003csub\u003e\u0026nbsp;\u003c/sub\u003eduring the \u0026nbsp;HER in the dark. Absence of any palladium underpotential deposition peaks characteristic of Pd\u003csup\u003e0\u003c/sup\u003e rules out the possibility of Pd\u003csup\u003e0\u0026nbsp;\u003c/sup\u003eat the operating negative potential. The Faradaic peak in cyclic voltammogram disappears when the HER is done under illumination (\u003cstrong\u003eSupplementary\u003c/strong\u003e \u003cstrong\u003eFigure S21\u003c/strong\u003e). The cyclic voltammetry suggests that no detectable Pd\u003csup\u003e2+\u003c/sup\u003e formation occurs during HER under light (\u003cstrong\u003eSupplementary\u003c/strong\u003e \u003cstrong\u003eFigure S21)\u003c/strong\u003e. The ICP-OES (\u003cstrong\u003eSupplementary Table S1\u003c/strong\u003e) of the solvent used for 100 hours of continuous operation did not show any detectable amount of Pd ion in the solution. We explain this stability in the schematic diagram shown in \u003cstrong\u003eFigure 4(c) and (d)\u003c/strong\u003e. During photo-generation of carriers, the electrons are used to reduce protons to hydrogen, which happens at the surface of the film. The photo-generated holes produced in film travel through the film and external circuit to the other counter electrode to complete the complementary reaction at that electrode. However, if any of the Pd\u003csup\u003e4+\u003c/sup\u003e is converted to Pd\u003csup\u003e2+\u0026nbsp;\u003c/sup\u003edue to electrons injected in the dark or photo-generated electrons, the holes revert it back to Pd\u003csup\u003e2+\u003c/sup\u003e while still maintaining charge neutrality. The light-assisted self-repair mechanism prevents material degradation and is responsible for its ultra-high stability under operational conditions. These materials pave the way for durable, single-component photoelectrocatalysts for hydrogen production. By leveraging their intrinsic water and thermal stability, catalytic activity and light assisted regeneration ability, future research can focus on scaling up Cs\u003csub\u003e2\u003c/sub\u003ePdX\u003csub\u003e6\u003c/sub\u003e films for industrial water-splitting applications, reducing the need for complex protective layers. Additionally, the versatility of our electrosynthesis approach enables exploration of compositional tuning with the Cs\u003csub\u003e2\u003c/sub\u003ePdX\u003csub\u003e6\u003c/sub\u003e family and other VODPs to further optimize the PEC HER performance.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThe EPIC-Fab method enables scalable fabrication of highly crystalline palladium-based VODP films, specifically Cs\u003csub\u003e2\u003c/sub\u003ePdCl\u003csub\u003e6\u003c/sub\u003e, Cs\u003csub\u003e2\u003c/sub\u003ePdBr\u003csub\u003e6\u003c/sub\u003e and Cs\u003csub\u003e2\u003c/sub\u003ePdI\u003csub\u003e6\u003c/sub\u003e, directly on conducting substrates using water as the only solvent. Highly crystalline films can be prepared at room temperature without requiring any post-processing or annealing, making it a single-step method for material synthesis cum film deposition. The morphology and the thickness of the films can be fine-tuned by varying the electrochemical deposition parameters and duration. This surface-agnostic method overcomes the limitations of typical solution- or vapour-phase deposition methods, enabling conformal film deposition even on tubular substrates for photo-electrochemical energy conversion applications. The wastage of precursor material is lower than that in spin-coating or vapour-phase deposition, which are commonly used to prepare continuous films. The resulting VODP films exhibit intrinsic resistance to thermal and aqueous degradation and sustain photoelectrochemical hydrogen evolution with light-to-hydrogen conversion efficiencies approaching 14%. The light-assisted self-repair mechanism ensures continuous performance without degradation, achieving operational stability exceeding 100 hours. This concept of electrosynthesis and deposition of VODPS opens up new possibilities for durable, lead-free perovskite materials in sustainable optoelectronics materials development.\u0026nbsp;\u003c/p\u003e\n"},{"header":"Methods and materials","content":"\u003cp\u003e\u0026nbsp;\u003cstrong\u003eMaterials:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ePalladium (II) chloride (ReagentPlus\u003csup\u003e\u0026reg;\u003c/sup\u003e, 99%), cesium chloride (ReagentPlus\u003csup\u003e\u0026reg;\u003c/sup\u003e, 99.9%), palladium (II) bromide (99%), cesium bromide (99.9% trace metal basis), %), palladium (II) iodide (99.98 trace metal basis%), cesium iodide (99.9% trace metal basis) and potassium iodide (ACS reagent, \u0026ge; 99.0%) were procured from Sigma-Aldrich. Sulfuric acid (about 98%) was procured from EMPARTA\u003csup\u003e\u0026reg;\u003c/sup\u003e, Merck Life Science Private Limited. Deuterium oxide was purchased from Sigma-Aldrich. FTO coated glasses (resistivity \u0026lt; 10 ohms/sq) were purchased from Dyesol.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMethods:\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eThin-film preparation:\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eBiologic SP-300 potentiostat was used for the deposition of the halide perovskites on the FTO substrates. \u0026nbsp;We prepared Cs\u003csub\u003e2\u003c/sub\u003ePdCl\u003csub\u003e6\u003c/sub\u003e films by dissolving 0.1 M PdCl\u003csub\u003e2\u003c/sub\u003e, 0.2 M CsCl, and 0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e in DI water, and deposited them on FTO or carbon paper by applying 1.5 V (vs SCE) for 5 minutes; film thickness was controlled by deposition time. For Cs\u003csub\u003e2\u003c/sub\u003ePdBr\u003csub\u003e6\u003c/sub\u003e, we used a solution of 0.1 M PdBr\u003csub\u003e2\u003c/sub\u003e, 0.2 M CsBr, and 0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u0026nbsp;\u003c/sub\u003ein DI water, and deposited films on similar substrates at 1.0 V for 5 minutes, with thickness again tuned via deposition time. To prepare Cs\u003csub\u003e2\u003c/sub\u003ePdI\u003csub\u003e6\u003c/sub\u003e, we dissolved 253 mg PdI\u003csub\u003e2\u003c/sub\u003e, 350 mg CsI, and 5 g KI in 5 mL DI water, and deposited the films on FTO by prepolarizing at \u0026ndash;0.5 V for 2 minutes, followed by 1.0 V for 2 minutes; the thickness was adjusted by varying the duration of the positive bias.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCharacterization techniques:\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eRaman spectroscopic investigation of the electrodeposited films was done using a Renishaw inVia\u003csup\u003eTM\u003c/sup\u003e confocal Raman microscope with a 532 nm laser. X-ray diffraction (XRD) was done using a Rigaku SmartLab\u003csup\u003e\u0026reg;\u0026nbsp;\u003c/sup\u003eX-ray diffractometer. Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) were done using JEOL JSM 7200F FE-SEM. Gas chromatography was done using a Thermo Scientific TRACE 1110 Gas Chromatograph. Mass spectrometric measurements were done using Stanford Research Systems (SRS) Residual Gas Analyzer (RGA) 200. Surface photovoltage measurements were done using a KP Technology Single-Point Kelvin Probe system (KP020). The photoelectrochemical measurements were performed using a SOLIS-525C High-Power LED (525nm, green, 2.4 W). ICP-OES was done using a Perkin-Elmer Avio 220 Max ICP-OES instrument.\u003c/p\u003e\n\u003cp\u003eAbsorbance measurements:\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFor performing absorption measurements, crystals (typical size\u0026nbsp;\u0026nbsp;) were extracted by scratching \u0026nbsp;the halide perovskite films grown on FTO and transferred to a\u0026nbsp;\u0026nbsp;\u0026nbsp;thick c-cut sapphire substrate. Optical absorption (\u0026nbsp;) spectra were recorded at room temperature (\u0026nbsp;) in the wavelength region\u0026nbsp;\u0026nbsp;\u0026nbsp;. A homebuilt setup\u003csup\u003e47\u003c/sup\u003e was used which could perform reflectance (\u0026nbsp;) and transmittance (\u0026nbsp;) spectroscopy on the same sample spot with a spatial resolution of\u0026nbsp;\u0026nbsp;. A tungsten halogen lamp acted as the light source for these measurements, while a\u0026nbsp;\u0026nbsp;\u0026nbsp;focal length spectrometer coupled with an EMCCD camera was used to record the spectra. Absorption spectra were deduced using the equation\u0026nbsp;\u0026nbsp;. Scattering losses were not accounted for in our measurements.\u003c/p\u003e\n\u003cp\u003eXPS valence band measurement:\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eHigh resolution XPS valence band measurements were performed using Al\u003cem\u003eK\u003c/em\u003e\u003cem\u003e\u003csub\u003ea \u0026nbsp;\u003c/sub\u003e\u003c/em\u003e(\u003cem\u003eh\u003c/em\u003e\u003cem\u003en\u003c/em\u003e\u003cem\u003e\u0026nbsp;\u003c/em\u003e= 1486.6\u0026nbsp;eV\u0026nbsp;)\u0026nbsp;radiation (energy) and a SCIENTA R4000 electron energy analyzer. The base pressure of the instrument was \u0026gt; 10\u003csup\u003e-10\u003c/sup\u003e mbar. As prepared samples were mounted on Cu sample holder and measured at room temperature with total instrumental resolution of ~ 400 meV.\u003c/p\u003e\n\u003cp\u003eComputational:\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eWe have employed Density Functional Theory (DFT) embedded\u003csup\u003e48,49\u003c/sup\u003e in Vienna Ab-initio Simulation Package (VASP\u003csup\u003e50\u003c/sup\u003e) for computing structural and electronic properties. The exchange correlation function is of the Perdew- Burke- Erzehenhorf (PBE\u003csup\u003e51\u003c/sup\u003e) type generalized gradient approximation. Projector Augmented Wave (PAW\u003csup\u003e52\u003c/sup\u003e) \u0026nbsp; pseudopotentials for Cs, Pd, Br, Cl, and I are 6s\u003csup\u003e1\u003c/sup\u003e, 4s\u003csup\u003e1\u003c/sup\u003e4d\u003csup\u003e9\u003c/sup\u003e, 4s\u003csup\u003e2\u003c/sup\u003e4p\u003csup\u003e5\u003c/sup\u003e, 3s\u003csup\u003e2\u003c/sup\u003e3p\u003csup\u003e5\u003c/sup\u003e, and 5s\u003csup\u003e2\u003c/sup\u003e5p\u003csup\u003e5\u003c/sup\u003e, respectively. The Brillouin zone has been sampled using 5 x 5 x 5 Monkhorst pack k-mesh. The cutoff energy is taken to be 500 eV throughout the calculations with plane wave basis set describing the valence electrons. The Hellmann-Feynman forces are taken to be 0.001 eV/\u0026nbsp;\u0026nbsp;to get the optimised structure with minimum energy configuration. The Bandstructure have been simulated using Hybrid functional of the type HSE06 for more accurate xc energy and spin orbit coupling (SOC) has been taken into account to consider the relativistic effect of heavy elements in the system.\u0026nbsp;\u003c/p\u003e\n"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData Availability Statement:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data that support the findings of this study are available from the corresponding author, [PN], upon reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgement:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFunding: P.K.N., TNN., AKR, and PRS acknowledge support from the Department of Atomic Energy (DAE), India, under project RTI 4007. P.K.N. also acknowledges support from the Department of Science and Technology (DST), India, via the Swarna Jayanti Fellowship. AA acknowledges financial support from the following projects funded by the Government of India: NM-ICPS of the DST through the I-HUB Quantum Technology Foundation (Pune, India), Project No. CRG/2022/007008 of SERB, MoE-STARS project No. MoE-STARS/STARS-2/2023-0912, CEFIPRA CSRP Project No. 7104-2, VAIBHAV fellowship number INAE/DST-VF/2024/I/03, DST National Quantum Mission project No. DST/QTC/NQM/QMD/2024/4 (G) and DST-CEFIPRA CSRP Project No. 7104-2. N.B. \u0026nbsp;acknowledge the CSIR India, for financial support through Awards No. 09/1020(0177)/2019EMR-I. The work was funded by the DAE, Govt. of India, through institutional funding to HRI. JK, PKP and SC acknowledge HRI for providing access to the infrastructure and high-performance computing facility.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003cstrong\u003eAuthor contributions:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ePKN, TNN and AKR conceived the idea. AKR performed all the synthesis, characterization and electrochemical studies under the supervision of PKN and TNN. AKR and PKN wrote the first draft of the manuscript and subsequently revised it based on input from all authors. \u0026nbsp;AG contributed to the development of electrochemical deposition methods. VA contributed to data interpretation and visualisation. \u0026nbsp;XPS measurements were performed by NB under the supervision of RSS. The quantum chemical calculations were performed by JK and PK under the supervision of SC. The microscopic Uv-Vis measurement was performed by AS and RMK under the supervision of AA. Mass spectrometry was performed by SKS and AKR under the supervision of PRS. PKN coordinated among the co-authors and supervised the project.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests:\u0026nbsp;\u003c/strong\u003eA patent based on this work has been granted by the Indian Patent Office (Application No /202541033941)\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eList of Supplementary Materials:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSupplementary\u0026nbsp;\u003c/strong\u003eFigures. S1-21\u003c/p\u003e\n\u003cp\u003eSupplementary Note S1-3\u003c/p\u003e\n\u003cp\u003eSupplementary Table S1\u003c/p\u003e\n\u003cp\u003eSupplementary Scheme1\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eCheng, W.-H. \u003cem\u003eet al.\u003c/em\u003e Monolithic Photoelectrochemical Device for Direct Water Splitting with 19% Efficiency. \u003cem\u003eACS Energy Lett.\u003c/em\u003e \u003cstrong\u003e3\u003c/strong\u003e, 1795\u0026ndash;1800 (2018).\u003c/li\u003e\n\u003cli\u003eMay, M. 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B\u003c/em\u003e \u003cstrong\u003e54\u003c/strong\u003e, 11169 (1996).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-9106903/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9106903/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"Semiconductors provide the material foundation for solar energy conversion, enabling the direct transformation of sunlight into electrical or chemical energy. Among emerging semiconductor classes, metal halide perovskites (HaPs) have attracted intense attention owing to their exceptional optoelectronic properties; however, their instability in aqueous media severely limits their application in photoelectrochemical energy conversion. We report electrosynthetic perovskite in-situ crystallization and film fabrication (EPIC-Fab), a scalable aqueous electrosynthesis and direct deposition strategy for palladium-based vacancy-ordered double perovskites (VODPs), specifically Cs2PdCl6, Cs2PdBr6, and Cs2PdI6. This method enables direct growth of highly crystalline films on a wide range of conductive substrates, including uneven tubular geometries, using water as the sole solvent and requiring no post-deposition annealing. Film thickness and morphology can be tuned through deposition parameters and duration. Unlike conventional lead halide perovskites, the resulting VODP films exhibit intrinsic high thermal and water stability. We demonstrate that these films function as efficient photoelectrocatalysts for hydrogen evolution (Faradic efficiency \u003e94%) without additional cocatalysts or protective layers. Notably, a light-assisted self-repair process enables maintaining operational stability during continuous photoelectrochemical hydrogen evolution for \u003e 100 hours. This work establishes an aqueous, surface-agnostic synthetic paradigm for vacancy-ordered double perovskites for optoelectronic and energy conversion applications.","manuscriptTitle":"In-situ electrosynthetic fabrication of highly crystalline, aqua-compatible palladium halide perovskite films for photoelectrocatalysis","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-03-18 08:06:53","doi":"10.21203/rs.3.rs-9106903/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"92a2016a-5d81-4f5a-ad81-dd9074d2022f","owner":[],"postedDate":"March 18th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":64631692,"name":"Physical sciences/Materials science/Materials for energy and catalysis/Photocatalysis"},{"id":64631693,"name":"Physical sciences/Materials science/Materials for devices/Electronic devices"},{"id":64631694,"name":"Physical sciences/Materials science/Condensed-matter physics/Semiconductors"}],"tags":[],"updatedAt":"2026-03-25T14:58:01+00:00","versionOfRecord":[],"versionCreatedAt":"2026-03-18 08:06:53","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9106903","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9106903","identity":"rs-9106903","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}