Electrochemical Growth of Copper Hydroxy Double Salt Films and

Jun 1, 2017 - Citation data is made available by participants in Crossref's Cited-by Linking service. For a more comprehensive list of citations to th...
0 downloads 0 Views 9MB Size
Download PDF
Article pubs.acs.org/Langmuir

Electrochemical Growth of Copper Hydroxy Double Salt Films and Their Conversion to Nanostructured p‑Type CuO Photocathodes Allison C. Cardiel,† Kenneth J. McDonald,‡ and Kyoung-Shin Choi*,† †

Department of Chemistry, University of WisconsinMadison, Madison, Wisconsin 53706, United States Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States

‡

ABSTRACT: New electrochemical synthesis methods were developed to produce copper hydroxy double salt(Cu-HDS) films with four different intercalated anions (NO3−, SO42−, Cl−, and dodecyl sulfate (DS)) as pure crystalline films as deposited (Cu2NO3(OH)3, Cu4SO4(OH)6, Cu2Cl(OH)3, and Cu2DS(OH)3). These methods are based on p-benzoquinone reduction, which increases the local pH at the working electrode and triggers the precipitation of Cu2+ and appropriate anions as Cu-HDS films on the working electrode. The resulting Cu-HDS films could be converted to crystalline Cu(OH)2 and CuO films by immersing them in basic solutions. Because Cu-HDS films were composed of 2D crystals as a result of the atomic-level layered structure of HDS, the CuO films prepared from Cu-HDS films have unique low-dimensional nanostructures, creating high surface areas that cannot be obtained by direct deposition of CuO, which has a 3D atomic-level crystal structure. The resulting nanostructures allowed the CuO films to facilitate electron−hole separation and demonstrate great promise for photocurrent generation when investigated as a photocathode for a water-splitting photoelectrochemical cell. Electrochemical synthesis of Cu-HDS films and their facile conversion to CuO films will provide new routes to tune the morphologies and properties of the CuO electrodes that may not be possible by other synthesis means.

■

INTRODUCTION

region for certain phases), the resulting structure is called HDS (Figure 1b).3 Because the structure of the HDS is maintained by weak electrostatic forces, van der Waals forces, and hydrogen bonding, this anion exchange can occur while preserving the structure. Because of these structural features, HDSs can be used for various applications such as catalysis,4 sensing,5,6 and ion exchange.4,7−11 Cu2+-based HDS materials can be particularly interesting for use in these applications because Cu is cheap, abundant, and nontoxic. The bulk Cu-HDSs have traditionally been prepared by precipitation, 4,6,9,12−14 hydrothermal10,11,15−17 and liquid−solid methods,7,8,18 and pulsed-laser ablation in liquids.19 However, synthesis methods that grow Cu-based HDSs directly from conducting substrates with good electrical continuity, which is necessary for use in electrochemical applications (e.g., electrocatalysis and electrochemical sensing), have been rare. The aims of this study are twofold. The first aim is to report new cathodic electrodeposition methods that allow for the direct growth of a variety of Cu-based HDSs on a conducting substrate to form film-type Cu-based HDS electrodes. Cathodic electrodeposition of pure Cu-based HDS can be challenging because Cu metal or Cu2O can be easily codeposited as a result

Hydroxy double salts (HDSs) are layered compounds that have a general formula of M2+(OH)2−x(An)x/n·zH2O (M = metal ions; A = anionic species).1 The structure of HDS is related to the brucite (Mg(OH2)) structure that is composed of M(OH)2 layers held together by van der Waals interactions (Figure 1a).2 In each M(OH)2 layer, divalent metal ions are stabilized in octahedral sites coordinated by six OH− ligands. When a portion of the OH− ions are replaced by other anions such as Cl−, NO3−, and SO42− (with H2O present in the interlayer

Special Issue: Fundamental Interfacial Science for Energy Applications Received: February 20, 2017 Revised: May 18, 2017

Figure 1. Crystal structures of (a) brucite (Mg(OH)2) and (b) rouaite (Cu2NO3(OH)3), as an example of an HDS. © XXXX American Chemical Society

A

DOI: 10.1021/acs.langmuir.7b00588 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir of the relatively positive reduction potentials of Cu2+ to Cu0 and Cu1+. However, our methods enable the deposition of Cubased HDS films containing NO3−, SO42−, or Cl− as pure and crystalline films as deposited. Additionally, we demonstrate the electrodeposition of Cu-HDS films containing dodecyl sulfate anions (DS−). Cu-HDSs containing larger organic anions, such as DS−, that considerably increase the interlayer distances can be useful for applications requiring the intercalation of larger species into the interlayer regions.12−14,18,20−22 The second aim of our study is to demonstrate the use of Cu-based HDS films as precursor films to form CuO films. CuO is a p-type semiconductor with a band gap of 1.2−1.8 eV, which along with its abundance and nontoxicity makes CuO ideal for use in solar energy conversion.23−30 Recently, its ability to produce H2 as a photocathode in a water-splitting photoelectrochemical cell (PEC) has been reported, confirming the thermodynamic feasibility of photoexcited electrons in the CB of CuO for water reduction.31,32 However, CuO suffers from poor charge-transport properties, which is typical for oxide-based photoelectrodes. Therefore, nanostructuring, which can shorten the distance that the minority carriers need to travel to reach the interface and thus minimize electron−hole recombination, can be a useful strategy. In this study, we show the use of 2D Cu-based HDS films as a precursor to form high-surface-area nanostructured CuO films. Because the microscale or nanoscale crystal morphologies often reflect their atomic-level crystal structures, the CuO films obtained using 2D Cu-HDS crystals as precursors show unique low-dimensional nanostructures, which cannot be easily obtained when CuO with a 3D crystal structure is prepared directly. The resulting CuO films demonstrate a considerable enhancement in photocurrent generation when used as a photocathode for a water-splitting PEC.

■

chosen because it allows for sufficient p-benzoquinone reduction current while avoiding the reduction of Cu2+ to Cu1+. Cu-HDS/SO42− films (Brochantite, Cu4SO4(OH)6) were deposited at 0.1 V at 80 °C for 10 min from an unstirred aqueous solution of 100 mM Na2SO4, 20 mM CuSO4, and 50 mM p-benzoquinone (average current density = −1.2 mA/cm2). The pH of the solution was adjusted to 5 before performing the deposition. Cu4SO4(OH)6 can exist as various hydrated forms (i.e., Cu4SO4(OH)6·H2O and Cu4SO4(OH)6· 2H2O) that contain water molecules in the interlayer regions. These hydrated forms can readily form as impurity phases when the deposition is performed at room temperature. Therefore, to obtain a pure anhydrated Cu4SO4(OH)6 phase, deposition was carried out at 80 °C. Cu-HDS/Cl− films (Botallackite, Cu2Cl(OH)3) were deposited at 0.1 V at room temperature for 10 min from an unstirred aqueous solution of 100 mM NaCl, 20 mM CuCl2, and 50 mM pbenzoquinone (average current density = −0.35 mA/cm2). The pH of the solution was adjusted to 5 before performing the deposition. Cu-HDS/DS− films (Cu2DS(OH)3) were deposited at 0.1 V at 65 °C for 10 min from an unstirred aqueous solution of 100 mM sodium dodecyl sulfate (SDS), 20 mM Cu(NO3) 2 and 50 mM pbenzoquinone (average current density = −0.3 mA/cm2). The pH of the solution was adjusted to 5 before performing the deposition. When the deposition was carried out at room temperature, amorphous films resulted, suggesting that because of the bulkiness of DS− the formation of crystalline Cu2DS(OH)3 was more difficult than other phases containing smaller anions. Crystalline Cu2DS(OH)3 films could be obtained by increasing the deposition temperature to 65 °C. In general, the crystallinity of deposits improves as the deposition temperature increases because it allows facile movement of ions and atoms to create long-range order. Conversion of Cu-HDS to CuO and Cu(OH)2. To convert CuHDS to CuO, a Cu-HDS/SO42− film was placed in a vial of 0.01 M NaOH solution (pH 12). The vial was covered with parafilm and heated at 60 °C. The pale green-blue color of the Cu-HDS/SO42− film became dark brown over time. The complete conversion of Cu-HDS/ SO42− to CuO was achieved within 2 h. To convert Cu-HDS to Cu(OH)2, a Cu-HDS/SO42− film was placed in a vial of 1 M NaOH solution (pH 13.8) at room temperature. The complete conversion of Cu-HDS/SO42− to Cu(OH)2 was achieved within 10−15 s. The film became bright sky blue. This film could then be annealed at 500 °C to produce fibrous CuO. The same procedures were also applied to Cu HDS/NO3−, Cl−, and DS− films to convert them to CuO and Cu(OH)2 films. In the case of the Cu HDS/NO3− and Cu HDS/Cl− films, Cu(OH)2 already started to form when the procedure used to convert Cu HDS/SO42− to CuO was applied. This suggests that the replacement of NO3− and Cl− ions by OH− ions is much more facile than the replacement of SO42−. Because the conversion of Cu-HDS to Cu(OH)2 occurred very rapidly, the morphologies of the Cu HDS/NO3− and Cu HDS/Cl− films remained intact, and the crystallinity of the resulting Cu(OH)2 films was poor. In the case of the Cu HDS/DS− film, the replacement of DS− ions by OH− ions appeared to be much slower, and crystalline Cu HDS/DS− remained unchanged when the procedure used to convert Cu HDS/SO42− to Cu(OH)2 was applied. When the procedure used to convert Cu HDS/SO42− to CuO was applied, the formation of crystalline CuO was observed only when the immersion time was increased to 12 h. Characterization. A Bruker D8 diffractometer (Ni-filtered Cu Kα radiation, λ = 1.5418 Å) was used to collect X-ray powder diffraction (XRD) patterns. Scanning electron microscopy (SEM) images were obtained using a LEO 1530 operated at an accelerating voltage of 5 kV. Energy-dispersive X-ray spectroscopy (EDS) spectra were obtained using the same SEM equipped with an EDS (Noran System Seven, Thermo Fisher) at an accelerating voltage of 15 kV. UV−vis absorption spectra were obtained using a Cary 5000 UV−vis−NIR spectrophotometer (Agilent) with an integrating sphere attachment, which allows the measurement of all light reflected and transmitted to calculate the absorbance of the sample electrode located in the center

EXPERIMENTAL SECTION

Materials. All chemicals used in this study had reagent-grade purity. Copper(II) sulfate pentahydrate, p-benzoquinone, sodium hydroxide, and sodium dodecyl sulfate were purchased from SigmaAldrich. Copper(II) nitrate trihydrate was purchased from Acros Organics, and copper(II) chloride dihydrate was purchased from Alfa Aesar. All solutions were prepared using deionized water further purified with a Barnstead E-pure (model D4631) purification system (resistivity >18 MΩ). Fluorine-doped tin oxide (FTO) slides (8−12 Ω resistance) were purchased from Hartford Glass, Inc. Platinum electrodes were prepared by sputter-coating a 100 nm Pt layer over a 20 nm Ti layer onto clean glass slides. Electrochemical Synthesis. All depositions used a conventional three-electrode setup in an undivided cell. Pt was used as the counter electrode, and FTO was used as the working electrode on which the films were deposited. The reference electrode was a Ag/AgCl in 4 M KCl double-junction electrode with a bridging solution of 4 M KCl. All potentials used for electrodeposition are reported vs Ag/AgCl in this study. All electrodeposition and electrochemical studies were performed using a Princeton Applied Research VMP2 multichannel potentiostat/galvanostat. The resulting films were rinsed with deionized water for 30 s and subsequently dried in a stream of air. The plating solution, deposition potential, and temperature used to produce each compound are described below. Cu-HDS films containing NO3−, SO42−, Cl−, and DS− were cathodically deposited from Cu2+-containing solutions using the electrochemical reduction of p-benzoquinone. Cu-HDS/NO3− films (Rouaite, Cu2NO3(OH)3) were deposited at 0.1 V at room temperature for 10 min from an unstirred aqueous solution of 100 mM NaNO3, 20 mM Cu(NO3)2, and 50 mM p-benzoquinone (average current density = −0.2 mA/cm2). The pH of the solution was adjusted to 5 before performing the deposition. This potential was B

DOI: 10.1021/acs.langmuir.7b00588 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

2).33−35 However, the reduction of nitrate, which should be thermodynamically more feasible than the reduction of Cu2+ to Cu+ or Cu0 (eqs 3 and 4), is kinetically more difficult and occurs at a more negative potential than that of Cu2+. Therefore, Cu-HDSs cannot be deposited as a pure phase without codepositing Cu2O or Cu as impurities using nitrate reduction. Similarly, O2 reduction, which is alternatively used to electrochemically generate OH− ions (eq 5), is also kinetically slower and often occurs at a more negative potential than Cu2+ reduction. The reduction of p-benzoquinone, on the other hand, is kinetically faster and occurs at a much more positive potential than the reduction of Cu2+.36 Figure 2 illustrates these

of the integrating sphere. Photocurrent measurements were performed using an SP-200 potentiostat/EIS (BioLogic Science Instrument) and simulated solar illumination obtained by passing light from a 300 W Xe arc lamp through neutral density filters, an AM 1.5G filter, and an IR filter (water filter) into an optical fiber. Films were illuminated through the FTO (back-side illumination). The light’s power density was calibrated to be 100 mW/cm2 at the FTO surface (before passing through FTO) by using an NREL-certified Si reference cell (Photo Emission Tech. Inc.). All sample electrodes were masked to make the exposed geometrical area (∼0.03−0.04 cm2) smaller than the illuminated area (0.06 cm2). An undivided three-electrode cell composed of a working electrode (CuO), a Pt counter electrode, and a Ag/AgCl (4 M KCl) reference was used. All photocurrent measurements were taken either while sweeping the potential at a scan rate of 10 mV/s (for J−V measurements) or while applying a constant bias (J−t measurements). All photocurrent measurements were performed in a 0.1 M KOH solution (pH 13) with continuous O2 (for O2 reduction) or N2 (for water reduction) bubbling, and the solution was stirred vigorously. Mott−Schottky measurements were performed using the aforementioned potentiostat setup and the same 0.1 M KOH solution with N2 bubbling and three-electrode cell as for the photocurrent measurements, except the solution was not stirred. A sinusoidal modulation of 10 mV was applied at 0.5, 1, and 2 kHz. The results from photoelectrochemical and Mott−Schottky measurements were reported against the reversible hydrogen electrode (RHE).

■

RESULTS AND DISCUSSION Cathodic Deposition of Cu-HDS Films. Various Cu-HDS films were deposited using Cu2+ solutions containing pbenzoquinone and proper anions that were intended to be incorporated into the Cu-HDS structure. The anions were provided from a copper salt and a supporting electrolyte. Therefore, the types of copper salt and supporting electrolyte were changed depending on the target formula of the Cu-HDS. For example, to prepare Cu-HDS/Cl−, CuCl2 and NaCl were used as the Cu source and the supporting electrolyte, respectively, whereas to prepare Cu-HDS/SO42−, Cu(SO4)2 and Na2SO4 were used. In the same manner, Cu(NO3)2 and NaNO3 were used as the Cu source and the supporting electrolyte to produce Cu-HDS/NO3−. For the case of CuHDS/DS−, Cu(NO3)2 and sodium dodecyl sulfate were used. The deposition mechanism involves the electrochemical generation of hydroxide ions on the WE, elevating the local pH at the WE surface. Consequently, the solubility of Cu2+ was decreased at the WE and was precipitated as Cu-HDS films on the WE. In this study, the electrochemical generation of hydroxide ions, which is equivalent to the electrochemical consumption of proton ions, was achieved by the reduction of p-benzoquinone to hydroquinone (eq 1).

Figure 2. LSVs for a pH 5 solution containing 100 mM NaNO3 purged with (a) N2 or (b) O2, LSVs of a pH 5 solution purged with N2 containing (c) 100 mM NaNO3 and 20 mM Cu(NO3)2, and (d) 100 mM NaNO3, 20 mM Cu(NO3)2, and 50 mM p-benzoquinone (scan rate = 10 mV/s).

trends by comparing linear sweep voltammograms (LSVs) for the reduction of p-benzoquinone, nitrate, O2, and Cu2+. The reduction current shown in the LSV that is obtained in a solution containing only 100 mM NaNO3 purged with N2 is unambiguously due to the reduction of nitrate because there is no other electrochemically active species in solution (Figure 2a). When the LSV of the same solution was obtained after the solution was purged with O2, the reduction onset was shifted in the positive direction, indicating that the reduction of O2 occurs more readily than the reduction of nitrate (Figure 2b). When 20 mM Cu(NO3)2 was added to the 100 mM NaNO3 solution, the reduction onset was shifted to the positive direction by 400 mV, indicating that the reduction of Cu2+ is much easier than the reduction of nitrate or O2 (Figure 2c). However, when pbenzoquinone was added to the solution containing nitrate and Cu2+, the onset potential for reduction shifted further in the positive direction, demonstrating that it is possible to reduce pbenzoquinone without reducing Cu2+ (Figure 2d). This means that if the deposition potential is chosen to be between the reduction onset of p-benzoquinone and the reduction onset of Cu2+, then Cu-HDS films can be deposited as a pure phase without codepositing Cu2O or Cu. A previous study has demonstrated the electrodeposition of copper hydroxyl sulfate hierarchical/hybrid structured materials by way of reducing dissolved O2.37 However, this deposition was performed successfully only with a limited type of WE (i.e., a Au WE with a polypyrrole−polystyrenesulfonate film on top); when a glassy carbon WE was used with the same deposition condition, deposition did not occur. This is probably because unlike p-benzoquinone reduction that is kinetically fast

NO3− + 3H+ + 2e− → HNO2 + H 2O E° = 0.94 V (2) 2+

−

+

+ e → Cu E° = 0.15 V

(3)

Cu 2 + + 2e− → Cu E° = 0.34 V

(4)

Cu

1 O2 (g) + 2H+ + 2e− → H 2O E° = 1.23 V 2

(5)

−

The electrochemical generation of OH has traditionally been performed through the use of nitrate reduction (eq C

DOI: 10.1021/acs.langmuir.7b00588 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

variations of the bilayer assembly of surfactant molecules, which can result in a variation in the interlayer spacing.38 The variations include a difference in the degree of overlap in the hydrophobic tail region (Scheme 1a) or in the tilting angles of the hydrophobic tails against the inorganic layers (Scheme 1b). The SEM images of the as-deposited Cu-HDS films show uniform coverage of the 2D crystals throughout the films with morphologies varying depending on the anion type present in the structure (Figure 4). Both the Cu-HDS/NO3− and the Cu-

regardless of the electrode type, the onset potential for O2 reduction, which is kinetically difficult and involves the formation of intermediates adsorbed on the electrode surface, is highly electrode-dependent. Characterization of Cu-HDS Films. XRD studies show that all types of Cu-HDS films produced in this study were pure and crystalline as deposited (Figure 3). Cu-HDS films

Figure 4. SEM images of as-deposited (a) Cu-HDS/NO3−, (b) CuHDS/Cl−, (c) Cu-HDS/SO42−, and (d) Cu-HDS/DS−. Figure 3. XRD patterns of as-deposited (a) Cu-HDS/NO3 − corresponding to Rouaite (Cu2NO3(OH)3) (JCPDS 015-0014), (b) Cu-HDS/SO42− corresponding to Brochantite (Cu4SO4(OH)6) (JCPDS 43-1458), (c) Cu-HDS/Cl− corresponding to Botallackite (Cu2Cl(OH)3) (JCPDS 8-88), and (d) Cu-HDS/DS− corresponding to Cu2DS(OH)3. The peaks from the FTO substrate are marked with an asterisk.

HDS/Cl− films contain platelike crystals that are 1 to 2 μm in diameter (Figure 4a,b). The Cu-HDS/SO42− films are composed of bladelike nanocrystals (Figure 4c). The platelike or bladelike crystals in these films are aligned not completely parallel to the substrate with ample space between the crystals, creating high surface areas. On the other hand, the Cu-HDS/ DS− films exhibit a dense conglomeration of nanoplates (Figure 4d). Conversion of Cu-HDS to CuO and Cu(OH)2. We discovered that when a Cu-HDS/SO42− film was soaked in a pH 12 solution at 60 °C for 2 h (Scheme 2) it could be completely converted to a pure crystalline CuO film, which was confirmed by XRD study (Figure 5a). (This film will be referred to as CuO I hereafter.) The simple and complete conversion of a Cu-HDS/SO42− film to CuO is probably owing to the fact that our Cu-HDS films are composed of extremely thin plates and have a high surface area. The morphology of the CuO I film is shown in Figure 6a. Although the Cu-HDS/ SO42− film was composed of 2D plates, the CuO I film was composed of micrometer-sized spiky balls that resemble burs. When CuO I was annealed for 3 h at 500 °C (the resulting CuO film will be referred to as CuO II hereafter), no morphological change was observed (Figure 6b). However, the XRD of the resulting film shows that the intensity of the CuO peaks increased while the full width at half-maximum decreased, suggesting an increase in the crystallinity of the CuO film (Figure 5b).

deposited with SO42−, NO3−, Cl−, and DS− were identified as Cu4SO4(OH)6, Cu2NO3(OH)3, Cu2Cl(OH)3, and Cu2DS(OH)3, respectively. The pattern for Cu-HDS/DS− shows a significant increase in basal spacing as expected from the size of DS−.7,20 The XRD pattern of Cu2DS(OH)3 also revealed that Cu2DS(OH)3 was formed with two slightly different basal spacings, 29 Å (indexed as 00l) and 25 Å (indexed as 00l′). This means that DS− in the interlayer region adopted two different bilayer assemblies. Scheme 1 shows a few possible Scheme 1. Possible Variations of Bilayer Assemblies of Cu2DS(OH)3, Including (a) an Increase in the Overlap of the Hydrophic Tails and (b) Tilting of the DS− Molecules

D

DOI: 10.1021/acs.langmuir.7b00588 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir Scheme 2. Conversion of the Cu-HDS/SO42− Film to Cu(OH)2 and CuO with Various Morphologies

the resulting Cu(OH)2 film was composed of nanofiber crystals (Figure 6c). When the nanofibrous Cu(OH)2 film was annealed for 3 h at 500 °C, a crystalline CuO film having the same nanofibrous morphology could be obtained (Figures 5d and 6d), which will be referred to as CuO III hereafter. The conversion processes used in this study to produce CuO I−III films using a Cu-HDS/SO42− film are summarized in Scheme 2. It should be noted that the nanostructured CuO films produced in this study are unique compared to the morphologies of CuO films that are directly electrodeposited, which typically results in dense, compact, featureless films.25,26,40−44 This is because the Cu-HDS films, which were used as precursors for the CuO preparation, were composed of 2D crystals owing to their layered atomic-level structures. As a result, the CuO films prepared from Cu-HDS films have unique low-dimensional nanostructures creating high surface areas. These nanostructured CuO films may be particularly beneficial for photoelectrochemical applications where the surface area and the particle size play critical roles in electron−hole separation. For oxide photoelectrodes, including CuO, which typically suffers from poor charge-transport properties and high electron−hole recombination, introducing nanostructures to increase the surface area and shorten the distance that the minority carriers need to travel to reach the interface can considerably decrease electron−hole recombination.45 Before examining the photoelectrochemical performances of the CuO I−III films, the UV−vis−NIR absorption spectra of the CuO I−III films were first investigated (Figure 7). The CuO I film shows an absorption onset at ∼860 nm, whereas CuO II−III films show an onset at ∼880 nm. The band gap

Figure 5. XRD patterns of (a) CuO I, (b) CuO II, (c) Cu(OH)2, and (d) CuO III. Black (hkl) indices are for CuO peaks, and green (hkl) indices are for Cu(OH)2 peaks. The peaks from the FTO substrate are marked with an asterisk.

Figure 6. SEM images of (a) CuO I, (b) CuO II, (c) Cu(OH)2, and (d) CuO III.

When the same Cu-HDS/SO42− film was soaked in 1 M NaOH solution at room temperature (Scheme 2), the film was immediately and completely converted to pure crystalline Cu(OH)2 in 15 s (Figure 5c).39 The SEM study revealed that

Figure 7. UV−vis−NIR absorption spectra of CuO I (red line), CuO II (blue line), and CuO III (black line). E

DOI: 10.1021/acs.langmuir.7b00588 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir energies for the CuO I film and the CuO II−III films were estimated to be 1.44 and 1.41 eV, respectively. The delayed absorption onset of the CuO I film compared to those of the CuO II−III films is due to the low crystallinity of CuO I that was obtained purely by a solution process without annealing. The absorption spectra also show that the CuO III film has slightly higher absorbance than the CuO II film in the region of 700−350 nm, although these two films were prepared from CuHDS films containing identical amounts of Cu4SO4(OH)6 and therefore should contain identical amounts of CuO. This difference in absorbance is due to a difference in the morphology of the CuO II and CuO III films affecting the amount of surface scattered light. The photocurrent density−potential (J−V) characteristics of the CuO I−III films for the photoelectrochemical reduction of water were obtained in a 0.1 M KOH solution (pH 13) purged with N2 (Figure 8a). The photocurrent produced by CuO I was not considerable and did not increase when the potential was

swept in the negative direction. This suggests that CuO I, which did not go through an annealing process, contains considerable defects in the bulk and on the surface of the electrode, which limited electron−hole separation and also resulted in Fermi-level pinning. On the other hand, CuO II and CuO III, which contain nanoporous morphologies that were treated by annealing, generated a considerable amount of photocurrent. For example, these films achieved ∼1.5 mA/cm2 at 0.6 V vs RHE. This level of photocurrent density at a potential as positive as 0.6 V vs RHE has not been achieved by any oxide-based photocathode. A portion of the photocurrent generated by CuO may be associated with the cathodic photocorrosion of CuO, as discussed below in the interpretation of the photocurrent−time (J−t) plots. However, even if the observed photocurrent is partially due to cathodic photocorrosion, the observed photocurrent can still be used to assess the enhanced capability of these nanostructured CuO electrodes in generating photoexcited electrons that reach the electrode surface. This is because the electrons used for cathodic photocorrosion must also be generated by photon excitation and electron−hole separation. Another notable feature of CuO II and CuO III was that their photocurrent onset potentials were more positive than 1 V vs RHE (1.05 V for CuO II and 1.15 V for CuO III) (Figure 8a, inset). This means that a photovoltage of more than 1 V for H2 evolution can be achieved, which is a highly advantageous feature. For comparison, the photocurrent onset potentials typically observed for p-Si and Cu2O photocathodes for water reduction are ∼0.4−0.5 V and ∼0.5 V vs RHE, respectively.46−51 The Mott−Schottky plots show that the flat band potentials of both CuO II and CuO III are approximately ∼1.22 V vs RHE (Figure 8b,c). The photocurrent−time (J−t) plots of the CuO II−III films obtained at 0.7 V vs RHE show that the photocurrent density gradually decreases over time (Figure 8d), which was expected as a result of the well-known cathodic photocorrosion problem of CuO.26,29−32 When the surface-reaching electrons are not quickly consumed and accumulate at the surface, these electrons tend to be used to reduce Cu2+ ions in the CuO lattice to form Cu2O or Cu. Indeed, XRD of CuO films after photocurrent measurement showed the formation of a crystalline Cu2O phase. With these bare CuO electrodes that rapidly photocorrode under illumination, we were unable to produce a detectable amount of H2 to calculate the Faradaic efficiency. However, recent studies show that the photocorrosion of Cu2O could be effectively suppressed by placing a thin TiO2 passivation layer and suitable catalysts on the Cu2O surface.46,47,52,53 We expect that the same strategy can be applied to our CuO films to prevent photocorrosion, which will be the focus of our future studies. The J−V characteristics of the CuO II−III films were also obtained for O2 reduction, which can consume surface-reaching electrons much more rapidly than water reduction (Figure 9a).54,55 Indeed, the photocurrents generated by CuO II and CuO III increased significantly compared to those observed for water reduction. For example, the photocurrent densities at 0.6 V vs RHE were ∼3.5 and ∼2.5 mA/cm2 for CuO II and CuO III, respectively, which is equivalent to increases in photocurrent density of 133 and 67%, respectively, in comparison with photocurrent densities observed during water reduction. The fact that the photocurrent increased considerably by introducing an electron acceptor with fast kinetics confirms that the photocurrents observed for the CuO photocathodes are not

Figure 8. (a) J−V plot (scan rate = 10 mV/s) of CuO I (red line), CuO II (blue line), and CuO III (black line). Mott−Schottky plots for (b) CuO II and (c) CuO III (circles for 2 kHz, squares for 1 kHz, and triangles for 0.5 kHz). (d) J−t plots at 0.7 V vs RHE for CuO II (blue line) and CuO III (black line). All measurements were carried out in 0.1 M KOH (pH 13) solution with N2 purging. AM1.5G (100 mW/ cm2) illumination was used for (a) and (d). F

DOI: 10.1021/acs.langmuir.7b00588 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

this study can be used completely for water reduction in a stable manner.

■

CONCLUSIONS We have established synthesis conditions to prepare uniform, high-surface-area Cu-HDS electrodes that contain four different anions: Cu2NO3(OH)3, Cu4SO4(OH)6, Cu2Cl(OH)3, and Cu2DS(OH)3. Depositing pure Cu-HDS without codepositing Cu or Cu2O was possible by utilizing p-benzoquinone reduction that has a much more positive reduction potential than Cu2+. The reduction of p-benzoquinone resulted in elevating the local pH on the surface of the working electrode, allowing for Cu2+ ions and appropriate anions to be precipitated as Cu-HDS films. We also demonstrated that Cu-HDS/SO42− films could be easily converted to crystalline Cu(OH)2 and CuO films with alkaline solution treatments. Because the Cu-HDS films were composed of 2D crystals due to the atomic-level layered structure of HDS, the CuO films prepared from Cu-HDS films have unique low-dimensional nanostructures, creating high surface areas. Because the nanostructure reduces the distance that photoexcited electrons have to travel to reach the photoelectrode/water interface, CuO films prepared from Cu-HDS films show great promise for use as photocathodes in a photoelectrochemical cell for solar water splitting; their photocurrent onset potentials (>1.0 V vs RHE) were very close to their flatband potentials (∼1.2 V vs RHE), and they generated photocurrent densities of ∼1.5 mA/cm2 at a potential as positive as 0.6 V vs RHE. Unfortunately, the bare CuO electrodes were unable to produce a detectable amount of H2 as a result of the limited catalytic ability of the CuO surface for H2 production and the resulting rapid photocorrosion. However, comparisons of photocurrents and photostabilities of CuO electrodes observed for water reduction and O2 reduction suggest that when the CuO films are coupled with efficient hydrogen evolution catalysts as well as a protection layer on their surfaces, more efficient and stable photocurrent generation by CuO for water reduction will be achieved while suppressing photocorrosion.

Figure 9. (a) J−V plots (scan rate = 10 mV/s) and (b) J−t plots at 0.7 V vs the RHE of CuO II (blue line) and CuO III (black line) for O2 reduction in 0.1 M KOH (pH 13) solution with O2 purging.

entirely due to cathodic photocorrosion. There are three competing reactions that consume the photoexcited electrons that reach the photoelectrode surface: interfacial charge transfer (water reduction or O2 reduction), photocorrosion, and surface recombination. Therefore, the magnitude of the total photocurrent and the portion of photocurrent associated with the desired interfacial charge-transfer reaction will be affected by the rate of the interfacial change-transfer reaction relative to the rate of cathodic photocorrosion and the rate of surface recombination. This means that when a proper hydrogen evolution catalyst is placed on the CuO film, which can make the interfacial charge transfer rate faster than the rates of photocorrosion and surface recombination, the level of photocurrent observed for O2 reduction may be achieved for water reduction while suppressing photocorrosion. We would also like to note that compared to water reduction the photocurrent onset potentials of the CuO II and CuO III films for O2 reduction were shifted to the positive direction closer to their flatband potential values (1.13 V for CuO II and 1.20 V for CuO III) (Figure 9a, inset). This is because the enhanced charge-transfer kinetics for O2 reduction reduces electron−hole recombination in the potential region very near the flatband potential. The J−t plots of the CuO II−III films obtained at 0.7 V vs RHE for the photoreduction of O2 are shown in Figure 9b. The decay rate is significantly slower compared to that of water reduction, confirming that facilitating the interfacial electron transfer and reducing electron accumulation on the CuO surface has a direct impact on retarding the cathodic photocorrosion of CuO. This result encourages us to further investigate the possibility of suppressing the photocorrosion of the nanostructured CuO electrodes with an appropriate protection layer and an efficient hydrogen evolution catalyst so that the considerable level of photocurrent demonstrated in

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Allison C. Cardiel: 0000-0001-7715-6642 Kyoung-Shin Choi: 0000-0003-1945-8794 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through grant DESC0008707. A.C.C. was supported by the National Science Foundation Graduate Research Fellowship under grant no. DGE-1256259.

■

REFERENCES

(1) Carbajal Arizaga, G. G.; Satyanarayana, K. G.; Wypych, F. Layered hydroxide salts: Synthesis, properties and potential applications. Solid State Ionics 2007, 178, 1143−1162. (2) Zigan, F.; Rothbauer, R. Neutronenbeugungsmessungen am brucit. N. Jb. Miner. 1967, 1967, 137−143.

G

DOI: 10.1021/acs.langmuir.7b00588 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

containing dodecyl sulfate anions. Polym. Adv. Technol. 2012, 23, 171− 181. (23) Koffyberg, F. P.; Benko, F. A. A Photo-Electrochemical Determination of the Position of the Conduction and Valence Band Edges of P-Type Cuo. J. Appl. Phys. 1982, 53, 1173−1177. (24) Chiang, C. Y.; Shin, Y.; Aroh, K.; Ehrman, S. Copper oxide photocathodes prepared by a solution based process. Int. J. Hydrogen Energy 2012, 37, 8232−8239. (25) Izaki, M.; Nagai, M.; Maeda, K.; Mohamad, F. B.; Motomura, K.; Sasano, J.; Shinagawa, T.; Watase, S. Electrodeposition of 1.4 eV Bandgap p-Copper (II) Oxide Film With Excellent Photoactivity. J. Electrochem. Soc. 2011, 158 (9), D578−D584. (26) Sagu, J. S.; Peiris, T. A. N.; Wijayantha, K. G. U. Rapid and simple potentiostatic deposition of copper (II) oxide thin films. Electrochem. Commun. 2014, 42, 68−71. (27) Masudy-Panah, S.; Moakhar, R. S.; Chua, C. S.; Tan, H. R.; Wong, T. I.; Chi, D. Z.; Dalapati, G. K. Nanocrystal Engineering of Sputter-Grown CuO Photocathode for Visible-Light-Driven Electrochemical Water Splitting. ACS Appl. Mater. Interfaces 2016, 8, 1206− 1213. (28) Lee, J. G.; Kim, D. Y.; Lee, J. H.; Kim, M. W.; An, S.; Jo, H. S.; Nervi, C.; Al-Deyab, S. S.; Swihart, M. T.; Yoon, S. S. Scalable BinderFree Supersonic Cold Spraying of Nanotextured Cupric Oxide (CuO) Films as Efficient Photocathodes. ACS Appl. Mater. Interfaces 2016, 8, 15406−15414. (29) Guo, X.; Diao, P.; Xu, D.; Huang, S.; Yang, Y.; Jin, T.; Wu, Q. Y.; Xiang, M.; Zhang, M. CuO/Pd composite photocathodes for photoelectrochemical hydrogen evolution reaction. Int. J. Hydrogen Energy 2014, 39, 7686−7696. (30) Emin, S.; Abdi, F. F.; Fanetti, M.; Peng, W.; Smith, W.; Sivula, K.; Dam, B.; Valant, M. A novel approach for the preparation of textured CuO thin films from electrodeposited CuCl and CuBr. J. Electroanal. Chem. 2014, 717-718, 243−249. (31) Jang, Y. J.; Jang, J. W.; Choi, S. H.; Kim, J. Y.; Kim, J. H.; Youn, D. H.; Kim, W. Y.; Han, S.; Lee, J. S. Tree branch-shaped cupric oxide for highly effective photoelectrochemical water reduction. Nanoscale 2015, 7, 7624−7631. (32) Septina, W.; Prabhakar, R. R.; Wick, R.; Moehl, T.; Tilley, D. S. Stabilized solar hydrogen production with CuO/CdS heterojunction thin film photocathodes. Chem. Mater. 2017, 29, 1735−1743. (33) Haynes, W. CRC Handbook of Chemistry and Physics, 92nd ed.; CRC Press: Boca Raton, FL, 2011−2012. (34) Therese, G. H. A.; Kamath, P. V. Electrochemical synthesis of metal oxides and hydroxides. Chem. Mater. 2000, 12, 1195−1204. (35) Kang, D.; Kim, T. W.; Kubota, S. R.; Cardiel, A. C.; Cha, H. G.; Choi, K. S. Electrochemical Synthesis of Photoelectrodes and Catalysts for Use in Solar Water Splitting. Chem. Rev. 2015, 115, 12839−12887. (36) Hill, J. C.; Ping, Y.; Galli, G. A.; Choi, K. S. Synthesis, photoelectrochemical properties, and first principles study of n-type CuW1‑xMoxO4 electrodes showing enhanced visible light absorption. Energy Environ. Sci. 2013, 6, 2440−2446. (37) Andreoli, E.; Rooney, D. A.; Redington, W.; Gunning, R.; Breslin, C. B. Electrochemical Deposition of Hierarchical Micro/ Nanostructures of Copper Hydroxysulfates on Polypyrrole-Polystyrene Sulfonate Films. J. Phys. Chem. C 2011, 115, 8725−8734. (38) Tan, Y. W.; Steinmiller, E. M. P.; Choi, K. S. Electrochemical tailoring of lamellar-structured ZnO films by interfacial surfactant templating. Langmuir 2005, 21, 9618−9624. (39) Jia, W. Z.; Reitz, E.; Sun, H.; Zhang, H.; Lei, Y. Synthesis and characterization of novel nanostructured fishbone-like Cu(OH)2 and CuO from Cu4SO4(OH)6. Mater. Lett. 2009, 63, 519−522. (40) Poizot, P.; Hung, C. J.; Nikiforov, M. P.; Bohannan, E. W.; Switzer, J. A. An electrochemical method for CuO thin film deposition from aqueous solution. Electrochem. Solid-State Lett. 2003, 6, C21− C25. (41) Nakaoka, K.; Ogura, K. Electrochemical preparation of p-type cupric and cuprous oxides on platinum and gold substrates from copper(II) solutions with various amino acids. J. Electrochem. Soc. 2002, 149, C579−C585.

(3) Effenberger, H. Verfeinerung der Kristallstruktur des monoklinen Dikupfer(II)-trihydroxi-nitrates Cu2(NO3) (OH)3. Z. Kristallogr. 1983, 165, 127−135. (4) Yamanaka, S.; Sako, T.; Seki, K.; Hattori, M. Anion-Exchange Reactions in Layered Basic Copper-Salts. Solid State Ionics 1992, 53-56, 527−533. (5) Andreoli, E.; Annibaldi, V.; Rooney, D. A.; Liao, K. S.; Alley, N. J.; Curran, S. A.; Breslin, C. B. Electrochemical Conversion of CopperBased Hierarchical Micro/Nanostructures to Copper Metal Nanoparticles and Their Testing in Nitrate Sensing. Electroanalysis 2011, 23, 2164−2173. (6) Xia, C.; Ning, W. A novel bio-electrochemical ascorbic acid sensor modified with Cu4(OH)6SO4 nanorods. Analyst 2011, 136, 288−292. (7) Meyn, M.; Beneke, K.; Lagaly, G. Anion-Exchange Reactions of Hydroxy Double Salts. Inorg. Chem. 1993, 32, 1209−1215. (8) Pinnavaia, T.; Thorpe, M. F.; Bruschini, C.; Hudson, M. Structure and Reversible Anion Exchange in Copper Hydroxy Double Salts. Access in Nanoporous Materials; Springer: 2002; pp 161−173. (9) Kandare, E.; Hossenlopp, J. M. Hydroxy double salt anion exchange kinetics: Effects of precursor structure and anion size. J. Phys. Chem. B 2005, 109, 8469−8475. (10) Fei, H. H.; Oliver, S. R. J. Copper Hydroxide Ethanedisulfonate: A Cationic Inorganic Layered Material for High-Capacity Anion Exchange. Angew. Chem., Int. Ed. 2011, 50, 9066−9070. (11) Biswick, T.; Jones, W.; Pacula, A.; Serwicka, E. Synthesis, characterisation and anion exchange properties of copper, magnesium, zinc and nickel hydroxy nitrates. J. Solid State Chem. 2006, 179, 49−55. (12) Kandare, E.; Chigwada, G.; Wang, D. Y.; Wilkie, C. A.; Hossenlopp, J. M. Nanostructured layered copper hydroxy dodecyl sulfate: A potential fire retardant for poly(vinyl ester) (PVE). Polym. Degrad. Stab. 2006, 91, 1781−1790. (13) Kandare, E.; Hossenlopp, J. M. Effects of Hydroxy Double Salts and Related Nanodimensional-Layered Metal Hydroxides on Polymer Thermal Stability. Polymer Degradation and Performance; American Chemical Society: Washington, DC, 2009; Vol. 1004, pp 209−218. (14) Fujita, W.; Awaga, K. Reversible structural transformation and drastic magnetic change in a copper hydroxides intercalation compound. J. Am. Chem. Soc. 1997, 119, 4563−4564. (15) Vilminot, S.; Richard-Plouet, M.; Andre, G.; Swierczynski, D.; Bouree-Vigneron, F.; Marino, E.; Guillot, M. Synthesis, structure and magnetic properties of copper hydroxysulfates. Cryst. Eng. 2002, 5, 177−186. (16) Vilminot, S.; Richard-Plouet, M.; Andre, G.; Swierczynski, D.; Guillot, M.; Bouree-Vigneron, F.; Drillon, M. Magnetic structure and properties of Cu3(OH)4SO4 made of triple chains of spins s = 1/2. J. Solid State Chem. 2003, 170, 255−264. (17) Guillou, N.; Louer, M.; Louer, D. An X-Ray and Neutron Powder Diffraction Study of a New Polymorphic Phase of Copper Hydroxide Nitrate. J. Solid State Chem. 1994, 109, 307−314. (18) Park, S.-H.; Lee, C.-E. Synthesis, Characterization and Magnetic Properties of a Novel Disulfonate-pillared Copper Hydroxide Cu2(OH)3(DS4)1/2, DS4 = 1,4-Butanedisulfonate. Bull. Korean Chem. Soc. 2006, 27, 1587−1592. (19) Roske, C. W.; Lefler, J.; Muller, A. Complex nanomineral formation utilizing kinetic control by PLAL. J. Colloid Interface Sci. 2017, 489, 68−75. (20) Rajamathi, J. T.; Arulraj, A.; Ravishankar, N.; Arulraj, J.; Rajamathi, M. Delamination of surfactant-intercalated brucite-like hydroxy salts of cobalt and copper and solvothermal decomposition of the resultant colloidal dispersions. Langmuir 2008, 24, 11164−11168. (21) Rajamathi, J. T.; Ravishankar, N.; Rajamathi, M. Delaminationrestacking behaviour of surfactant intercalated layered hydroxy double salts, M3Zn2(OH)8(surf)2.2H2O [M = Ni, Co and surf = dodecyl sulphate (DS), dodecyl benzene sulphonate (DBS)]. Solid State Sci. 2005, 7, 195−199. (22) Zhu, L. Y.; Su, S. P.; Hossenlopp, J. M. Thermal stability and fire retardancy of PMMA (nano)composites with layered metal hydroxides H

DOI: 10.1021/acs.langmuir.7b00588 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir (42) Siegfried, M. J.; Choi, K. S. Conditions and mechanism for the anodic deposition of cupric oxide films in slightly acidic aqueous media. J. Electrochem. Soc. 2007, 154, D674−D677. (43) Switzer, J. A.; Kothari, H. M.; Poizot, P.; Nakanishi, S.; Bohannan, E. W. Enantiospecific electrodeposition of a chiral catalyst. Nature 2003, 425, 490−493. (44) Joseph, S.; Kamath, P. V.; Upadhya, S. Electrochemical Synthesis of Oriented CuO Coatings on Stainless Steel Substrates: Solution-Mediated Control over Orientation. J. Electrochem. Soc. 2009, 156, E18−E22. (45) Kim, T. W.; Choi, K. S. Nanoporous BiVO4 Photoanodes with Dual-Layer Oxygen Evolution Catalysts for Solar Water Splitting. Science 2014, 343, 990−994. (46) Luo, J. S.; Steier, L.; Son, M. K.; Schreier, M.; Mayer, M. T.; Gratzel, M. Cu2O Nanowire Photocathodes for Efficient and Durable Solar Water Splitting. Nano Lett. 2016, 16, 1848−1857. (47) Paracchino, A.; Laporte, V.; Sivula, K.; Gratzel, M.; Thimsen, E. Highly active oxide photocathode for photoelectrochemical water reduction. Nat. Mater. 2011, 10, 456−461. (48) Morales-Guio, C. G.; Liardet, L.; Mayer, M. T.; Tilley, S. D.; Gratzel, M.; Hu, X. L. Photoelectrochemical Hydrogen Production in Alkaline Solutions Using Cu2O Coated with Earth-Abundant Hydrogen Evolution Catalysts. Angew. Chem., Int. Ed. 2015, 54, 664−667. (49) Yang, Y.; Wang, M.; Zhang, P. L.; Wang, W. H.; Han, H. X.; Sunt, L. C. Evident Enhancement of Photoelectrochemical Hydrogen Production by Electroless Deposition of M-B (M = Ni, Co) Catalysts on Silicon Nanowire Arrays. ACS Appl. Mater. Interfaces 2016, 8, 30143−30151. (50) Zhao, Y.; Anderson, N. C.; Zhu, K.; Aguiar, J. A.; Seabold, J. A.; van de Lagemaat, J.; Branz, H. M.; Neale, N. R.; Oh, J. Oxidatively Stable Nanoporous Silicon Photocathodes with Enhanced Onset Voltage for Photoelectrochemical Proton Reduction. Nano Lett. 2015, 15, 2517−2525. (51) Berglund, S. P.; He, H. C.; Chemelewski, W. D.; Celio, H.; Dolocan, A.; Mullins, C. B. p-Si/W2C and p-Si/W2C/Pt Photocathodes for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2014, 136, 1535−1544. (52) Wang, P.; Wen, X. M.; Amal, R.; Ng, Y. H. Introducing a protective interlayer of TiO2 in Cu2O-CuO heterojunction thin film as a highly stable visible light photocathode. RSC Adv. 2015, 5, 5231− 5236. (53) Li, Z. Z.; Xin, Y. M.; Zhang, Z. H. New Photocathodic Analysis Platform with Quasi-Core/Shell-Structured TiO2@Cu2O for Sensitive Detection of H2O2 Release from Living Cells. Anal. Chem. 2015, 87, 10491−10497. (54) Kang, D.; Hill, J. C.; Park, Y.; Choi, K. S. Photoelectrochemical Properties and Photostabilities of High Surface Area CuBi2O4 and AgDoped CuBi2O4 Photocathodes. Chem. Mater. 2016, 28, 4331−4340. (55) Read, C. G.; Park, Y.; Choi, K. S. Electrochemical Synthesis of pType CuFeO2 Electrodes for Use in a Photoelectrochemical Cell. J. Phys. Chem. Lett. 2012, 3, 1872−1876.

I

DOI: 10.1021/acs.langmuir.7b00588 Langmuir XXXX, XXX, XXX−XXX