Effect of sputter-coated platinum on the photostability of nanoporous sol–gel CuO thin film photocathodes

Abstract

Copper oxide-based materials are promising candidates as photocathodes for photoelectrochemical (PEC) hydrogen production due to their suitable band gap energy for visible light absorption and appropriate band edge positions for driving the hydrogen evolution reaction. However, the application of CuO suffers from inherent drawbacks, such as poor photostability upon illumination, caused by the position of the Cu²⁺ reduction potential within the energy band gap of CuO. To protect CuO from photocorrosion, Pt was sputter-coated on nanoporous sol–gel-based CuO thin films prepared by dip-coating using Pluronic® F-127 as a structure-directing agent and, for the first time, citric acid as a complexing agent. Electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy collectively confirmed that Pt can be homogeneously deposited with controlled thicknesses on the nanoporous CuO surface, possessing pore dimensions between 10 nm and 100 nm. The impact of the Pt coating thickness on the PEC performance and stability of CuO was investigated in a neutral aqueous electrolyte. PEC analysis showed that Pt can act as a protective layer, which was indeed found to slow down the photodegradation process of CuO substantially (57% versus 2.9% of the initial photocurrent retained after 30 min of PEC operation). However, Pt coating did not increase gas evolution under irradiation and applied potential, when compared to pristine CuO. The optimum Pt layer thickness in terms of photostability was found to be approximately 20 nm. The empirically derived optoelectronic properties of CuO thin film photocathodes (indirect optical E_gap = 1.5 eV) were supported by density functional theory calculations, providing fundamental insight into the electronic band structure. The outcomes of this study demonstrate a reproducible and up-scalable synthetic approach to increase the photostability of CuO-based materials under operating conditions in PEC cells.

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Introduction

A profound transformation is taking place in the global energy sector, as nations come together to address climate change and strengthen energy production capacity to move towards cleaner, more sustainable energy sources. Due to its excellent energy storage capacity and carbon-free nature, hydrogen is gaining an increasingly significant role amid the ongoing transformation of the global energy structure. Within this framework, photoelectrochemical (PEC) water-splitting has emerged as a promising strategy for producing green hydrogen by harnessing sunlight and utilizing a semiconductor photoabsorber in contact with water. This process involves the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), which occur at spatially separated photoelectrodes. As a viable pathway for generating low-emission hydrogen from carbon-free energy sources, PEC water-splitting offers several notable advantages over alternative water-splitting approaches. For instance, its integration into a single device simplifies the overall system configuration, thereby reducing associated costs when compared to systems such as photovoltaic-based electrochemical (PV-EC) water-splitting.^1,2

Since the first TiO₂ photoanode was reported in 1972,³ significant advancements in the field of solar water-splitting have been achieved.^4,5 Transition metal oxides have been intensively investigated as photoabsorbers, among which BiVO₄ has shown highly promising photoelectrochemical properties.^6–8 In particular, the abundance and cost-saving due to less complex synthesis strategies make transition metal oxides economically viable for large-scale applications.⁶ However, it has been reported that the PEC performance and properties of open 3d-shell transition metal oxides, such as the charge carrier lifetime and electronic band gap energy, are dramatically limited by the unoccupied 3d-states⁹ and the formation of polaron defect states,¹⁰ respectively. Among the most efficient transition metal oxide photocathodes, copper(II) and (I) oxide-based materials are promising candidates as photocathodes for PEC hydrogen production, due to their p-type characteristics and suitable band gap energies for visible light absorption.¹¹ CuO has a small band gap of ∼1.5 eV, enabling good solar absorption, and thus, can theoretically achieve a high photocurrent density of 35 mA cm⁻².^12,13 However, the unoccupied 3d states and formation of polaron defects limit the observed photovoltages, photocurrents, and photostability of (sol–gel-derived) CuO photocathodes.¹⁴ As illustrated in Fig. S1, for cupric and cuprous oxide in contact with water-based electrolytes, the Cu²⁺/Cu⁺ (0.16 V vs. NHE) and Cu⁺/Cu (0.53 V vs. NHE) reduction potentials are located close to the reduction potential of H⁺/H₂ at 0 V vs. NHE.^15–17 Therefore, these competitive reactions (photocorrosion) occur instead of hydrogen evolution, leading first to the formation of reduced copper species (Cu⁺) and finally to metallic copper (Cu⁺ to Cu), as observed for CuO under operating conditions.^18–20

Various approaches have been explored to minimize the occurrence of competitive reactions and to enhance the stability of photoelectrodes against photocorrosion in aqueous electrolytes, including nanostructuring and applying protection layers to the surface of CuO (see Table S1). In the study by Guo et al.,²¹ Pd nanoparticles were used as a protection layer on CuO photocathodes, which enhanced the photocurrent by up to 110% and more than doubled the long-term stability compared to bare CuO. Xing et al.²² investigated the use of TiO₂ as a protective layer on CuO photocathodes, a widely adopted strategy to mitigate photocorrosion. It is demonstrated that TiO₂ improved the photostability significantly, with an enhancement of 32% compared to pristine (unprotected) CuO. Jeong et al.^23,24 improved the photostability of CuO by depositing a NiO_x capping layer as both a protective layer and a cocatalyst. The modified CuO achieved a photocurrent density of −1.86 mA cm⁻² at −0.55 V vs. SCE and a significantly improved photostability, about three times higher than that of pristine CuO. Meng et al.²⁵ reported that using Cu₃N as a protection shell can stabilize CuO photocathodes against photocorrosion. The CuO/Cu₃N photocathode retained 80% of its initial current density after 20 minutes, compared to only 10% for bare CuO. Li et al.²⁶ fabricated CuO nanowires on Cu foil through a simple thermal-treatment process. The nanostructured CuO photocathode achieved a photocurrent density of ∼1.4 mA cm⁻², among the highest reported for bare CuO. Additionally, electrochemical deposition of Pt nanoparticles improved the stability by accelerating surface reactions and minimizing charge accumulation. Despite these advancements, achieving a uniform protection layer on nanoporous CuO remains a significant challenge. Conventional methods, such as electrochemical deposition²³ or chemical synthesis,²⁵ often suffer from non-uniform coverage or pore blockage, which inevitably degrades the original hierarchical morphology and reduces the active surface area. Furthermore, simple loading of cocatalyst nanoparticles often fails to provide long-term photostability due to particle aggregation.^21,26 Herein, we report a facile and efficient sputter-coating strategy to decorate nanoporous CuO with a conformal Pt protection layer of optimal thickness. Notably, this approach successfully preserves the pristine nanoporous architecture, thus allowing for thorough electrolyte infiltration and efficient mass transport.

Our group has been working on the nanostructuring of CuO photocathodes prepared by dip-coating and the evaporation-induced self-assembly (EISA) process with the goal of minimizing the migration distances of photogenerated charge carriers and thus decreasing bulk recombination within the photocathode.¹⁴ However, in accordance with the literature,²⁷ the photoactivity of CuO is mainly controlled by surface recombination and photocorrosion. Hence, nanostructuring of metal oxide photoelectrodes can only be beneficial for PEC applications, when the surface is passivated and surface recombination is inhibited.^27,28 Based on that, the present study aims to investigate how the deposition of a platinum layer on nanoporous CuO photocathodes acts as a protection layer against photoreduction, particularly when applied in photoelectrochemical hydrogen production. A thickness-dependent study on sputter-coated platinum and how it affects the photostability of nanostructured CuO photocathodes was conducted. An optimized deposition time was identified based on the tradeoff between homogeneous Pt layer deposition for protecting the CuO surface from photocorrosion and – at the same time – efficient light absorption for photoexcitation of charge carriers.

Experimental section

Materials

All chemicals were utilized as received: copper(II) nitrate hemi(pentahydrate) (Cu(NO₃)₃·2.5H₂O, ≥99.99%, Sigma-Aldrich), citric acid (≥99.5%, Carl Roth), 2-methoxyethanol (≥99.5%, Carl Roth), ethanol (≥99.8%, Carl Roth), and Pluronic® F-127 (Sigma-Aldrich). A Pt target (99.99%, 54 mm diameter × 0.2 mm thickness, Baltic Präparation) was used for sputter coating on CuO samples. Silicon (100) wafers (Siltronic) and fluorine-doped tin oxide (FTO) coated Pilkington TEC glass slides (Xop Glass) were used as substrates.

Synthesis of CuO thin films

Nanoporous CuO photocathodes were prepared by the sol–gel and dip-coating method. Prior to the synthesis, polar and non-polar substrates were cleaned with a mixed solution of acetone and ethanol in the volume ratio of 1 : 1 in an ultrasonication bath for 15 minutes, after which the polar fluorine-doped tin oxide (FTO) substrates were additionally ozone-cleaned (with an Ossila L2002A3-EU UV ozone cleaner) for 10 minutes. As illustrated in Fig. 1, the CuO thin film was synthesized through the following procedure: 45 mg of Pluronic® F-127 was dissolved in 0.7 mL of 2-methoxyethanol. In another vial, 0.092 g of citric acid (CA) and 0.384 g of Cu(NO₃)₃·2.5H₂O were dissolved in 0.7 mL of 2-methoxyethanol. Subsequently, the Pluronic® F-127 solution was added dropwise to the metal precursor solution under gentle stirring. Following this, 0.8 mL of ethanol and 0.1 mL of deionized water were introduced into the mixture, which was further stirred for 30 minutes. The dip-coating (with an Ossila L2006A1-EU dip coater) was conducted in an enclosed home-made chamber controlling the relative humidity (RH) at 18 ± 2% integrated with a regulated air-flow system. This environment ensured a consistent evaporation rate of the solvent and controlled the chemical transformation and arrangement of the precursor and porogen into a well-developed, amorphous hybrid network. The withdrawal speed of substrates was set between 2 mm s⁻¹ and 4 mm s⁻¹. In the next step, all samples were air-dried for 4 minutes and transferred into a pre-heated (125 °C) muffle furnace (Nabertherm LT 3/11). After stabilization of the samples at 125 °C for 15 minutes, the temperature was raised to 250 °C and held for 24 hours. Afterwards, the sample was heated up to 600 °C and calcined for another 2 hours.

Fig. 1 Flow chart of the synthesis of nanoporous CuO thin films consisting of (1) preparation of dip-coating solution, (2) dip-coating, and (3) calcination.

Deposition of platinum on the CuO thin films

Pt was deposited onto CuO using a sputter-coater (Quorum Technologies Q300TD). In this process, pristine nanoporous CuO films were coated in a vacuum chamber with a current of 30 mA per second for various deposition times (10 s, 20 s, 40 s, 60 s, 80 s, and 100 s), resulting in distinct layer thicknesses of the Pt layer. In this work, Pt-coated CuO is denoted as Pt-CuO unless otherwise specified.

Physicochemical characterization

The thicknesses of the CuO-based thin films and the Pt protection layers were analyzed by profilometry (Bruker Dektak XT) with a resolution of 0.1 µm per point and a scan range of 6.5 µm. The surface morphology of the as-prepared photocathodes was characterized by scanning electron microscopy (SEM, Phillips XL 30 electron microscope) utilizing an acceleration voltage of 30 kV and a working distance of 3–4 cm. Prior to the SEM analyses, samples were coated with Pt between 30 s and 120 s (unless otherwise specified) for increasing the electrical conductivity. The UV-vis absorption spectra were acquired (with a UV-vis spectrophotometer PerkinElmer LAMBDA 365+) in the scan range between 350 nm and 1100 nm, with a scan speed of 240 nm min⁻¹. The crystal structure of the materials was investigated by X-ray diffraction (XRD, Rigaku Smartlab) utilizing copper Kα radiation at 20 kV and 170 mA. Raman spectroscopy (WITec alpha300 R) was carried out at a power density of 12.5 mW, a laser wavelength of 532 nm, and an integration time of 3 s. X-ray photoelectron spectroscopy (XPS) analyses were performed on a ULVAC-PHI VersaProbe II under a chamber pressure below 5 × 10⁻⁹ mbar. The applied excitation source was a monochromatized Al-Kα X-ray source (1486.6 eV). Detailed spectra were acquired at a pass energy of 23.5 eV and a step size of 0.1 eV per step. For survey spectra, the pass energy and step size were adjusted to 187.9 eV and 0.8 eV per step, respectively. For binding energy calibration, the binding energy of Au 4f_7/2 emission of 84.0 eV from gold foil was applied. To examine the volume of hydrogen production, gas chromatography (GC, SHIMADZU Nexis GC-2030) measurements were performed together with the chronoamperometry under simulated sunlight illumination (as described in the (Photo)electrochemical measurements section), during which the potential applied on the photocathode was at 0.45 V vs. RHE. The electrolyte was degassed for 30 min prior to the measurements, then poured into the PEC cell and purged with N₂ for another 15 min to remove all air from the setup. To make sure there is no more overpressure from the gas purging in the setup, multiple GC measurements for 45 min (10 points every 4.5 min intervals) without illumination or applied voltage were performed. Subsequently, the analysis was recorded for 20 min, and any produced gas during these 20 min was stored/collected in the PEC cell and released as an overpressure visible in the GC by an increased signal.

(Photo)electrochemical measurements

All (photo)electrochemical measurements were performed using a three-electrode system (Zahner cell PECC-2), in which a built-in platinum wire ring serves as the counter electrode, and the photocathode samples serve as the working electrode. For the evaluation of the electrochemically active surface areas (ECSAs) of thin films, scan rate-dependent cyclic voltammetry (CV, GAMRY Interface 1000E) analysis was performed in the non-faradaic region (+0.75 to +0.95 V vs. RHE for pristine CuO and +0.92 to +1.12 V vs. RHE for Pt-coated CuO). Briefly, the OCP of the samples was determined, and consecutive CV cycles at scan rates ranging from 200 to 1000 mV s⁻¹ were performed to extract the capacitive current at OCP and the difference in the forward (anodic) and backward (cathodic) scan current; the linear fitting slope of the current difference vs. the scan rate yielded the double-layer capacitance.²⁹ A Hg/HgO electrode was utilized as the reference electrode and 1 M KOH (pH ≈ 13.6) was used as the electrolyte. To characterize the flat-band potential and the acceptor concentration, dark Mott–Schottky measurements (M–S, ZENNIUM PRO potentiostat) were conducted in the range of +0.6 to +1.2 V vs. RHE at a frequency of 100 Hz and an amplitude of 5 mV. Dark linear sweep voltammetry (LSV, GAMRY Interface 1000E potentiostat) was employed to investigate the current density in the cathodic region (+0.2 to −1.0 vs. RHE). The adapted step size was 1 mV and the scan rate was 10 mV s⁻¹. To assess the photoelectrochemical performance and photocurrent density, chopped-light voltammetry (CLV, Zahner LSW-2, potentiostat PP212) under white light illumination (AM 1.5, with a wavelength of 536 nm and intensity of 1000 W m⁻²) was applied from +0.85 to +0.3 V vs. RHE at a scan speed of 5 mV s⁻¹. For the evaluation of photostability, chronoamperometry was performed (using the same light source as for CLV) at +0.6 V vs. RHE over 30 minutes. M–S, LSV, and CLV measurements, and chronoamperometry were all performed using an Ag/AgCl reference electrode in an aqueous 0.2 M Na₂SO₄ electrolyte including a 0.1 M KH₂PO₄ and 0.1 M K₂HPO₄ buffer.

Density functional theory (DFT) calculations

The spin-polarized Density Functional Theory (DFT) calculations in this study were carried out using the Vienna Ab initio Simulation Package (VASP).^30,31 The valence electrons considered for solving the Kohn–Sham equations included 11 electrons from Cu (3d¹⁰4p¹) and 6 electrons from O (2s²2p⁴), while the core electrons were represented using projected augmented wave (PAW) pseudopotentials.³² The effects of electron exchange and correlation were accounted for using the generalized gradient approximation (GGA), specifically employing the functional formulated by Perdew, Burke, and Ernzerhof (PBE).³³ The plane wave expansion employed a kinetic energy cut-off of 520 eV. The electronic convergence criterion is established at 1.0 × 10⁻⁵ eV, while the force convergence for ionic relaxation is set at 1.0 × 10⁻³ eV Å⁻¹. The PBE method often underestimates the band gap. However, in this study, Dudarev's approach demonstrated that applying a U_eff value of 7 eV to Cu atoms produces much more precise results in terms of band gap calculation.³⁴ An 11 × 15 × 10 Monkhorst–Pack k-point mesh was generated using VASPkit for the calculations.³⁵ The Vesta software was utilized for visualizing the structure.³⁶

Results and discussion

Morphological surface characterization

For the preparation of the nanoporous CuO thin films, the well-established EISA process was carried out (see Fig. 1). In principle, the structure-directing agent Pluronic® F-127 and the copper nitrate precursor were separately dissolved in alcoholic solvents and subsequently mixed to prepare the dip-coating solution. Citric acid is well-known to act as a complexing agent during the preparation of nanoporous thin film^37,38 and powder³⁹ metal oxides and helps to form stable carbonates, which allows for preservation of the nanoporous architectures upon annealing in air. In this study, citric acid was used for the first time as a complexing agent in conjunction with Pluronic® F-127 for the preparation of nanoporous CuO. Our previous work reported on the synthesis of nanoporous CuO without the necessity of using citric acid,¹⁴ however, we found that these films can only be produced as a well-developed nanoporous CuO network, when the atmospheric humidity (outside of the dip-coating chamber) is below 30% upon solution preparation. Under high atmospheric humidity (larger than 40%), an interconnected and structurally intact CuO framework can hardly be realized under otherwise the same synthesis conditions.

To investigate the impact of cooperatively utilizing citric acid in the presence of both copper nitrate and Pluronic® F-127, scanning electron microscopy (SEM) was utilized to study the surface morphology (Fig. 2). The images in Fig. 2a–d depict a noticeable difference in the material's porous network structure brought by the addition of citric acid. By using citric acid (Fig. 2a and b), the CuO structure becomes considerably more condensed and possesses interconnected structural features after calcination at 600 °C, in contrast to the formation of spatially separated CuO agglomerates in the absence of CA (Fig. 2c and d). The results are in accordance with the report from Burroughs et al.,⁴⁰ in which it was concluded that the addition of citric acid enables fabrication of structurally intact mesoporous copper oxide through the formation of copper carbonate intermediates. The processing parameters, especially the heating temperature, when converting the nitrate–citrate complex into the carbonate intermediate, enable the facile control of pore size, pore distribution, and the propagation of cracks. Notably, in the conventional route of soft-templated metal oxides, in which the decisive state of porous network formation is the modulable steady state (MSS), the mesoporous structure forms within seconds during room temperature drying.⁴¹ In contrast, when soft-templating is carried out via a metal carbonate intermediate, the decisive state of porous network formation lies within the whole carbonate formation period during which the temperature is significantly higher than room temperature (250 °C in our work). This elevated temperature minimizes the influence of external parameters—such as ambient humidity—that typically affect hydrolysis and condensation reactions, and thereby the overall reaction kinetics.⁴²

Fig. 2 SEM images of pristine CuO films synthesized (a and b) with and (c and d) without citric acid as a complexing agent at low (left) and high (right) magnifications. (e and f) SEM images of 80 s Pt-CuO at low (left) and high (right) magnifications. All CuO samples were prepared under the same experimental conditions, including the humidity (18 ± 2% RH).

To investigate the influence of the Pt protection layer thickness on the photoelectrochemical activity of CuO photocathodes, the deposition time of Pt during sputter coating was varied from 10 s to 100 s. Fig. S2a shows the cross-section of a pristine CuO sample on a FTO substrate. The thickness of CuO was estimated to be 150 (±30) nm by profilometry (Fig. S2). Despite the increase in interconnectivity within the nanoporous network achieved through soft-templating via the formation of metal carbonate intermediates, the final film thickness remains susceptible to external preparation factors. These include, for instance, the relative humidity within the dip-coating chamber, the ambient humidity outside the chamber/in the lab, and the nature of the substrate employed.^41,43,44 As shown in Fig. S2b, the film thickness increased from 120 nm to 145 nm after a 100 s Pt coating, indicating the formation of Pt layers on CuO. A linear correlation (Fig. S2c) was observed between the Pt deposition time and the resulting film thickness. Based on this correlation, it can be inferred that deposition durations of 10 s and 100 s yield film thicknesses of ∼2.5 nm and 25 nm, respectively. In this context, the Pt deposition time is used as a reference for the corresponding film thickness throughout this work. To investigate how Pt deposition influences the surface morphology, top-view SEM images with 80 s and 100 s of Pt-coating on nanoporous CuO were examined (Fig. 2e, f and S3). In comparison with the pristine CuO (Fig. 2a and b), consisting of a worm-like network of CuO nanodomains and cavities/pores of about 30–50 nm in length, after 80 s (Fig. 2e and f) and 100 s (Fig. S3) of Pt coating, the nanostructure seems to be partially filled and covered by a secondary phase. The pore size range and distribution were further analyzed based on the SEM images (histogram). All samples revealed comparable nanopore sizes between 10 nm and 100 nm (Fig. S4), independent of the Pt coating time. The average pore size of the pristine, 80 s, and 100 s coated CuO was determined to be 40 ± 16 nm (Fig. S4b), 36 ± 12 nm (Fig. S4d), and 43 ± 16 nm (Fig. S4f), respectively. In conjunction with profilometry and SEM investigations, imaging of the surface topography by atomic force microscopy (AFM, Fig. S5) further confirms that the Pt protection layers were formed uniformly across the CuO surface. The low root-mean-square roughness (S_q) over a 1 µm × 1 µm and a 2 µm × 2 µm area was found to be 5.6 nm and 6.7 nm for pristine CuO (Fig. S5a and b) and 7.7 nm and 8.1 nm for 80 s Pt-coated CuO thin films (Fig. S5c and d), respectively. This indicates that Pt coating increases the surface roughness of CuO, also visualized by larger grains/structural features (Fig. S5). Interestingly, and in accordance with SEM images (Fig. 2), the grain boundaries/pore walls of CuO remain visible, with no indication of particle clustering.

Crystallographic characterization

The grazing-incidence X-ray diffraction (GIXRD) on silicon substrates was performed to identify the crystallographic structure of pristine CuO and Pt-coated CuO thin films with various deposition times, as depicted in Fig. 3a. For each sample, only the monoclinic CuO phase with the space group C2/c was identified. The most prominent Bragg reflections appeared at 2θ values of 32.5°, 35.5°, 38.8°, and 53.5°, corresponding to the (110), (002), (111), and (020) crystal planes (JCPDS #05-0661), respectively. Two diffraction peaks observed at 2θ = 39.8° and 46.3°, corresponding to the (111) and (200) lattice planes, respectively, are attributed to the cubic platinum phase with Fm3̄m space group symmetry (JCPDS #87-0646). As the Pt coating time increases from 0 s to 100 s, the intensity of these peaks also increases, indicating a progressive accumulation of platinum on the surface. This trend, as expected, suggests that the surface concentration of platinum increases with longer deposition durations. Moreover, the relatively broad Bragg peaks imply the presence of nanosized crystallites that constitute the mesoporous network.^14,45,46

Fig. 3 (a) XRD patterns of CuO samples coated for deposition times between 0 s and 100 s, including the corresponding reference bars (JCPDS # 05-0661 and JCPDS # 87-0646) indicating the diffraction peaks of CuO and Pt. (b) Raman spectra recorded under identical acquisition conditions for CuO-based samples, in which a pristine CuO sample and a Si substrate were taken as a reference. Baseline correction was applied to remove fluorescence background. (c) Monoclinic crystal structure of pristine CuO; red: Cu atoms and blue: O atoms.

Raman spectroscopy was also conducted to exclude the formation of side phases. In the spectra of CuO-based samples with varied Pt deposition times prepared on silicon (Fig. 3b) and FTO substrates (Fig. S6), the Raman modes A_g (291 cm⁻¹), B⁽¹⁾_g (339 cm⁻¹), and B⁽²⁾_g (624 cm⁻¹) confirmed solely the existence of the monoclinic CuO phase (see Fig. 3c).⁴⁷ The peaks originating from the Si substrate were located at 520 cm⁻¹ and 935–990 cm⁻¹ (Fig. 3b), and those from the FTO substrate were located at 552 cm⁻¹ and 1197 cm⁻¹ (Fig. S6). The broad peak ranging from 935 to 990 cm⁻¹ might be ascribed to the multi-phonon scattering effects from the silicon substrate.^48,49 For the Pt-CuO samples, the absence of any Pt-related Raman peaks is in agreement with other studies⁵⁰ reporting that metals like Pt, serving as a uniform layer, lack discrete vibrational modes in the visible range of Raman spectra. Possible explanations include the shielding effect of conductive electron pairs on ionic vibrations and the strong coupling between phonons and electrons.^51,52 Moreover, thicker Pt deposition results in a decrease in peak intensity, which is reasonable since the consecutive Pt layers exhibit significant reflection and absorption (compare Fig. 5), resulting in reduced excitation and light intensity reaching the CuO layer. Simultaneously, Raman-scattered light from the CuO layer undergoes further absorption and reflection as it traverses the Pt layers, thereby diminishing the detectable Raman signals in both directions.^52,53 In essence, Raman spectroscopy supports the hypothesis that increasing coverage (with increasing deposition time) of the porous CuO surface by platinum has occurred.

For the investigation of the chemical composition and oxidation states, X-ray photoelectron spectra of the pristine and Pt-CuO samples were acquired, as illustrated in Fig. 4 and S7. For pristine CuO samples, the peaks located at the binding energy of 953.6 eV and 933.7 eV correspond to the spin-orbital splitting of Cu 2p (see Fig. 4a) with an energy difference of about 19.9 eV, which is in accordance with the literature.^54,55 The two strong satellite peaks centered at 962.4 eV and 942.9 eV indicate the existence of Cu²⁺ as the only metal species present at the surface. In conjunction with the Cu LMM Auger signal (see Fig. S7), which is located at around 917.7 eV, the presence of Cu²⁺ is further supported and the formation of CuO as the dominant phase is confirmed. The O 1s spectrum (Fig. 4c) showed the main peak at 529.7 eV, which can be assigned to the existence of lattice oxygen, and the shoulder at 531.5 eV originates from defective (undercoordinated) oxygen or surface-adsorbed hydroxyl groups.^14,56,57

Fig. 4 (a) Cu 2p, (b) Pt 4f, (c) O 1s, and (d) VB XPS spectra of pristine CuO (black) and 80 s Pt-CuO (yellow) thin films.

For the 80 s Pt-CuO sample, the 4f_5/2 and 4f_7/2 peaks were observed in the Pt 4f core level spectra (Fig. 4b) at 74.7 eV and 71.4 eV, respectively.⁵⁸ The presence of these photoemission lines confirms the metallic character of the 80 s sputter-coated Pt protection layer (with measured thickness of up to 18 nm, beyond the detection depth of the XPS applied in this work) at the surface of the nanoporous CuO films. The absence of photoemission lines of both Cu 2p and Cu LMM Auger signals (in Fig. 4a, b and S7) is evidence for the complete and conformal coverage of Pt on the nanoporous oxide surface. In the O 1s spectrum of the Pt-CuO sample (Fig. 4c), the broad peak at around 531.4 eV is likely derived from surface adsorbed species on the Pt overlayer.⁴⁶ From the VB spectra shown in Fig. 4d, the valence band maximum (VBM) of pristine CuO is determined to be located at 0.18 eV; after depositing Pt on the surface, the VBM shifted to the vicinity of the Fermi level (0 eV), confirming the metallic character of the Pt layers. These results collectively allow the assumption that the surface is completely covered by Pt, considering the maximum detection depth of the utilized XPS setup.

Optical characterization

To evaluate the light absorption behavior of pristine and Pt-CuO thin films, UV-vis spectroscopy was carried out. As shown in Fig. 5a and S8a, longer deposition times (corresponding to thicker Pt layers, see Fig. S2c) continuously enhance UV-vis absorbance across 400–1100 nm (visible light and near-infrared region (NIR)) owing to the introduction of thicker Pt layers on the CuO surface. Hence, CuO samples with 60 s to 100 s deposition time of Pt show the strongest absorption behavior (maxima around 500 nm). CuO is known as an indirect band gap semiconductor,^20,59 which was verified by fitting the Tauc plots in Fig. 5b. For all samples, despite the surface treatment, an indirect optical band gap energy value of about 1.5 eV was determined, which is in line with the literature.^60,61 Since nanostructures contain a large portion of the surface area (related to the bulk), the breaking of crystal lattice symmetry at the surface is induced,⁶² direct optical transitions in the material can also be expected. On the other hand, the direct optical band gap of Pt-coated CuO, ranging from 1.8 to 2.0 eV (see Fig. S8b), was estimated to be slightly smaller than that of pristine CuO (2.1 eV). The introduction of Pt led to a slight reduction in the direct optical band gap energy of CuO, resulting in a red-shift in the absorption spectrum, as well as a stronger absorbance in the visible light and NIR regions.⁶³ Moreover, the increase in absorbance in the NIR region can also be attributed to the plasmonic effects of platinum interacting with the incident light.^64,65

Fig. 5 (a) UV-vis absorbance spectra and (b) the derived Tauc plots for an indirect optical transition of 0 s, 10 s, 20 s, 40 s, 60 s, 80 s, and 100 s Pt-CuO thin films.

To elucidate the flat band potential and charge carrier densities, Mott–Schottky (M–S) measurements in an aqueous 1 M Na₂SO₃ + 0.2 M Na₂SO₄ electrolyte (with 0.1 M KH₂PO₄/K₂HPO₄ buffer) were performed for the CuO-based samples (Fig. 6a and S9). The linear part of all plots was analyzed based on the Mott–Schottky equation (eqn (S1)).⁶⁶ Negative slopes were obtained for all plots, indicating p-type conductivity for all CuO samples. The negative slopes also suggest that the charge carrier density is attributed to the acceptor density (N_A) for both the pristine and Pt-CuO samples. Throughout this paper, the term ‘charge carrier density’ refers to the density of majority carriers. For the p-type CuO-based photocathodes studied here, this corresponds to the acceptor density unless otherwise specified. As illustrated in Fig. 6a and Table S2, no significant flat band potential (V_FB) shift was observed upon Pt coating, independent of the deposition time. All CuO-based samples showed V_FB of 1.0–1.1 V vs. RHE, which is consistent with the literature reports.^14,60 The invariant V_FB can be attributed to the dominant effect of Fermi level pinning due to the high density of surface states provided by the nanoporous CuO samples, originating, for example, from the presence of intrinsic dangling bonds, oxygen and copper vacancies, and (adsorbed) hydroxyl groups.^67,68 This assumption is also in line with the V_FB values determined after conducting 3 CLV cycles (Fig. S9 and Table S2). For all samples, an energetic decrease of about 200 mV was found for the V_FB position, suggesting a photodynamically induced population of surface states through charge carriers, leading to a downshift of the band edges.^28,69 Additionally, Pt nanoparticles may promote reconfiguration of surface states on CuO during illumination, particularly by forming new charge transfer pathways between Pt and CuO.⁷⁰

Fig. 6 (a) Mott–Schottky analysis for the 10 s, 20 s, 40 s, 60 s, 80 s, and 100 s Pt-coated samples measured at a frequency of 100 Hz under dark conditions in the same electrolyte as for CLV. (b) Schematic drawing of the energy diagram of CuO and Pt-CuO, including the redox potentials of the water-splitting reaction and the redox couples Cu²⁺/Cu⁺ and Cu⁺/Cu.

Notably, the charge carrier density (N_A) within the CuO photocathode is considerably increased after the Pt deposition, with the order of magnitude being elevated from 10²⁰ to 10²² cm⁻³ (Table S2). While previous reports on Pt-coated photoelectrodes have primarily focused on the enhancement of photocurrent and H₂ evolution rates, the direct impact of Pt deposition on the charge carrier density of p-type photocathodes evaluated by M–S measurements remains unexplored.^71–76 Interestingly, Mott–Schottky analyses reveal an increase of N_A by two orders of magnitude, which is ascribed to the enhanced electric (surface) conductivity imparted by Pt deposition and consistent with prior reports for NiO_x electrocatalysts on CuO.²³ The introduction of Pt particles can also facilitate the PEC performance of the photocathode by efficient spatial separation of photogenerated electrons and holes in the space charge region, thereby decreasing the surface recombination rate, and thus increasing the observed photocurrent density.^26,77

The evaluated flat band potential (1.1 V vs. RHE for both CuO and Pt-coated CuO) derived from M–S measurements and the optically determined band gap energy values (1.5 eV for both) from UV-vis spectroscopy were utilized to construct an energy diagram of CuO and Pt-CuO (Fig. 6).⁷⁸ The potential in solution, E_A (V vs. RHE), was converted into the energy E (eV) with respect to the vacuum level as depicted in eqn (1),⁶⁴ in which ΔE_F is the energy difference (0.13 eV for CuO) between the Fermi level and E. V_H is the Helmholtz potential built up between the CuO electrode and the solution, which equals 0.059 mV (pH_z − pH_a), with pH_z being the pH of an isoelectric point (9.5 for CuO) and pH_a being the pH value of the actual solution (7.8 for buffer solution with 0.1 M KH₂PO₄, 0.2 M Na₂SO₄, and 0.1 M K₂HPO₄). Based on the values above, eqn (1) can be converted to eqn (2), yielding the position of the CuO and Pt-CuO Fermi levels at −5.1 eV vs. the vacuum energy level. In both diagrams, the position of the VBM can be estimated in relation to the positions of the Fermi level (also presented through the flat band potentials) by calculation of the charge carrier (hole) density (p) in eqn (3). Assuming the effective mass of a hole in CuO to be 1.87 m₀,⁷⁹ the energetic difference between the Fermi level and VBM was determined to be 40 meV and 170 meV for pristine CuO and Pt-CuO, respectively.

E = −(E_A + 4.4 + ΔE_F + V_H) (1)

E = −(E_A + 4.0) (2)

The energy diagram depicted in Fig. 6b indicates that, despite a slight shift of the conduction band minimum (CBM) owing to Pt deposition, both CuO and Pt-CuO thin films possess – in principle – suitable band edge levels for photocathodic reduction of H⁺ to H₂ since the position of the CBM is located above the H₂/H⁺ potential (0 V vs. RHE). However, since both energy band edges straddle the potential of Cu²⁺/Cu⁺ and Cu⁺/Cu, competitive reactions such as the reduction of Cu²⁺ and Cu⁺ are well-known and likely to occur during PEC operation.^14,20

DFT calculations

Insight into the electronic band structure by DFT analysis is important to understand the photoelectrochemical properties and to support the empirical data derived for CuO photocathodes. The result from the DFT calculations (see Fig. 7) also indicates that CuO is a p-type semiconductor (the Fermi level seems to be close to the VBM, while the CBM seems to be far from the Fermi level). To validate this observation, additional Cu-vacancies were considered for DFT calculations, showing that after introduction of Cu-vacancies the Fermi level further shifts towards the VBM and the effective band gap decreases in the electronic band structure (Fig. S10), which underscores the p-type character of CuO.

Fig. 7 (a) The calculated band structure of CuO along high-symmetry K-points, with the Fermi energy (E_F) set as the reference at 0 eV. (b) The projected density of states (PDOS) that exhibits the contribution of different atomic orbitals.

The unit cell in the monoclinic crystal structure contains eight atoms, consisting of 4 copper atoms and 4 oxygen atoms. Using DFT + U optimization, the relaxed lattice parameters are a = 4.55 Å, b = 3.65 Å, and c = 5.19 Å, with an angle of 95.61°. The Cu–O bond length is found to be 1.96 Å, and the unit cell volume is 86.12 ų, aligning well with previously published data.^80,81 Moreover, prior studies have identified CuO as exhibiting antiferromagnetic behavior, which is consistent with our computed magnetic moment of 0.72µ_B with an anti-ferromagnetic nature.^81,82 As shown in Fig. 7, the electronic properties of CuO with DFT + U calculations reveal its semiconducting nature with a well-defined band structure and density of states (DOS). The band structure plot (see Fig. 7a) shows the energy dispersion of electronic states along different high-symmetry points in the Brillouin zone. The Fermi level is set at 0 eV, and the conduction and valence bands are clearly separated, indicating the presence of a bandgap. Notably, the direct bandgap at the Γ-point is observed to be 1.30 eV, while an indirect transition of 1.02 eV is observed, suggesting that CuO exhibits characteristics of an indirect bandgap semiconductor.^14,83 A Hubbard potential (U_eff) of 7 eV was applied to the strongly localized Cu 3d electrons in CuO, as this value is the most appropriate for addressing the band gap issue in CuO, which was taken from previously reported papers.^81,84 However, most previous studies have used U_eff = 7 eV, whereas Su et al.⁸⁵ increased the U_eff to 9 eV to better match the experimental band gap. The projected density of states (PDOS) in Fig. 7b provides meaningful deep insight into the orbital contributions of different elements in CuO. The valence band is predominantly formed by oxygen (O 2p) states, while the conduction band is mainly composed of copper (Cu 3d) states, confirming strong hybridization between Cu and O atoms. The PDOS plot also distinguishes between spin-up and spin-down states, demonstrating possible magnetic interactions within the material. The total DOS shows a distinct energy gap, supporting the semiconducting behavior observed in the band structure. The presence of Cu d-orbitals near the conduction band edge suggests that CuO has strong electronic correlations, which may influence its electronic properties. This characteristic makes CuO a potential candidate for applications in photocatalysis and energy storage devices. Also, the spin-polarized nature of the DOS suggests that CuO may have interesting magnetic properties, which could be useful for spintronic applications. Overall, the DFT-calculated band structure and DOS of CuO confirm its role as a correlated semiconductor with mixed orbital contributions and hybridization between Cu and O orbitals plays a critical role in shaping its electronic properties.

Photoelectrochemical characterization

To investigate the photocurrent response of the pristine and Pt-CuO photocathodes under illumination and applied potential, chopped-light voltammetry (CLV) analyses were conducted in a phosphate buffer solution. As shown in Fig. 8, CLV data revealed that during the 1st cycle (Fig. 8a, b and S11), the photocurrent density constantly decreases as the Pt coating time increases. Specifically, the photocurrent density decreases from −2.4 mA cm⁻² to −1.0 mA cm⁻² to −0.25 mA cm⁻² at 0.4 V vs. RHE for the CuO samples deposited for 0 s, 40 s, and 100 s, respectively. Since it was reported that the photoresponse under identical operational conditions of sol–gel-derived nanoporous CuO thin films was solely assigned to photocorrosion,¹⁴ it is likely that the underlying trend of the dependence of photocurrent on the deposition time is based on suppressed surface photodegradation due to the presence of Pt as a protective layer. In other words, the thicker the Pt layer, the lower the observed photoresponse due to less photocorrosion. Interestingly, an opposite trend emerged in the 3rd CLV cycle (see Fig. 8c, d and S11), demonstrating that samples that underwent longer Pt deposition periods (e.g. 80 s and 100 s) displayed increased photocurrent densities up to −0.9 mA cm⁻² at +0.4 V vs. RHE in contrast to samples subjected to shorter Pt deposition periods (e.g. 0 s and 10 s). This observation suggests that for pristine CuO thin films, the photocurrent density is mainly assigned to photocorrosion, which proceeds much faster than for unprotected CuO samples. However, for increasing Pt layer thickness (here from 10 s to 100 s), the photocurrent density response continuously increases during the third cycle, which might be interpreted as a decelerated photocorrosion process.

Fig. 8 (a) Photocurrent density analysis and (b) corresponding plots of the current density at 0.4 V vs. RHE as a function of the Pt deposition time at the 1st cycle. (c) Photocurrent density analysis and (d) corresponding plots of current density (at 0.4 V vs. RHE) vs. Pt deposition time at the 3rd cycle. All samples were measured in 1 M Na₂SO₃ + 0.2 M Na₂SO₄ electrolyte using 0.1 M KH₂PO₄ and 0.1 M K₂HPO₄ as buffer; the applied light source was a white light LED with a light period time of 10 s and intensity of 1000 W m⁻².

It can be concluded that the addition of a Pt coating on the surface protects the CuO from photocorrosion, and thus reduces the photodegradation rate, expressed by slower photocurrent densities. To systematically examine the photostability of the CuO-based photocathodes, chronoamperometry under irradiation was conducted for the pristine CuO and 80 s Pt-deposited CuO thin film (Fig. 9). Since the 80 s Pt-CuO sample showed the highest photocurrent density after the 3rd CLV (see Fig. 8c and d), indicating the most stable performance, it was used as a control sample. The photocurrent densities of pristine and 80 s Pt-CuO thin films were measured over 30 minutes at a fixed potential of 0.6 V vs. RHE under white-light illumination, as illustrated in Fig. 9. It is shown that the photocurrent density of pristine CuO, initially reaching 1.0 mA cm⁻² at 0.6 V vs. RHE, declines rapidly over the course of 30 minutes to 0.02 mA cm⁻², which is only 2.9% of the initial photocurrent density, indicating severe photodegradation already after 10 minutes under operational conditions. Compared to this, the 80 s Pt-CuO retained over 57% of the initial photocurrent density after 30 minutes, allowing the assumption that the Pt indeed protects the CuO surface against photocorrosion, which, however, is still likely to occur considering the energy band positions of CuO and Pt-CuO that straddle the reduction potentials of Cu²⁺/Cu⁺ and Cu⁺/Cu, and additionally, the presence of an ill-defined nanoporous CuO network (see Fig. 2).

Fig. 9 Photocurrent density as a function of time for pristine and 80 s Pt-deposited CuO thin films recorded at 0.6 V vs. RHE and measured in 1 M Na₂SO₃ + 0.2 M Na₂SO₄ electrolyte, including a phosphate buffer. The applied light source was a white light LED using intermittent light mode with a period of 10 s and an intensity of 1000 W m⁻².

To verify if the photodegradation process is correlated with the surface area of CuO-based photocathodes, the electrochemical surface area (ECSA) in the dark was determined based on the scan rate-dependent cyclic voltammetry analysis (Fig. S12a and b) according to McGrory et al.⁸⁶ As illustrated in Fig. S12c, the electrochemical double-layer capacitance of pristine CuO and 80 s Pt-CuO was calculated to be 0.016 mF and 0.029 mF, and the corresponding ECSAs were calculated to be 0.4 cm² and 0.725 cm², respectively, assuming a specific capacitance of 0.040 mF cm⁻² (typical value for the specific capacitance of CuO in alkaline solution). Note that the specific capacitance values within 0.022–0.130 mF cm⁻² have been reported for transition metal oxides in alkaline solutions, and the obtained ECSA may vary accordingly.⁸⁶ The 80 s Pt-CuO sample showed a significantly higher ECSA compared to the pristine sample, confirming that the surface area was affected by deposition of Pt through formation of (agglomerated) nanoparticles that increased the density of electrochemically active and accessible sites. These findings are in agreement with the surface analyses made by SEM (Fig. 2) and AFM (Fig. S5), presenting smoother surfaces, but higher roughnesses of the Pt-CuO samples. The predominant role of the deposited platinum is most likely the passivation of CuO surface sites, which are prone to photoreduction. As a consequence, the Pt-CuO samples were found to be more (photo-)stable than pristine CuO (Fig. 8, 9 and S11).

To further probe the electrocatalytic implications of Pt deposition, linear sweep voltammetry (LSV) measurements (see Fig. S12d) were performed. The pristine CuO sample produced a current density of −9.1 mA cm⁻² at −0.4 V vs. RHE, whereas the current density was increased up to −13.1 mA cm⁻² for Pt-CuO under identical conditions. However, and importantly, according to Fig. S10d, platinum only shows an additional electrocatalytic effect/higher current density for potentials below 0.175 V vs. RHE, suggesting that Pt as a cocatalyst does not contribute catalytically to the observed dark currents (≡light off in CLVs) in Fig. 8 and 9.

To confirm the evolution of hydrogen, chronoamperometry was performed under illumination and at a fixed potential of 0.45 V vs. RHE under otherwise the same experimental conditions. The accumulated gas volumes, produced by the pristine and Pt-CuO samples upon PEC operation for 20 min, were detected by gas chromatography (GC) (Fig. S13). The average detected gas volume of pristine CuO and Pt-CuO thin films was evaluated to be 4.6 µL and 4.2 µL, respectively. Compared with the 4.1 µL of detected gas in the background measurement (recorded without utilizing any photoelectrode), the pristine CuO presents a very low photocathodic activity, which, however, was still analyzed on a very low absolute scale (a few µL of detected gas). On the other hand, the gas evolution of the Pt-CuO sample showed a negligible increase compared to the background measurement. The Pt-coating on CuO does not appear to have a positive impact on the gas evolution at potentials above 0.175 V vs. RHE (see Fig. S12d) for 20 min of operation time, which is most likely related to the overall very low gas concentrations produced by the Pt-CuO samples (within the detection limit of the instrument). As a conclusion, even though deposition of Pt layers was presented to slow down the overall photoinduced degradation effect on/in nanoporous CuO photocathodes (see Fig. 9) – based on the underlying data – it does not facilitate the production of hydrogen at 0.45 V vs. RHE significantly, when compared to pristine CuO thin films. Further research is indicated to maintain the protective effect of Pt-coatings for suppression of the photodegradation process, while simultaneously, the solar-to-hydrogen efficiency is further improved.

Conclusions

In this study, a dip-coating route is presented using citric acid as a complexing agent to obtain nanoporous CuO photocathodes with uniform structures independent of the atmospheric humidity. The effect of sputter-coated platinum as a protective layer on CuO thin films used for photoelectrochemical applications was studied. Structural characterization (SEM, AFM, XRD, Raman, and XPS) confirmed the formation of a nanoporous network containing pores averaging 30–40 nm in size, crystallization of the pore walls in a monoclinic structure, and uniform coverage of Pt at the surface. UV-vis spectroscopy revealed an indirect optical band gap energy of 1.5 eV, while the effective electronic band gap energy was calculated to be ∼1.0 eV by DFT. Deposition of Pt on CuO led to enhanced light absorption, particularly in the near-infrared region due to plasmonic effects. PEC measurements revealed that Pt-CuO retains over 57% of its initial photocurrent compared to 2.9% observed for pristine CuO after 30 minutes of chronoamperometry experiments. An optimized platinum thickness of 20 nm was identified to balance effective protection and light absorption of the photocathode, resulting in significantly suppressed surface photocorrosion and a twenty-fold enhancement in PEC photostability compared to unprotected CuO. Mott–Schottky analysis showed a charge carrier density of 10²⁰ and 10²² cm⁻³ for pristine and Pt coated CuO photocathodes, respectively, indicating a substantial increase in acceptor density owing to the presence of Pt. The flat band potential was determined to be 1.1 V vs. RHE independent of Pt coating. PEC characterization suggested that the Pt layers act as electron sinks, facilitating charge separation and suppressing electron–hole recombination in the space charge region. Despite the enhanced photostability in alkaline media, no improved hydrogen evolution could be detected, most likely owing to low absolute gas evolution produced by the low amounts of active mass provided by the mesoporous CuO framework. In conclusion, the sputter-coated Pt acts as a multifunctional material, enhancing absorption of light within 400 nm to 1100 nm, promoting spatial charge carrier separation between CuO and Pt via band-edge alignment at the interface, and improving the photostability of CuO cathodes by preventing direct physical electrolyte contact.

Conflicts of interest

There is no conflicts of interest to declare.

Data availability

All raw data and files used for this paper are openly accessible in the Zenodo repository: https://doi.org/10.5281/zenodo.20763676.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d6na00111d.

Acknowledgements

Marcus Einert acknowledges funding by the German Federal Ministry of Research, Technology, and Space (BMFTR), project no. 033RC036, and the support from the China Scholarship Council (CSC), project no. 202208320036 (Qingyang Wu). The authors appreciate the support of Jean-Christophe Jaud (Department of Structural Research, Institute of Materials Science at TU Darmstadt), Kerstin Sticker (Soil Mineralogy and Soil Chemistry Department, Institute for Applied Geosciences at TU Darmstadt), and Barbara Gonzalez-Navarrete (Department of Surface Science, Institute of Materials Science at TU Darmstadt) for conducting and/or assisting with XRD, Raman spectroscopy, and AFM measurements, respectively.

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