An Efficient CuxO Photocathode for Hydrogen Production at Neutral

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An efficient CuxO photocathode for hydrogen production in neutral pH: new insights from combined spectroscopy and electrochemistry Tomasz Baran, Szymon Wojty#a, Cristina Lenardi, Alberto Vertova, Paolo Ghigna, Elisabetta Achilli, Martina Fracchia, Sandra Rondinini, and Alessandro Minguzzi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03345 • Publication Date (Web): 29 Jul 2016 Downloaded from http://pubs.acs.org on July 29, 2016

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ACS Applied Materials & Interfaces

An efficient CuxO photocathode for hydrogen production in neutral pH: new insights from combined spectroscopy and electrochemistry Tomasz Baran,a Szymon Wojtyła,b Cristina Lenardi,c Alberto Vertova,a,d Paolo Ghigna,d,e Elisabetta Achilli,e Martina Fracchia,e Sandra Rondininia,d and Alessandro Minguzzia,d * a. Dipartimento di Chimica Università degli Studi di Milano, via Golgi 19, 20133 Milan, Italy b. Department of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krakow, Poland; and SajTom Light Future, Czaniec, Poland c. CIMAINA, Dipartimento di Fisica, Università degli Studi di Milano, Via Giovanni Celoria, 16, 20133 Milan, Italy d. INSTM, Consorzio Interuniversitario per la Scienza e Tecnologia dei Materiali, Via San Giusti 9, Firenze. e. Dipartimento di Chimica, Università degli Studi di Pavia, Viale Taramelli 13, 27100, Pavia, Italy

KEYWORDS. Copper oxide, photocathode, hydrogen evolution reaction, photoelectrochemical water splitting, scanning electrochemical microscopy, XANES, EXAFS, XPS

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ABSTRACT. Light driven water splitting is one of the most promising approaches for using solar energy in view of a more sustainable development. In this paper, a highly efficient p-type copper(II) oxide photocathode is studied. The material, prepared by thermal treatment of CuI nanoparticles, is initially partially reduced upon working conditions and soon reaches a stable form. Upon visible light illumination, the material yields a photocurrent of 1.3 mA cm-2 at a potential of 0.2 V vs RHE at mild pH under illumination by AM 1.5G and retains 30% of its photoactivity after 6 hours. This represents an unprecedented result for a non-protected Cu oxide photocathode in neutral pH. The photocurrent efficiency as a function of the applied potential was determined using scanning electrochemical microscopy - SECM. The material was characterized in terms of photoelectrochemical features; XPS, XANES, FEXRAV and EXAFS were carried out on pristine and used samples and allowed to explain the photoelectrochemical behavior. The optical features of the oxide are evidenced by DRS and fluorescence spectroscopy, while Mott-Schottky analysis at different pH explains the exceptional activity at neutral pH.

INTRODUCTION The progressive lack of fossil fuels and environmental pollution boosts the search for a more efficient employment of renewable energy sources. In this respect, solar energy has been demonstrated to be storable in the form of energy vectors (solar fuels), such as H2, produced through photoelectrochemical (or photocatalytic) water splitting, or methane (and other C1 compounds) produced via CO2 reduction.1–3 The use of sunlight to convert water into fuel is very attractive and ambitious since H2 is considered to be the energy carrier of the future due to its high mass energy density and its environmental friendliness.1 Water splitting consists in the


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Hydrogen Evolution Reaction (HER) at the cathode and the Oxygen Evolution Reaction (OER) at the anode. In the perspective of attaining an efficient hydrogen production from sunlight, the possibility of carrying it out in neutral media further improves the benefits of this technology. In photoelectrocatalytic water splitting, the anode and/or cathode of a cell are constituted by photoactive materials, usually semiconductors. Promising, efficient and stable photoelectrocatalytic cells for water splitting should cope with the following criteria: (i) thermodynamics of light absorption and charge transport at Semiconductor/Electrolyte Junction (SEJ); (ii) band gap energy, band bending, and bands edge potentials; (iii) kinetics of charge transfer at the SEJ; (iv) charge recombination processes in the bulk and at the interfaces; (v) stability of the photoelectrode; (vi) cost and abundance of the materials involved. N-type semiconductors such as Fe2O3, TiO2, BiVO4 or ZnO have been reported to be efficient photoanodes for OER.4 Photocathode materials have been largely less investigated in comparison with photoanodes. The most efficient materials, e.g. p-doped silicon, silicon carbide, nickel oxide, gallium arsenide phosphide or indium phosphide showed current density up to -30 mA cm-2 (for example in case of InP nanopillar in acid solution or nickel-coated Si in alkaline solution) but they have a number of shortcomings such as low stability, high cost or large band gap energy.5–10 In comparison, copper oxides-based photocathodes are characterized for displaying lower photocurrent densities but are attractive for their absorption in the visible range, low cost, high abundance and easy synthetic protocols.11–16 Copper oxide exists with two semiconducting compounds, CuO and Cu2O. Compared with CuO, Cu2O received noticeable more attention as a photocathode, due to its direct band gap equal to ca. 2 eV, which is suitable for water splitting


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under visible light irradiation, a suitable conduction band potential for hydrogen generation as well as theoretical photocurrent density of -14.7 mA cm-2 and a high light-to-hydrogen conversion yield of 18.7%.11,12 CuO is also a p-type semiconductor with a smaller band gap (1.2 – 1.5 eV)17 implying that CuO is in principle capable of generating a higher cathodic photocurrent density, amounting at ca. -35 mA cm-2.18 Moreover, copper(II) oxide has an appropriate conduction band alignment for water reduction to hydrogen with an edge potential in the range from −3.3 to −3.8 eV (pH 14) with respect to the vacuum.18,19 The efficiency values reported for copper oxide-based photocathodes are wide-ranging.20 Thin films of Cu2O and CuO incorporated as photocathodes in a photoelectrochemical cell achieved photocurrents of -0.28 mA cm-2 and -0.35 mA cm-2, respectively, at 0.05 V vs. a RHE (Reversible Hydrogen Electrode).18 A Cu2O photocathode covered with a thin layer of CuO showed photocurrent densities in the range from -0.05 to -0.82 mA cm-2 at 0 V vs. RHE depending on the preparation technique.21 Another example of copper oxides composite (Cu2O/CuO) showed a photocurrent density of -1.54 mA cm-2 at a potential of 0 V vs. RHE),11 with highly stable photocurrents, but it was tested only for 30 min. The highest current density (3.5 mA cm-2 , at 0.05 V vs. a RHE) was reported for branch-shaped cupric oxide, however its activity was evaluated only for 1200 s.22 Paracchino et al. reported photocurrents higher than -2.2 mA cm-2 for Cu2O, but the current density decreased dramatically within the first minutes.12 Here we show how CuxO composites can exhibit both high current densities and long-term stability. As expected from literature, neat CuO undergoes the photocorrosion processes.11 Therefore initial loss of photocurrent density of a photocathode composed mainly on CuO (due to its reduction to Cu2O) is not surprising. However, after an initial transient, we observe the simultaneous presence of CuO and Cu2O and the stabilization of the photocurrent. We suggest


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the onset of an electronic coupling between the two phases, where the charge carriers not transferred through the SEJ are conveniently transferred at the CuO/Cu2O interface, where recombination is likely favored and further oxidation/reduction of the material itself is prevented (see fig S1). These photocathodes show good efficiency toward hydrogen evolution in both neutral and alkaline environments. High photocurrent density (up to -1.3 mA cm-2 at mild potentials ≈0.2 V vs. RHE) and long-term stability in neutral pH make this material very promising for future applications. The material can be obtained through a simple procedure by direct deposition of copper(I) iodide on a FTO (Fluorine-doped Tin Oxide) glass, and a subsequent mild thermal treatment in air. The facile preparation of the cathodes and their reproducibility are among the major advantages of the proposed system.

EXPERIMENTAL SECTION Synthesis Copper iodide was synthesized according to the following procedure. 100 ml of aqueous 0.01 M Na2SO3 were added to 60 ml of 0.05 M CuSO4 while stirring. 300 ml of aqueous 0.01 M KI were then added dropwise to the resultant green suspension. The obtained a white precipitate (CuI nanoparticles) was separated by centrifugation, washed 3 times with water and ethanol, and dried in air at about 80°С for 8 hours. The CuI suspension was prepared in ethanol (3 mg ml-1) by sonication for 5 minutes. Subsequently, 100 µl (if not stated otherwise) of the suspension were drop-casted on a FTO


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(Sigma Aldrich, surface resistivity ~7 Ω/sq) glass plate, previously washed with H2SO4, water and ethanol under ultrasounds. The electrodes were then dried in air at room temperature and subsequently annealed in air at 400 °C (if not stated otherwise). Highly active materials are obtained only by placing samples directly in the oven when the temperature is stabilized at 400 °C. The resulting photoelectrodes will be denoted CuxO-E within the manuscript. Characterization of materials X-ray Absorption Spectra (XAS) were acquired in the fluorescence mode at the LISA beamline at ESRF (European Synchrotron Radiation Facility) at the Cu K-edge. A Si(311) double crystal monochromator was used; the harmonic rejection was realised by Pd mirrors with a cut-off energy of 20 keV, and a High Purity Germanium fluorescence detector array (13 elements) was employed. The energy calibration was performed by measuring the absorption spectrum of metallic copper foil at the Cu K-edge (Cu-K: 8979 eV). All data were obtained at room temperature. Spectra of CuO and Cu2O, used as standard materials, were acquired in the transmission mode. For the measurement a proper amount of sample (as to give a unit jump in the absorption coefficient) was mixed to cellulose and pressed to pellet. The signal extraction was performed by means of the ATHENA code.23,24 The EXAFS (Extended X-ray Absorption Fine Structure) data analysis was performed by using the Excurve code, using a k2 weighing scheme and full multiple scattering calculations.25 The goodness of fit (GOF) is given by the Ffactor parameter: F = 100 ∑

[, , ]²

(1)


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For the X-ray Absorption Near Edge Structure (XANES) analysis, the raw spectra where first background subtracted using a straight line, and the normalized to unit absorption at 800 eV above the edge energy, where the EXAFS oscillations are not visible anymore. FEXRAV (FixedEnergy X-ray Absorption Voltammetry) experiments were carried out in dark or under illumination by a 400 nm light emitting diode (LED). The electrode potential was swept in the range 0.65 to -0.8 V vs RHE, with a scan rate of 1 mV s-1. Sample were analyzed by fixing the X-ray energy at 8981 eV, corresponding to the first maximum in the Cu2O absorption edge. X-ray photoelectron spectroscopy (XPS). The electrodes were characterized in a Leybold LHS 10/12 UHV apparatus equipped with a hemispherical electron analyzer and a conventional X-ray source (Mg Kα = 1253.6 eV). The high resolution spectra were acquired in constant step energy mode, with Epass= 30 eV. The overall energy resolution was 0.9 eV. The pressure in the experimental chamber during experiments was below 5·10-9 mbar. All spectra are scaled to the Fermi level and the binding energy scale is calibrated via the Au 4f5/2 core level line (located at 88.5 eV) of a clean polycrystalline Au sample. Charging effects of the samples under investigation were observed during the measurements. The spectra were aligned to adventitious carbon peak, to which a value of 285.0 eV is assigned. This in turn implies the shift of all spectra of about 10 eV. The line shapes were fitted with mixed singlets obtained by a linear combination of a Gaussian and a Lorentzian profiles sited on a Shirley background. UV-Vis Diffuse Reflectance Spectra (DRS) were recorded using a UV-3600 spectrophotometer (Shimadzu) equipped with an integrating sphere (15 cm diameter). Barium sulfate was used to dilute the sample and as a reference material. Samples were grounded together with the desired amount of BaSO4 used as internal standard (1:50 wt. ratio). The commercial samples used for


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DRF are: CuO (+99%, Acros), Cu2O (POCH), BaSO4 (dried, Sigma-Aldrich). The spectrum of CuxO-E was obtained directly form the electrode. Solid state photoluminescence spectra were collected at ambient temperature using a FluoroLog-3 (Horiba) spectrofluorimeter equipped with a R928 photomultiplier tube (Hamamatsu) and a cut-off filter. Electrochemical Impedance Spectroscopy (EIS) was performed in the following N2-saturated aqueous electrolyte solutions: 1) 0.1 M K2HPO4, pH=9, 2) 0.1 M K2HPO4 + KH2PO4, pH=7, 3) 0.1 M potassium hydrogen phthalate, pH=4. EIS was recorded at a constant frequency while sweeping the electrode potential using a 1260A Impedance/Gain-Phase Analyzer coupled to a 1287A potentiostat (Solartron). Photoelectrochemistry experiments A three-electrode quartz cell equipped with a home-made light chopper (frequency 0.25 Hz) was used in PEC measurements, applying potentials profile at the photocathode equivalent to linear or cyclic voltammetry. CuxO-E were the working electrode and a saturated calomel electrode (SCE) and a platinum plate were used as the reference and the counter electrodes, respectively. A solar simulator (LOT-Oriel, AM 1.5 - Air Mass filter) or a LED (λ = 400 nm, LED ENGINE LZ1-00UA00) was used as light source. The electrolyte was bubbled with nitrogen prior to each measurement and a gas blanket was kept on top of the solution during all experiments Concentration, composition and pH of the electrolytes are given together with the results of each experiment. Scanning electrochemical microscopy


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The substrate-generation/tip-collection mode of scanning electrochemical microscopy (a CH Instrument 920c) was used for the photocurrent efficiency determination. In this experiment, a 63 μm diameter Pt microdisk tip is approached until the substrate-tip distance was set to 20 μm by means of approach curves in the presence of oxygen (negative feedback). The substrate electrode is identical to those described in the manuscript, with the exception that only a fraction of the surface is exposed to the electrolyte solution (0.1 M K2HPO4 + KH2PO4, pH = 7 or 0.1 M K2HPO4, pH=9) by means of a 3.5 mm radius o-ring. The setup includes a glassy carbon and a AgCl/Ag (in 3 M KCl) as the counter and reference electrode, respectively. After the approach, N2 is bubbled in the solution for 15 minutes and the substrate potential was scanned under chopped illumination by a 400 nm LED, while the tip was held at 0.65 V vs. RHE, or both substrate and tip were held at constant potential, 0.65 or 0.45 V vs. RHE and 0.65 V vs RHE respectively.

RESULTS AND DISCUSSION Calcination of nanocrystalline copper iodide (particle sizes in the range 10 to 40 nm – EDX analysis and TEM images presented in figure S2) directly on FTO glass, under the conditions described in the Experimental section, leads to a dark brown, mechanically stable electrode. This thermal treatment of CuI leads directly to CuO, as demonstrated by XRD (see figure S3) and confirmed by XPS, XANES and EXAFS measurements (see below). The behavior of the precursor (CuI) during the heat treatment was investigated by TGA analysis as shown in figure S4. Using CuI as starting material guarantees the formation of stable, nanostructured particles


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and allows the formation of the oxide after calcination, avoiding the use of complex synthetic pathways. As anticipated in the Introduction, CuxO-E photocathodes result to be active in neutral pH. A typical linear sweep voltammetry registered at pH 7 under chopped irradiation using the solar simulator as a light source is shown in figure 1. Irradiation of the photoelectrode leads to a cathodic photocurrent in the whole range of the considered potential window. The photocurrent increases with decreasing applied potential reaching the value of –1.2 mA cm-2 at 0.1 V. The current sign confirms the p-type nature of the CuxO electrode.26 It has been determined that the optimal temperature for CuxO-E preparation was 400°C (see Fig. S5). Figure 1 also shows cyclic voltammetries in dark and illumination conditions for the CuxO-E at pH = 7, recorded between 0.3 and 1.3 V vs. RHE: while the CV in the dark presents a flat characteristics, upon irradiation (λ = 400 nm) a cathodic photocurrent appears for potentials lower than about 0.80 V vs. RHE. As it will be described below, this potential is quite close to the valence band edge, since the latter is located just below the Fermi level in p-type semiconductors. The “bumps” observed in the cathodic scan are likely due to the onset of photoreduction of CuO to Cu2O, a mechanisms that possibly leads to the exceptional stability of the final CuxO material. During the anodic scan, the positive peak suggests the re-oxidation of Cu(I) to Cu(II).


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Figure 1. Current density-potential curves of CuxO-E in 0.1 M K2HPO4 + KH2PO4 (pH=7) electrolyte. A - cathodic potential sweep under chopped AM 1.5 light illumination, B - cyclic voltammetry in dark and under irradiation.

The CuxO photocathode also showed an excellent photoresponse under LED (λ=400 nm) irradiation in a broad range of potentials from 0.65 V to -0.1 V vs RHE as shown in figure S6. The onset of a dark current is evident at about E=0.4 V. The photocurrent density at 0 V vs RHE was -2.5 mA cm-2, but was accompanied to an intense current density in the dark. The PEC performance of the CuxO-E composite photocathodes (figure S7) evaluated at 0 V vs. RHE with chopped illumination at 400 nm with N2 bubbling shows decent stability in the time scale of the


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experiment (no decrease in current density after 1 h) and higher photocurrent density (up to 2.44 mA cm-1 while the dark current varies from 0.42 to 0.52 mA cm-1) in comparison with PEC experiment at higher potential (0.35 V or 0.44 V vs. RHE, see below). In order to verify the effective production of H2 and to evaluate the efficiency, we carried out scanning electrochemical microscopy (SECM) experiments. The setup is schematically described in figure S8. In a typical substrate generation/tip collection experiment, the product generated at the substrate can be partially collected by the using a second electrochemical reaction. In the present case, the tip was held at 0.65 V vs. RHE, a potential at which H2 can be oxidized with a mass transport limited kinetics. The substrate was held at constant potential (0.65 V or 0.45 vs. RHE) or scanned under chopped illumination at 400 nm. The tip current is proportional to the local concentration of H2 according to the following equation: (2)

Itip=4nFCDr

where I is the tip current intensity, n the number of exchanged electrons, F the Faraday constant, C and D the concentration and the diffusion coefficient of the reacting species (H2), respectively and r the electrode radius. The tip then acts as an amperometric sensor with a very short response time (of the order of ms), due to its micrometric dimensions.27 As evident from the figure 2 and the figure S9, the tip current increases when a photocurrent is observed at the substrate. According to the data shown in figure S9, a net increase of the photocurrent efficiency is observed at about 0.7 V, but decreases as soon as a dark current is observed (below 0.4 V). The current efficiency can be determined by comparison with a reference system. In a preliminary experiment, we adopted the same setup, with the exception of


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using a Pt plate instead of CuxO substrate. The potential at the substrate is kept constant until the current is stable and the tip current is then registered. In this way, we could determine the ratio between the tip and substrate currents when the current efficiency for H2 evolution at the substrate is 100% while maintaining the same experimental conditions (cell type and dimensions, electrolyte, tip, counter and reference electrodes). The tip-to-substrate current ratio obtained based on this experiment is 1.2·10-4. Another approach to determine the collection efficiency is through finite elements simulation, that we carried out using COMSOL Multiphysics® and that is described in the supporting information (Fig. S10). This approach was used to further confirm the calibration experiment and output a tip/substrate current ratio of 0.9·10-4, being in excellent agreement with the experiment. This approach has to be considered as a direct method for the hydrogen detection, since hydrogen is detected after its production at the electrode and by means of its oxidation reaction. This allows to overcome the limitation of other methods (e.g. chromatographic determination). In fact, the use of SECM allows to determine the presence of the reactant of interest with high selectivity (other species released in solution are easy spotted by means of a tip voltammetry), quantitative (provided that a calibration experiment is carried out) and in real time (the tip response is rather quick and in the present case depends on the diffusion of the reactant from the sample) thus allowing to carry out determination during potential or light on/off cycles. The evaluation of the photocurrent efficiency was performed at 0.65 and 0.45 V vs. RHE, as shown in figure 2. The tip current increases during illumination, when the substrate photocurrent appears, because of H2 oxidation at the tip. The ratios between tip and substrate currents were: 3.7 10-5 for the measurement at 0.45 V and 7.1 10-5 in the case of potential 0.65 V. Considering the


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tip/substrate current ratio obtained by calibration, the faradaic efficiency for the photoelectrochemical generated H2 is 30% at 0.45 V and 59% at 0.65 V vs RHE.

Figure 2. Substrate and tip current as a function of time in dark/light mode (illumination with LED λ = 400 nm). Tip potential: 0.65 V vs RHE, substrate potential: A. 0.65 or B. 0.45 V vs RHE in 0.1 M K2HPO4 + KH2PO4 (pH = 7). Red arrows indicate the time chosen to determine the faradaic efficiency.

In view of better understanding the exceptional activity observed at neutral pH, we point to define the role of pH in the photoelectrochemical activity, and it is necessary to determine the flat band potential in the various electrolytes considered in the present work. Mott–Schottky measurements (by EIS) were employed to this aim. For p-type semiconductors the capacitance of the space charge layer depends on the applied potential according to the equation

!

"#

$ (3)

where ε is the electric permittivity of the sample, ε0 is the vacuum permittivity, e is the charge of an electron, N the is hole concentration, V is the applied potential, VFB is the flat band potential, k


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is the Boltzmann constant, T is the temperature and Csc is the capacitance of the space charge layer.28 Csc has been derived from EIS fixed frequency potential scans as follows

%&' () "

(4)

where ω is the frequency and Z” is the impedance imaginary part. For each electrolyte, the determination of VFB was carried out by Mott-Schottky analysis considering three values of ω. An example, for pH=7, is shown in figure 3.

Figure 3. Mott-Schottky plots at a fixed frequencies (10, 500 and 1000 Hz) on CuxO-E photoelectrode registered in 0.1M K2HPO4 + KH2PO4 electrolyte (pH = 7). Flat band is an average of three values obtained by extrapolation of plots.

The Mott-Shottky plot exhibits a negative slope, confirming the p-type nature of the semiconductor. The value of Vfb obtained at pH=7 is 0.33 V vs SCE, corresponding to 0.97 V vs.


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RHE. Considering the band gap energy (EBG = 1.31 eV, see below), it follows that the Conduction Band (CB) edge lies at -0.34 V vs. RHE. In aqueous solution, the flat band potential of oxide semiconductors shifts by 0.059 V per pH unit due to the fact that protons are potentialdetermining ions. Surprisingly, as shown in figure 4, the slope of lines interpolating the MottSchottky results indicate that the material behavior deviates from the expected value and assumes the value of 0.093 V/pH unit. Similar, non-nernstian (or so-called “super-nernstian”) behavior (pH sensitivity larger than 60 mV) was reported in literature for iridium oxide and hematite electrodes.29,30

Figure 4. Conduction and valence bands in various pH values determined from Mott-Schottky analysis and diffuse reflectance spectroscopy.


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As the equilibrium potential of the H2O/H2 redox couple lies at 0 V vs. RHE at pH=1, the conduction band edge potential of the material is more negative than the redox potential of H2O/H2 and photogenerated electrons can thus be transferred to reduce water to hydrogen. Therefore, the studied material can be employed as a photocathode for photoelectrochemical water splitting, provided that an external bias is given to compensate for the non-correct position of the valence band with respect to the O2/H2O couple and for the insufficient value for the band gap of the material (1.31 eV, see below), that is in principle high enough considering the thermodynamics, but not to overcome the overpotential of interface reaction(s). On the other hand, photogenerated electrons are able to reduce the semiconductor itself since VCB is more negative than the potential of the CuO/Cu2O redox couple (0.84 V vs. RHE).31 The dependence of CuxO-E flat band potential on pH can be attributed to the redox processes involving oxygen-containing surface groups, which participate in acid–base equilibria when in contact with aqueous solution, in turn influencing the potential drop across the double layer. Accordingly, the photoactivity of the studied photocathode also depends on pH. Irradiation of electrode under solar simulated light (AM 1.5) at pH = 4, 7 and 9 leads to various efficiencies of light conversion as shown in figure 5. Note that similar experiments were carried out while bubbling N2 in solution during the measurements. Neglecting the obvious increase of noise, the value of the observed photocurrent is very similar, thus excluding the contribution of O2 reduction. The most important outcome from figure 5 is that photocurrent densities in alkaline and neutral pH are similar but significantly higher than in the case of the acidic electrolyte. In fact, at pH 4 (i.e. in presence of a higher concentration of H+ that could in principle improve the kinetics), the location of the conduction band edge corresponds to a lower overpotential for H2 evolution with respect to higher pH values. The opposite happens for the oxygen evolution


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reaction at the counter electrode but one has to consider that the latter reaction is typically favored at increasing pH.32 Furthermore, as shown in figure 5 (inset) and particularly in the case of pH 9, the characteristic overshot of photocurrent (spiked cathodic pulses) can be interpreted in terms of trap-mediated electron/hole recombination followed by slow detrapping.33,34 Trap-mediated recombination is one of the proposed recombination models responsible for carrier annihilation in semiconductors (other models include band-to-band recombination and Auger recombination). Trap-assisted recombination occurs when an excited electron from the conduction band indirectly recombines with a hole from the valence band at a trap state.35 The neutral (not charged) trap may capture an excited electron from the conduction band resulting in an energy loss (as light or heat) but also may capture an electron from the valence band thus it acquiring a negative charge. As a consequence, the negative trap may easily capture an excited hole from the valence band.36 The photocurrent due to charge recombination sums to the one resulting in the chemical reaction but the first decreases exponentially and eventually becomes negligible when surface traps are saturated. When the light is turned off, the carriers trapped on surface traps continue to recombine thus causing an inversion of the current sign since no minority carriers are produced and only majority ones (holes, for p-type semiconductors) can reach the traps.37 In the present case, the less significant spikes observed at neutral pH suggests a less effective recombination in this medium.


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Figure 5. Influence of pH on the photocurrent generation. Linear potential sweep in 3 various electrolytes: pH 4, 7 and 9. Inset: zoom on selected potential range. Light source: LED, λ = 400 nm.

The photocurrent density strongly depends on the wavelength of incident light. The Incident Photon to Current Efficiency (IPCE) of CuxO-E photocathodes was measured at 0.355 V vs RHE (chronoamperometric measurements for each wavelength- integrated currents are summarized in table S1) at pH = 7 (1 M KClO4) using the following equation IPCE /%1

23 ∙567 8 ∙ 9

$ ∙ 100

(5)

where λ is the wavelength of the incident light, Iph and J are the measured photocurrent density and the measured irradiance at the selected wavelength, respectively. As shown in figure 6A, IPCE exceeds 22% for λ lower than 450 nm.


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Figure 6. Influence of incident light wavelength on the activity of CuxO-E in 0.1 M KClO4 +KOH, pH = 7 (to avoid specific adsorption of PO43-). A – Incident photon to current efficiency (IPCE) measured at 0.355 V versus RHE for CuxO-E and its precursor (CuI); B – dependence of current density on wavelength and applied potential. Both, A and B, under AM 1.5 light illumination.

The IPCE vs. λ curve does not correspond to the absorption spectrum of the material (fig. 7A), as IPCE unavoidably includes the contribution of light intensity, that varies with the wavelength.


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In addition, the dependence of photocurrent intensity on incident light wavelength and applied potentials, as shown in Fig. 6B, proved that the studied material (CuxO-E) is active in the broad range of wavelengths and the largest current density was observed for 550 nm. The influence of incident light wavelength on the activity of the material is strongly related with its optical properties. To study these features, UV–Vis reflectance and emission spectroscopy has been used. Diffuse reflectance spectra of CuxO-E, as well as spectra of commercially available Cu2O and CuO were transformed to the Kubelka-Munk function (KM), defined as: /?@ 1

/ AB 1 AB

C/DE1

(6)

F

where R∞ is the diffuse reflectance referred to a layer of infinite thickness, S the scattering coefficient and α(hν) the absorption coefficient. The results are shown in figure 7. The spectrum of CuxO-E shows a broad absorption range, similar to that of copper(II) oxide, with the absorption onset slightly shifted towards higher photon energies. The band gap was estimated from the Tauc plot ([FKM(R∞) E]n vs. E) shown in Figure 7B since /GℎI1J K/ℎI L!M 1

(7)

where E is the photon energy and EBG the band gap, while the exponent n depends on the inter band transition mechanism, being n=2 or n=1/2 when direct or indirect transitions are allowed, respectively.38 The values of EBG of commercial copper oxides (derived using the presented above DRS methods), summarized in Table 1, are in a good agreement with literature data:18 this supports the reliability of the results reported here for CuxO-E (1.31 eV).


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Figure 7. Diffuse reflectance spectra of the materials CuxO-E, and commercial Cu2O and CuO transformed to the Kubelka-Munk function (A) and the method of band gap energy estimation from Tauc plot in case of CuxO-E (B).

Table 1. Band gap energies of studied materials estimated from Tauc plot.

material

index n

EBG ± 0.01 eV

CuxO-E

1/2

1.31 eV


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Cu2O

2

2.05 eV

CuO

1/2

1.23 eV

Moreover, the optical properties of the CuxO-E have been studied also by means of solid state fluorescence spectroscopy. Fig. 8 presents the emission spectrum of CuxO-E, with the photocathode excited at 400 nm: the strong luminescence peak at about 460 nm is related with the band gap transition. The material also shows weak emission bands in the red light range (with a maximum at ca. 700 nm). According to literature this band could be related to the recombination of electrons and holes at vacant sites, the so-called specific surface effect.39 Copper oxide is a p-type semiconductor due to the existence of Cu vacancies thus the radiative recombination involving acceptor level defect centers is likely at the basis of the observed emission peak.

Figure 8. Solid state emission spectrum of CuxO-E measured with λexc = 400 nm with a cut-off filter 420 nm.


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It is well-known that photocatalytic properties strongly depend on the material’s morphology, particle size and structure. For example, the crystallite size influence photoactivity through changes in surface area, light scattering and light absorptivity (including quantum size effects). Particles of size ≈ 30 nm are too small to invoke the presence of a charge depletion layer - no band bending has to be considered because of the small size of the crystallites making up the electrode. However, transitions from micro- to nano-sized particles result in increases in surface energy and affect the interaction of water molecule and stabilization of charge carriers at the surface. Charge carrier trapping at surface sites depends on the particle size, with smaller particles exhibiting less stable surface sites for electron trapping.40 In the present case, the material is highly crystalline and the average crystallite size calculated using Scherrer equation from XRD data (figure S3) is 30 nm. The structure of the sample was first investigated using XAS. Figure 9 shows the XANES spectra of CuxO-E as well as CuxO-E used as a photocathode (working conditions: after 8 h of irradiation with λ = 400 nm while biased at 0.3 V vs. RHE). Comparison with standard CuO and Cu2O shows immediately that the freshly prepared electrode is composed only of copper(II) oxide: this evidences that the annealing procedure effectively oxidizes CuI to CuO. After its activity upon photocathodic conditions, the composition of the sample changes: CuO is partially reduced to Cu2O. Indeed, the XANES spectrum of the sample kept under working conditions can be fitted as a linear combination of the spectra of Cu2O and CuO, the weights of the linear combination being 0.8(1) and 0.2(1), respectively.


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Figure 9. Cu-K edge XANES spectra of CuxO-E (b) and CuxO-E kept at 0.3 V vs RHE for 8 h upon irradiation with λ = 400 nm (c, red line). The spectra of CuO (a) and Cu2O (d) are also shown for comparison. The black line is a fit of the experimental spectrum with a linear combination of a and d, as described in the text.

As recently introduced, the XANES features and their severe modifications after oxidation state transitions can be conveniently exploited for the real time monitoring of electrochemical and photoelectrochemical reactions by means of the so-called fixed energy x-ray absorption voltammetry (FEXRAV).41,42 In the present case, the method consists to fix the energy of X-ray photons at a value, 8981 eV (Cu-K edge), at which the maximum contrast between the absorption coefficient, µ, of Cu2O and CuO is observed (see figure S11). In this way, the electrode potential can be varied at will and any modification of µ indicates a significant change of the oxidation state of the electrode. At the chosen energy, an increase of µ indicate the formation of Cu2O


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In the present case, FEXRAV was carried out both in the dark (figure 10 A) and under illumination (figure 10 B) while the photocathode potential is varied cyclically in a wide potential window.

Figure 10. Fixed energy x-ray absorption voltammetry of CuxO electrodes at 8981 eV in the dark (A) and under LED illumination (B). 0.1M K2HPO4 + KH2PO4, pH = 7, was used as an electrolyte. Shadow zones indicate the instability potentials. Black curves: µ; blue or brown curves: j (current density). Starting points are indicated by red arrows.

In both cases, the experiment is carried out on a freshly prepared electrode for assuring the initial presence of pure CuO only. In figure 10a (dark) the increase of µ, and thus the onset of the material reduction, is clearly observed at about 0.25 V. A similar conclusion can be drawn under illumination (figure 10 B) but with the onset shifted to less positive potentials, at about 0.4 V, where photoreduction begins. In both figures, the region of instability is indicated by the shading. It is worth noting that the FEXRAV signal


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clearly indicates an increasing decomposition during cycling, as µ constantly increases: as 8981 eV corresponds to the first maximum of the absorption edge of Cu2O, this has the obvious meaning that an increasing fraction of the sample is reduced to Cu2O. The similar trend of the currents is then mainly assigned to the reduction of the material: the current onset clearly corresponds to the drift in the XAS signal. For what concerns the photocurrents shown in figure 10 B, their constant decrease with subsequent cycles indicated the decrease of photoelectrochemical activity, in turn due to the material severe reduction. However, the increase in the FEXRAV signal decreases from cycle to cycle, indicating that the amount of Cu2O formed in a single cycle decreases with time. The nature of the material after use can be further confirmed by the EXAFS data fittings, as shown in Fig. 11. The structural parameters of both electrodes, obtained after the refinement are shown in the tables S2 and S3 in SI: while the spectrum of the as-prepared sample could be fitted with a cluster derived from the structure of CuO only, the sample kept 8 h in working conditions shows the simultaneous presence of both CuO and Cu2O, with weights 0.3(1) and 0.7(1), respectively.


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Figure 11. Cu-K edge EXAFS spectra (A, B) of CuxO-E and CuxO-E kept at 0.3 V vs RHE for 8 h upon irradiation with λ = 400 nm, respectively. C and D panels show the corresponding Fourier Transforms. Black line: experimental; red line fits according to the structural model described in the SI (Tab. S2 and S3)

XAS is bulk sensitive, and the above described results are compatible with the surface composition of the samples and its evolution occurring upon photoelectrolysis as demonstrated by XPS. XPS spectra of a new sample and after typical working condition for 30 s and 1 h (see experimental details) were collected and shown in Figure 12. The full scale spectra are shown in Fig. S12. The Cu 2p3/2 core level was employed for studying the oxidation state. On the left of Figure 10A, the whole Cu 2p edge is shown after subtraction of the background and compared with reference CuO and Cu2O,43 while on the right the spectra in the region of the Cu 2p3/2 peaks, normalized to the intensity of the main peak of the pristine sample, are shown. The Cu(II) oxide is characterized by intense shake-up satellites at ~9 eV higher binding energy than the main 2p3/2 and 2p1/2 peaks. In the detailed figure this feature is indicated, after fitting, as table S1


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and S2. Those satellites are very weak in Cu2O and totally absent in pure metallic copper. The photocathodes show an evident reduction upon irradiation in photoelectrochemical conditions, from a CuO to a Cu2O-like state, as assessed by the strong decrease of the satellite intensity. If a certain amount of surface CuO is present after 30 s, it is almost vanished after 1 h. The fully reduction yields also a broadening of the peaks as assessed by the shape and fitting of the Cu 2p3/2, together with a small shift towards higher binding energy of the peak centroid. The Cu 2p3/2 peaks position is observed to be at 932.5 eV for operated samples and 933.7 for the new sample, in full agreement with the values reported in literature for Cu2O and CuO respectively.43. However the chemical state differentiation between Cu(I) oxide and Cu metallic could maintain a certain degree of uncertainty due to the superposition of Cu 2p peaks. The ambiguity is fully dispelled by the X-ray excited Auger lines of Cu LVV, shown in Figure 12b): the kinetic energy of the pristine sample is larger (918.1eV) with respect the treated samples (916.5eV), that is in full agreement with the assignments of Cu(II) and Cu(I) oxides respectively.44 The metal copper LVV peak would be expected at a kinetic energy (918.6 eV) even higher than that of the pristine sample, but in the spectra of operated samples no feature can be observed at that position. Also the O 1s edge (Figure 12C) confirms the observed trend with the peaks of the new and operated samples assigned at 529.9 and 530.2 eV respectively.


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Figure 12. XPS spectra of CuxO-E as well as CuxO-E used previously as a photocathode at 0.3 V vs RHE for 30 s or 1 h upon irradiation with λ = 400 nm. A – Cu 2p spectra, B – Cu LVV Auger spectra, C – O 1s spectra. Abbreviation Ref refers to literature.43 Wide spectra and 1s C spectra are presented in SI (fig S12 and S13).

These results show that the differences between the spectra of new material and the one used for 30 s are more significant than in the case of comparison between electrodes used for 30 s and 1 h. This in turn suggests that the main changes occur during the first seconds, while after this period the electrode remains more stable (see prolonged photoelectrolysis results below).


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The long-term stability at pH=7 was investigated on a photocathode prepared at neutral pH. The experiment was carried out under chopped light (frequency 0.03 Hz) at 0.35 V vs. RHE. As demonstrated in figure 13A, the efficiency of light conversion decreases during the first hour and then the material reaches a nearly stationary condition since the photocurrent density does not change significantly with time, up to more than 5 h. Also in the case of this measurement, the characteristic decay and overshot of the photocurrent at points of shutter opening and closure spiked cathodic pulse with contribution of charge/discharge components (see figure 13 A inset). Stability and photostability in alkaline solution were also investigated, upon applying a potential of 0.44 V vs. RHE for at least 1 hour (see figure 13 B) and 0.73 V (approximately -0.10V vs OCP) for more than 12 h (data not shown). As demonstrated by figure 13B, in alkaline solution the photomaterial is less stable (dark current going towards more negative values) and photostable than in neutral pH (photocurrent going towards more positive values). In order to confirm the efficiency of the material towards the hydrogen evolution reaction, we carried out a SECM analysis on sample used for 6 hours (irradiation with LED 400 nm, pot. 0.42 V vs RHE). The result, shown in figure S13, indicates a decent faradaic efficiency (83% at 0.42 V vs RHE) likely indicating that photoreduction of the material is indeed decreased after the initial use of the material , in agreement with the FEXRAV results.


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Figure 13. Stability test. A – at 0.35 V vs. RHE in 0.1 M K2HPO4 + KH2PO4 (pH = 7) under chopped illumination with LED λ = 400 nm; B – at 0.44 V vs. RHE in 0.1 M KClO4 + NaOH (pH = 11). Inset: zoom on initial time scale.

DISCUSSION As shown, the present work aims at describing the facile synthesis and the performances of a CuxO photocathode that shows high photocurrents. High values of photocurrents are retained after hours of operation, an unexpected result considering previous literature reports.11,21 In the present case, the as-prepared sample is composed of CuO as demonstrated by XAS and XPS.


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However, the chemical composition of changes as a results of photoelectrochemical activity. In detail, the material is partially reduced and the amount of copper(I) oxide increases under working conditions. XPS analysis proved the almost complete reduction of CuO to Cu2O on the surface while XANES, being bulk sensitive, indicates the presence of a mixture of copper(I) and copper(II) oxides. Based on the XPS measurements it can be concluded that the photocathode changes significantly at the beginning of its working life (the most important degradation occurring within 30 s) and then a stationary state is reached, where the composition of the electrode does not significantly change with time. Similarly, stability tests proved that the photocurrent density decreases during the first 60 minutes and afterwards it stabilizes. This supports the conclusion that a Cu2O layer initially forms on the core of the CuO particles but also the “bulk” is significantly reduced upon reaching a stable condition. Zhang and Wang suggested, that the top layer of CuO in the Cu2O/CuO composite minimized the Cu2O photocorrosion and served as a recombination inhibitor for the photogenerated electrons and holes from Cu2O, which collectively explained a much enhanced stability and activity of the composite.11 In our case, an opposite architecture is formed, with Cu2O in the top layer. Previously is was demonstrated that, while Cu2O itself provides the right conditions for water splitting, its stability is relatively low.14 However, the present material present a unique combination of the two oxides that is likely at the bases of the exceptional stability. In fact, considering the potentials of conduction and valence band edges for both oxides,18 the hydrogen evolution from water reduction is still favored on Cu2O. At the same time, the latter’s band edges straddle those of the internal CuO layer. Our best suggestion for explaining the prolonged operation of the photoelectrode together with the observed stabilized composition (XPS and XAS results) is that copper(II) oxide can act as


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internal protector that prevents further Cu2O reduction: photogenerated electrons and holes formed in Cu2O and not participating in water splitting can be transferred to CuO and eventually recombine or be transported in the same phase and move towards the electrolyte(e)/FTO(h). CONCLUSIONS In this paper, a facile method to prepare copper oxide photocathode by a thermal treatment at 400˚C of CuI nanocrystals has been presented. The cathodic photocurrent observed during linear sweep voltammetry experiments in neutral media reaches the value of –1.2 mA cm-2 at 0.1 V under solar simulating light. This is in line with the high values of IPCE measured in the visible spectra. Moreover, the material possesses an exceptional stability, and its chemical composition remains unvaried after the first minutes of operation (as shown by XPS and XAS results). This interesting result is in apparent contrast with previous reports on Cu oxides that showed very poor stability. In the present case, the coexistence of CuO and Cu2O can originate by the reaching of a phase equilibrium or stationary state, where further reduction of CuO is hidden by the Cu2O layer. Still, we cannot ignore the newly formed band structure of the composite material. Our best explanation is the onset of an electronic coupling between the internal CuO core and the external Cu2O layer in which copper(II) oxide acts as internal protector that prevents further Cu2O reduction acting as a “buffer” for photogenerated charges that are not transferred across the semiconductor/electrolyte junction. The band edges of CuO in fact straddle those of Cu2O, and excess photogenerated charges can in principle move back toward the current collector and recombine before acting on the material itself.


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ASSOCIATED CONTENT Supporting Information. Electronic Supplementary Information (ESI) available: EDX and TEM analysis of CuI, XRD analysis, TGA analysis, the structural parameters of CuxO-E from EXAFS, wide XPS spectra, aligned C 1s XPS spectra, optimization of annealing temperature, SECM of electrode used previously for 6 h, XANES of standards, integrated current used for IPCE. AUTHOR INFORMATION Corresponding Author * Alessandro Minguzzi, Dipartimento di Chimica, Universita degli Studi di of Milano, via Golgi 19, 20133 Milan, Italy, e-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources The authors gratefully acknowledge Ministero dell’Istruzione, dell’Università e della Ricerca (MIUR) for funding (Futuro in Ricerca 2013, project RBFR13XLJ9). S. W. thanks to the financial support from Polish Innovation Economy Operational Program co financed by European Regional Development Fund (contract no. POIG.02.01.00-12-023/08). ACKNOWLEDGMENT


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The authors acknowledge beamline BM08 “LISA” at the European Synchrotron Radiation Facility for provision of beam time (experiment 08-01-1004). They are also greatly indebted with Dr. Francesco D’Acapito for kind assistance in using the BM08 beamline, to Dr. Alberto Naldoni and Dr. Francesco Malara (ISTM-CNR) for assistance during IPCE measurements, to Dr. Marcello Marelli (ISTM-CNR) for TEM analysis. Acknowledgments to Dr Ottavio Lugaresi for TGA measurements. REFERENCES (1)

Centi, G.; Perathoner, S. Towards Solar Fuels from Water and CO2. ChemSusChem 2010, 3 (2), 195–208. (2) Baran, T.; Wojtyła, S.; Dibenedetto, A.; Aresta, M.; Macyk, W. Zinc Sulfide Functionalized with Ruthenium Nanoparticles for Photocatalytic Reduction of CO2. Appl. Catal., B 2015, 178, 170–176. (3) Caputo, C. A.; Wang, L.; Beranek, R.; Reisner, E. Carbon nitride–TiO2 Hybrid Modified with Hydrogenase for Visible Light Driven Hydrogen Production. Chem. Sci. 2015, 6 (10), 5690–5694. (4) Hisatomi, T.; Kubota, J.; Domen, K. Recent Advances in Semiconductors for Photocatalytic and Photoelectrochemical Water Splitting. Chem. Soc. Rev. 2014, 43 (22), 7520–7535. (5) van Dorp, D. H.; Hijnen, N.; Di Vece, M.; Kelly, J. J. SiC: A Photocathode for Water Splitting and Hydrogen Storage. Angew. Chem. Int. Ed. 2009, 48 (33), 6085–6088. (6) Hu, C.; Chu, K.; Zhao, Y.; Teoh, W. Y. Efficient Photoelectrochemical Water Splitting over Anodized P-Type NiO Porous Films. ACS Appl. Mater. Interfaces 2014, 6 (21), 18558–18568. (7) Feng, J.; Gong, M.; Kenney, M. J.; Wu, J. Z.; Zhang, B.; Li, Y.; Dai, H. Nickel-Coated Silicon Photocathode for Water Splitting in Alkaline Electrolytes. Nano Res. 2015, 8 (5), 1577–1583. (8) Maier, C. U.; Specht, M.; Bilger, G. Hydrogen Evolution on Platinum-Coated P-Silicon Photocathodes. Int. J. Hydrogen Energy 1996, 21 (10), 859–864. (9) Wu, J.; Li, Y.; Kubota, J.; Domen, K.; Aagesen, M.; Ward, T.; Sanchez, A.; Beanland, R.; Zhang, Y.; Tang, M.; Hatch, S.; Seeds, A.; Liu, H. Wafer-Scale Fabrication of SelfCatalyzed 1.7 eV GaAsP Core–Shell Nanowire Photocathode on Silicon Substrates. Nano Lett. 2014, 14 (4), 2013–2018. (10) Lee, M. H.; Takei, K.; Zhang, J.; Kapadia, R.; Zheng, M.; Chen, Y.-Z.; Nah, J.; Matthews, T. S.; Chueh, Y.-L.; Ager, J. W.; Javey, A. P-Type InP Nanopillar Photocathodes for Efficient Solar-Driven Hydrogen Production. Angew. Chem. Int. Ed. 2012, 51 (43), 10760–10764. (11) Zhang, Z.; Wang, P. Highly Stable Copper Oxide Composite as an Effective Photocathode for Water Splitting via a Facile Electrochemical Synthesis Strategy. J. Mater. Chem. 2012, 22 (6), 2456–2464.


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An Efficient CuxO Photocathode for Hydrogen Production at Neutral

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