Stable and Efficient CuO Based Photocathode through Oxygen-Rich

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Stable and Efficient CuO Based Photocathode through Oxygen-Rich Composition and Au−Pd Nanostructure Incorporation for SolarHydrogen Production Saeid Masudy-Panah,†,‡,§,# Roozbeh Siavash Moakhar,†,∥,# Chin Sheng Chua,† Ajay Kushwaha,†,⊥ and Goutam Kumar Dalapati*,† †

Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Innovis, #08-03, Singapore 138634 ‡ Low Energy Electronic System (LEES), Singapore-MIT Alliance for Research and Technology (SMART), 1 CREATE Way, #09-01/02 CREATE Tower, Singapore 138602 § Electrical and Computer Engineering, National University of Singapore, Singapore 119260 ∥ Department of Materials Science and Engineering, Sharif University of Technology, Tehran 11155-9466, Iran ⊥ Discipline of Metallurgy Engineering and Materials Science, IIT Indore, Indore, Madhya Pradesh 453552, India S Supporting Information *

ABSTRACT: Enhancing stability against photocorrosion and improving photocurrent response are the main challenges toward the development of cupric oxide (CuO) based photocathodes for solar-driven hydrogen production. In this paper, stable and efficient CuO-photocathodes have been developed using in situ materials engineering and through gold− palladium (Au−Pd) nanoparticles deposition on the CuO surface. The CuO photocathode exhibits a photocurrent generation of ∼3 mA/cm2 at 0 V v/s RHE. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis and X-ray spectroscopy (XPS) confirm the formation of oxygen-rich (O-rich) CuO film which demonstrates a highly stable photocathode with retained photocurrent of ∼90% for 20 min. The influence of chemical composition on the photocathode performance and stability has been discussed in detail. In addition, O-rich CuO photocathodes deposited with Au−Pd nanostructures have shown enhanced photoelectrochemical performance. Linear scan voltammetry characteristic shows ∼25% enhancement in photocurrent after Au−Pd deposition and reaches ∼4 mA/cm2 at “0” V v/s RHE. Hydrogen evolution rate significantly depends on the elemental composition of CuO and metal nanostructure. The present work has demonstrated a stable photocathode with high photocurrent for visible-light-driven water splitting and hydrogen production. KEYWORDS: oxygen-rich CuO, stability, photoelectrochemical water splitting, Au−Pd decorated CuO thin films, solar-hydrogen production, CuO-photocathode

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INTRODUCTION Global energy demand is growing rapidly. Reliable, sustainable, and affordable renewable energy is critical to meet basic human needs. Therefore, significant technological developments are imperative to secure clean renewable energy with low carbon emission.1 Conversion of solar energy by photoelectrochemical (PEC) water splitting is a potential technology that can be used extensively to produce clean energy and meet the rising energy requirements.2−4 However, development of suitable photocatalytic materials is challenging to realize hydrogen production by water splitting. Copper oxide is an inexpensive, nontoxic, and easy to synthesize material. It has a suitable band gap (varying from 1.2 to 2.5 eV) and is a potential candidate for visible-lightdriven PEC water splitting. However, its photocorrosion with time and low photocurrent are the key concerns to implement it as a photocathode in PEC water splitting. Toward this, a more © 2017 American Chemical Society

stable and efficient photocathode has been developed through in situ material engineering comprising a nanostructured coating on the sputter grown cupric oxide (CuO). In a PEC water splitting cell, solar energy is harvested by the photocatalytic electrode material and generates electron−hole pairs, followed by chemical reaction at the electrode−electrolyte interface to produce hydrogen.5 Light absorption capability, photocorrosion stability in electrolyte, and cost of the electrode material are the key parameters to determine the efficiency of the PEC water splitting cell.6 Copper oxide is a highly promising p-type candidate for water splitting applications because of its high optical absorption, abundance of raw material, nontoxicity, Received: February 23, 2017 Accepted: July 21, 2017 Published: July 21, 2017 27596

DOI: 10.1021/acsami.7b02685 ACS Appl. Mater. Interfaces 2017, 9, 27596−27606

Research Article

ACS Applied Materials & Interfaces

annealing treatment at 550 °C was carried out in nitrogen ambient for 1 min to improve the crystal quality of the as-deposited O-rich and Cu-rich CuO film. The films’ details and corresponding sputtering power and working pressure were tabulated in Table 1. To incorporate

and ease of production.7−19 The optical band gap energy of CuO is around 1.5 eV, which is very suitable for sunlight absorption.20−23 Theoretically predicted photocurrent of CuO is ∼35 mA cm−2 under standard AM1.5G irradiation with maximum conversion efficiency of 31%.24,25 Despite the attractive features of CuO for PEC application, only a handful of works have been reported on CuO based PEC water splitting owing to instability and poor performance.9,10,26−35 Issues with copper oxide instability have been addressed by using a protecting layer.36,37 In our previous report, it was shown that by improving the crystal quality of the CuO film the photocurrent and photocorrosion stability of the CuO photocathode can be improved significantly.33,34 With the increase of sputtering power from 30 to 300 W, the photocurrent and stability of the CuO photocathode were enhanced remarkably.33 Furthermore, thermal treatment of the as-deposited CuO film also improved the PEC performance. The optimum annealing temperature for sputtered CuO thin film on FTO-coated glass substrate was shown to be ∼550 °C.34 Recently, Fishman et al.38 reported the impact of oxygen composition for band gap and reactivity of CuO film. However, tuning of material composition without changing the optical properties and its impact on the photocatalytic performances have not been addressed yet in previous reports. Thus, investigating the origin of instability of CuO electrodes during solar-driven water splitting is of high importance which can provide new opportunities to realize a stable CuO based PEC cell. The stability of the photocathode can be significantly controlled by tuning the elemental composition and stoichiometry of the CuO film. Sputter deposition is comprehensively used methods in the semiconductor industry. Chemical composition and structural property can be controlled precisely.20−23 By controlling the working pressure during sputtering, oxygen-rich (O-rich) CuO and copper-rich (Cu-rich) CuO are sputtered on the FTO-coated glass substrate. PEC water splitting measurements show that the stability of the CuO electrodes is considerably influenced by the amount of Cu or O elements in the films. The oxygen-rich CuO electrode shows enhanced photocurrent and stability against photocorrosion. In addition, Au−Pd nanostructures are coated on an oxygen-rich CuO film to harvest a wider solar spectrum.39,40 Because of the interaction of plasmonic nanostructures with the semiconductor, the light trapping and electron−hole pair production in the nearby semiconductor were enhanced, which results in improvement of the energy conversion efficiency.41−43 The present work demonstrates a stable and efficient photocathode using a sputter grown O-rich CuO thin film for PEC water splitting applications. The performance of the O-rich CuO photocathode is further significantly improved through Au−Pd nanoparticles deposition on the CuO surface.

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Table 1. List of the CuO Films and Corresponding Sputtering Power and Working Pressure films B1 B2 B3 B4 B5 B6

sputtering power

working pressure

CuO (W)

Cu (W)

mTorr

300 300 300 300 300 300

0 0 0 0 3 6

3.3 8 15 40 40 40

gold−palladium (Au−Pd) nanoparticles on the O-rich CuO surface, bimetallic Au−Pd was sputtered from Au−Pd (60:40) target using a JEOL smart coater machine with the current of 30 mA. TOF-SIMS was used to analyze the chemical composition and elemental distribution of copper (Cu) and oxygen (O) in the prepared samples. Optical properties of the sputter deposited CuO film were measured by a Shimadzu UV-3101PC scanning spectrophotometer. The VG ESCALAB 220i-XL XPS system was used to determine the chemical composition and phase of the O-rich and Cu-rich CuO films. Structural property of the sputtered CuO films was investigated by the GADDS XRD system using a CuKα (λ = 0.154 18 nm) radiation. The photoelectrochemical characteristics of a CuO photocathode of different composition were evaluated in 0.1 M Na2SO4 electrolyte solution with a pH value of 5.84 using chronoamperometry (CA) and linear sweep voltammetry (LSV). An Autolab 302N galvanostat/ potentiostat with reference (Ag/AgCl), counter (Pt), and working electrode (CuO) was employed to measure the electrochemical characteristics. The active area of the CuO and Pt electrodes is 1 cm2.

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RESULTS AND DISCUSSION TOF-SIMS analysis was conducted to compare the relative amounts of the copper (Cu) and oxygen (O) species present in the films grown at different working pressure (B1−B4) and films grown at different Cu cosputtering power (B4−B6). Figure 1a,b shows the intensity of Cu and O which is considerably varied with working pressure. With the increase of working pressure, the intensity of O increases, while the corresponding intensity of the Cu element decreases, indicating the formation of O-rich CuO thin films. Figure 1c,d shows the distribution of O and Cu of B4−B6 samples. With the increase of sputtering power of Cu from 0 to 6 W, the intensity of Cu enhances, while the intensity of O is reduced which suggests the formation of O-poor (Cu-rich) CuO thin film. Chemical composition of the CuO thin films was investigated by XPS analysis. The deconvolution of O 1s spectra of the sample B1 is also presented in this figure. The O 1s spectra exhibit two strong peaks (denoted as “I”) at the binding energy of ∼529.4−529.6 eV and the satellite peak (denoted as “II”) at the binding energy of ∼531.7−532.0 eV. Peaks I and II are ascribed to the “O2−” ions of the crystalline network (Cu−O) and subsurface “O−” species, respectively.44,45 As shown in Figure 2b, the intensity of the “O−” signal increases, while the intensity of “O2−” remains the same with the increase of working pressure from 3.3 mTorr to 40 mTorr (films B1−B4). These results suggest the presence of more subsurface “O−” species in the films sputtered at high working pressure and form oxygen-rich CuO film. However, by increasing the Cu content in the film through the increase of cosputtering power of Cu from 0 to 6 W, intensity of

EXPERIMENTAL SECTION

Stoichiometric cupric oxide (CuO) target and pure copper (Cu) target with purity 99.99% were used to deposit CuO, Cu-rich CuO, and O-rich CuO films on fluorine-doped tin oxide (FTO) coated glass substrates. FTO coated glass substrates were cleaned ultrasonically in isopropyl alcohol (IPA) before deposition. After being purged with nitrogen, substrates were loaded into the sputtering chamber. CuO thin film was deposited at 25 °C using stoichiometric CuO target in argon (Ar) ambient at a sputtering power of 300 W. Sputtering was performed at different working pressures ranging from 3.3 mTorr to 40 mTorr by changing the Ar gas flow to vary the chemical composition in the deposited film. The Cu-rich CuO film was synthesized with cosputter of Cu. The sputtering power of Cu was varied from 0 to 6 W. Rapid thermal 27597


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Table 2. Value of Cu and O Percentage in the Films B1−B6 Calculated Using XPS Analysis Cu (%) O (%) O/Cu

B1

B2

B3

B4

B5

B6

55 45 0.80

51 49 0.96

47 53 1.12

44 56 1.27

49 51 1.04

53 47 0.89

excessive oxygen existence on the surface, most likely in the O2(ads) form.46 Structural properties of the sputter deposited O-rich and Opoor CuO thin films were then analyzed by XRD measurements. The XRD peaks located at 32.548°, 35.482°, 38.764°, and 53.459° in Figure 3a are associated with the CuO (110), CuO

Figure 1. TOF-SIMS analysis of (a) oxygen and (b) copper distribution in B1−B4 samples (films grown at different working pressure) and (c) oxygen and (d) copper distribution in B4−B6 samples (films grown at different Cu cosputtering power).

Figure 3. (a) XRD spectra of samples B1−B6 and (b) enlargement of (a) to show the variation of peaks width. (c) fwhm of B1−B4 samples and (d) fwhm of B4−B6 samples.

(002), CuO (111), and CuO (020), respectively (JCPDS# 050661). For better illustration, the magnified views of the XRD peaks of the dominant plains of (002) and (111) are shown in Figure 3b. The corresponding full-width half-maximum (fwhm) of the two main XRD peaks of CuO (111) and CuO (002) of films B1−B6 are illustrated in Figure 3c,d. All films have the CuO dominant phase while no trace of Cu2O phases is observed. When the working pressure is increased from 3.3 mTorr to 40 mTorr, fwhm value is also similar, which indicates that the crystal quality of the deposited films is comparable. It is worth noting that the crystal quality of the sputter grown CuO film increases with sputtering power and annealing temperature.33,34 In the present work, the CuO film was deposited at a sputtering power of 300 W and annealed at 550 °C in ambient nitrogen. Co-sputtering of the Cu at the high working pressure of 40 mTorr does not significantly affect the XRD intensity and fwhm of the prepared CuO thin films, maintaining similar crystalline quality of O-rich and O-poor CuO films deposited at 40 mTorr working pressure. Furthermore, high-resolution transmission electron microscopy (HR-TEM) analysis was carried out to investigate the crystallinity of the prepared samples. Figure 4 shows the HR-TEM images of the samples. As shown in Figure 4a−d, the

Figure 2. XPS analysis of CuO thin film grown at different working pressure. (a) Deconvolution of O 1s XPS spectra of B1 sample, (b) O 1s XPS spectra of B1−B4 samples, and (c) O 1s XPS spectra of B4−B6 samples.

the “O−” signal decreases without significant change of intensity of “O2−”, as shown in Figure 2c (samples B4−B6). This exhibits growth of O-poor (Cu-rich) CuO by incorporating more Cu species in the films. The surface atomic compositions of the films B1−B6 were also calculated from the XPS spectra and the extracted values listed in Table 2 to provide more information about the chemical composition of the sputtered samples. Sample B4 gives the highest O/Cu atomic ratio sputtered at a working pressure of 40 mTorr which implies the 27598


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Figure 5. (a) PEC current−voltage measurements for B1 sample (deposited at 3 mTorr), B2 (deposited at 8 mTorr), B3 (deposited at 15 mTorr), and B4 (deposited at 40 mTorr). (b) PEC current−voltage measurements for B4 sample (deposited at 40 mTorr without Cu cosputter), B5 (deposited at 40 mTorr with Cu cosputtering power of 3 W), and B6 (deposited at 40 mTorr with Cu cosputtering power of 6 W). (c) Current−voltage measurements for B4 samples with thickness of 150 and 500 nm. For LSV and CA measurement, scan rate and potential were set to 50 mV s−1 and 0.2 V vs RHE, respectively.

Figure 4. HR-TEM images of CuO thin films grown at different working pressure of (a) 3.3 mTorr, (b) 8 mTorr, (c) 15 mTorr, and (d) 40 mTorr. HR-TEM of CuO thin films grown at 40 mTorr at Cu cosputtering power of (e) 3 W and (f) 6 W.

lattice fringe of the films deposited at different working pressure can be clearly seen, indicating the formation of similar crystalline films. From Figure 4d−f it can be seen that the lattice fringes look similar in all three films. Indeed, the cosputtering of Cu does not significantly affect the crystal quality. The influence of the formation of O-rich and O-poor CuO on the optical characteristics of the sputtered thin films on glass substrate was examined by an UV−vis-NIR scanning spectrophotometer. Transmittance spectra of the samples were measured at normal incidence (0°). Figure S1 of Supporting Information, SI, illustrates optical transmittance and the Tauc curves of (αhν)0.5 versus photon energy (hν). As presented, the optical property of O-rich and O-poor CuO thin film is comparable. The impact of Cu-rich and O-rich CuO thin films on the PEC water splitting properties was investigated by PEC current− voltage characteristics in the dark (“light off”) and under sunlight of 100 mW/cm2 and AM 1.5 (“light on”) for samples B1−B6. For all samples, the thickness of the CuO is ∼150 nm, measured by using cross-sectional TEM as presented in Figure S2. Sample B1 with O/Cu ratio of 0.80 produces a photocurrent density of −0.92 mA/cm2 at 0 V vs RHE, and the photocurrent density is increased up to −1.53 mA/cm2 when O/Cu ratio is increased to 1.27 for the sample B4 (Figure 5a). The photocurrent density of the samples B5 (O/Cu ratio = 1.04) and B6 (O/Cu ratio = 0.89) is lower than the sample B4 (Figure 5b). Since the structural and optical properties were found to be comparable in all the CuO films (Figure S1), it can be concluded that the compositional change of the CuO films significantly affects the PCE performance. Indeed, the photocurrent density reduces from 1.53 mA/cm2 (in sample B4) to 1.24 mA/cm2 (in sample B6) with the increase of Cu composition in the film, although the samples have similar crystal quality. Thus, as the photocurrent is dominated by electron transfer in the p-type photoelectrodes, the enhancement in photocurrent with the increase of O/Cu ratio

can be mainly attributed to improvement of separation of electrons from holes and longer lifetime of photoinduced electron− hole pairs.47 The thickness of the CuO electrode significantly influences the absorption of light and properties of charge carrier transport. It is shown that the optimum thickness of CuO is around 500− 600 nm.26,33,34,48 Therefore, the film thickness is optimized for the best quality sample (O-rich CuO film, B4) for PEC water splitting properties. Figure 5(c) illustrates the PEC current− voltage characteristics of the 500-nm thick CuO electrode under light ON and OFF conditions. The photocurrent density of around −3.1 mA/cm2 at 0 V vs RHE is observed, and it is found to be the highest reported photocurrent using a plain CuO based thin film photocathode with the thickness of ∼500 nm (Table 3). The realization of the CuO based photocathode for PEC water splitting is hindered due to the poor stability against photocorrosion. To test the stability of the prepared photocathodes, amperometric current density (J) vs time (t) (J−t) measurements were conducted under chopped visible illumination. Figure 6 illustrates the transient current density profile over a time scale of 1250 s at a potential of 0.125 V vs RHE. Photocathodic current overshoots immediately upon illumination with visible light and is mainly originated from sudden generation of photo charge carriers and electron accumulation at the surface of photocathode. Current overshoot decays rapidly and reaches the stable state owing to surface recombination of electrons and holes at the surface of the photoelectrode and the balance of the process of recombination and generation.49 As shown in Figure 6a−c, sample B1 could retain almost 68% of the initial photocurrent after 1250 s, whereas sample B4 retained 89% of the initial photocurrent. It is important to note that the sample B1 is O-poor CuO as observed from XPS and SIMS analysis. 27599


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ACS Applied Materials & Interfaces Table 3. Photocurrents of CuO Electrodes Fabricated by Different Synthesis Approach for PEC Water Splitting sn.

type of photocathode

1 2

CuO thin film CuO thin film

3 4

CuO thin films Ni doped CuO nanoparticles CuO nanowire Ti-alloyed CuO thin film

5 6 7 8 9 10 11 12 13

CuO nanoparticles films CuO thin film heterojunction CuO nanowire CuO nanoparticle Li-doped CuO nanoparticles CuO thin film Au−Pd decorated CuO thin film

fabrication process

photocurrent

stability

thickness of CuO

ref.

sputter deposition electrolysis deposition-thermal oxidation sol−gel process flame spray pyrolysis

−1.8 mA/cm2 @ −0.55 V (Ag/AgCl) −1.8 mA/cm2 @ 0 V (RHE)

not provided 8% in 20 min

520 nm not provided

8 9

−0.55 mA/cm2 @ 0.05 V (RHE) −1.07 mA/cm2 @ −0.55 V (Ag/AgCl)

90% in 5 min not provided

600 nm 850 nm

25 26

electrochemical deposition radio frequency (RF) magnetron sputter solution process RF-magnetron sputtering thermal oxidation and hydrothermal growth flame spray pyrolysis flame spray pyrolysis

−0.25 mA/cm2 @ −0.5 V (Ag/AgCl) −0.09 mA/cm2 @ −0.5 V (Ag/AgCl)

95% in 2 min not provided

1000 nm 500 nm

27 28

−1.2 mA/cm2 @ −0.7 V (Ag/AgCl) −2.5 mA/cm2 @ 0 V (RHE) −0.65 mA/cm2 @ −0.45 V (Ag/AgCl)

not provided 68% in 20 min ∼75% in 3 min

1340 nm 500 nm ∼10 μm

30 32 34

−1.2 mA/cm2 @ −0.55 V (Ag/AgCl) −1.7 mA/cm2 @ −0.55 V (Ag/AgCl)

not provided not provided

387 nm 1700 nm

35 46

RF-magnetron sputtering

−3.1 mA/cm2 @ 0 V (RHE)

89% in 20 min

500 nm

RF-magnetron sputtering

−3.88 mA/cm2 @ 0 V (RHE)

91% in 20 min

500 nm

present work present work

Figure 6. Photocurrent stability of the thin film grown at (a) 3.3 mTorr, (b) 15 mTorr, and (c) 40 mTorr. (d) Stability of CuO thin film grown at 40 mTorr of thickness 150 and 500 nm.

Figure 7. Photocurrent stability of CuO photocathode grown at (a) 40 mTorr without Cu cosputtering (sample B4). (b) Cu-rich CuO grown at 40 mTorr with cosputtering power of (b) 3 W (sample B5) and (c) 6 W (sample B6), respectively.

The O-rich CuO photocathode (sample B4) is more stable in the photo-oxidation process. The comparative study of the Cu-rich and O-rich CuO photocathodes shows that higher O/Cu ratio results in better photocorrosion stability. The thicker CuO photocathode (500 nm) also exhibits stable photocurrent and retained ∼90% of the initial photocurrent after 20 min, as shown in Figure 6d. Figure 7 shows the stability of Cu-rich CuO photocathode. With the increase of Cu amount in the films, stability decreases. Since the charge transfer property of CuO is poor, the CuO based photocathodes are not stable during the PEC water splitting. Therefore, improving the charge transfer properties remains the key factor to prepare a stable and efficient photocathode. The semiconducting properties and charge transport behavior of prepared CuO photoelectrodes were analyzed by

electrochemical impedance spectroscopy (EIS). Mott−Schottky measurement was conducted to determine the carrier density (NA) and flat band position (VFB) of the CuO electrodes using the following M-S equation.16 1 2 ⎡ kT ⎤ = ⎥ ⎢⎣(VS − VFB) − 2 NAeεε0 e ⎦ C

(1)

where NA, k, e, C, ε0, ε, T, and VS are the hole carrier density; Boltzmann constant; elemental charge value; space charge capacitance in the semiconductor; permittivity of the vacuum; relative permittivity of the semiconductor (ε of CuO is 10.26); temperature; and applied potential. Since the flat electrode model was used to derive M-S, it just provided a qualitative 27600


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ACS Applied Materials & Interfaces comparison of the different samples with similar material morphology, composition, and device geometry. The M-S plot of the samples B1−B6 as 1/C2 vs potential is presented in Figure 8. The linear part of the M-S curves has the negative slope,

Figure 9. Nyquist characterization of the (a) B1−B4 samples and (b) B4−B6 samples.

the films with higher O/Cu ratio facilitates photoinduced charge migration to the reaction sites on the surface of CuO. This result indicates that the formation of the films with higher O/Cu ratio significantly improves the interfacial charge transport and the efficiency of separation of photoinduced charges under light-on conditions which results in the improvement of the PEC performance. By increasing the Cu vacancy (O-rich condition), the improvement in electrical properties can be achieved in CuO film that leads to better stability of photocathode. The O-rich CuO photocathode shows higher stability against photocorrosion. Higher stability of O-rich CuO based electrodes might have originated from the slower self-reduction of CuO to Cu2O, which facilitates faster electron transfer to the electrolyte. To investigate the origin of the instability of the CuO photocathode, XPS, XRD, and SEM analysis were performed before and after PEC tests. Cu 2p XPS spectra of the samples B1−B6 were measured before and after amperometry measurement and compared in Figure 10. The observed peaks at 933.8 and 954 eV

Figure 8. Mott−Schottky characterization of the (a) B1−B4 samples and (b) B4−B6 samples.

indicating that sputter grown CuO thin films are p-type. The x-intercept of the extrapolated linear part of the M-S curve to the 1/C2 is the estimated value of VFB. Generally in the p-type semiconductors the value of VFB is close to the position of the valence band. The estimated value of NA of the prepared samples can also be determined from the slope of the extrapolated linear part of the M-S curve to the 1/C2. The values of VFB and NA of the films B1−B6 are listed in Table 4. It is observed that with the increase of O/Cu ratio NA and VFB increase. Significant enhancement of the carrier concentration in the samples with high O/Cu ratio is mainly ascribed to the enhancement of density of copper vacancies which improves the charge transport in CuO. The formation energy of intrinsic copper vacancies reduces as O/Cu ratio increases and thus carrier concentration and p-type conductivity of O-rich films increase.50 Moreover, the increased carrier density can enhance the band bending degree at the CuO surface and assist separation of charge at the interface between CuO and electrolyte.51 EIS was carried out to analyze the charge transfer activity at the photoelectrode/solution interface and conductivity of the photoelectrode at a potential of 0 V vs RHE under standard lighton condition. Figure 9 presents the Nyquist plot of the samples B1−B6. The semicircular characteristic of the Nyquist plots at high frequencies is indicative of the process of charge transfer, and the charge transfer resistance (Rct) is related to the diameter of the semicircle.49 The Rct of the sample B1 is highest, and Rct of the B4 is lowest among different samples. The lower resistance of

Figure 10. Cu 2p XPS spectra of B1−B6 samples (a) before and (b) after the amperometry measurement.

correspond to Cu 2p3/2 and Cu 2p1/2 peaks of Cu2+, respectively, indicating the formation of CuO, while the peaks at 952.8 and 932.7 eV, which are observed after amperometry measurement, are ascribed to the Cu 2p1/2 and Cu 2p3/2 characteristic peaks of Cu+, respectively, representing the formation of Cu2O. The existence of shake up peaks at higher binding energies of around 940.8, 943.8, and 962.5 eV in samples B1−B4 before amperometry measurement originates from multiple excitations in copper oxide and ascribes to the partially filled Cu 3d9 shell of Cu2+,54−56 which clearly indicates the formation of a CuO

Table 4. Charge Carrier Density (NA) and Flatland Potential (VFB) of of CuO Film B1−B6 Calculated from M-S Plots

NA VFB

B1

B2

B3

B4

B5

B6

7.41 × 1019 0.75

2.05 × 1020 0.8

8.2 × 1020 0.83

1.57 × 1021 0.94

8.42 × 1020 0.86

4.87 × 1020 0.82

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From the XRD, XPS, and SEM analysis of the samples before and after water splitting test, it can possibly be concluded that the key factor which significantly affects the photocorrosion stability of the CuO based photocathodes is the reduction of Cu2+ to Cu+. The reduction process of oxide is mainly described by different kinetics model where the rate of reaction depends on the area of the reduced oxide (interface-controlled model) or on the initial nucleation of the reduced oxide (nucleation model).58 In the interface-controlled model (ICM), a continuous layer of reduced oxide is formed quickly. The solid reactant is entirely covered by a uniform reduced-oxide/oxide interface, and reduction of oxide rate is related to the area of such interface. While in the nucleation model (NM), small aggregate generation of the reduced oxide clusters is the most influential factor.59 SEM images of the samples after water splitting test suggested that the reduction process of our prepared CuO films fits well with the NM. The main factor of the kinetics of oxide reduction in the NM is existence of induction period.58 It is also shown that the magnitude of induction time is a function of defects in oxide.59 Indeed, the magnitude of the induction time is significantly reduced by the introduction of defects in the oxide. Another parameter which plays the key role in the reduction process of oxide is the oxygen (O) vacancies.60 In our earlier work,23 we have shown that the defect density of the sputtered CuO thin films at higher working pressure is significantly reduced. Reduction of defect density of the sputter deposited samples at high working pressure (O-rich samples) increases the induction time and slows down the phase reduction. As a result O-rich samples are more stable. Long-term photocorrosion stability test of the sample B4 (O-rich sample) and sample B1 (O-poor sample) are compared in Figure S4. As shown in the figure, the O-rich sample is more stable than O-poor sample. This is mainly due to the faster reduction process of CuO in the O-poor sample. XPS spectra of the O-rich B4 sample and O-poor B1 sample before and after stability test for 20 and 120 min are presented in Figure S5. After 120 min of stability test the O-poor B1 sample was almost completely reduced to Cu2O, while the dominant phase of the O-rich B4 sample was CuO. These results are also supported by XRD analysis (data presented in SI). To further investigate the effects of the reduction of CuO on the stability of the prepared photocathode, the J−V characteristics and photocorrosion stability of the control CuO sample (sample B1) are compared with the sputter deposited Cu2O photocathode. Fabrication conditions of Cu2O and CuO thin films (working pressure, sputtering power, annealing conditions, and film thickness) are similar. Cu2O was synthesized by cosputtering the Cu at sputtering power of 60 W. Figure S6 shows the J−V characteristics and photocorrosion stability of CuO and Cu2O photocathodes. The CuO photocathode tends to be more stable than Cu2O and has the higher photocurrent compared with Cu2O. These results are in agreement with the results reported by Han et al.14 and Lim et al.26 To enhance the performance of O-rich CuO photocathode, metal nanostructures were decorated on the surface of CuO. It is well-known that surface plasmon resonance (SPR) can create strong electromagnetic field on the noble metal nanostructures. The light interaction between CuO and nanostructures can enhance optical absorptions and charge separation which may result in improvement of solar conversion efficiency and photocatalytic processes. The O-rich CuO photocathode decorated with Au−Pd nanoparticles was investigated under different coating conditions of Au−Pd nanoparticles. Bimetallic Au−Pd

dominant phase in all of these samples. Before amperometry measurement, the spin−orbit splitting separation between Cu 2p3/2 and Cu 2p1/2 is also around 19 eV which further confirms the formation of the CuO dominant phase. By reducing the O/Cu ratio, the satellite peaks of samples B1−B6 after amperometry measurement begin to disappear, and the core level binding energy of Cu 2p1/2 and Cu 2p3/2 shifts from 954 and 933.8 eV to 952.8 and 932.7 eV, respectively. These results indicate the reduction of CuO into Cu2O during the water splitting process. It can also be observed in these figures that the O-rich CuO photocathodes are more stable against phase transformation and CuO reduction. To investigate further of photocorrosion stability, XRD spectra of samples B1−B6 were measured after PEC test. The results are presented in Figure S3. As shown in the figure, the XRD peak at 36.45° is ascribed to the Cu2O (111) phase (JCPDS# 05-0667). With the increase of Cu/O ratio, the intensity of the Cu2O peak is also increased. Indeed in the samples with higher Cu/O ratio, the dominant phase is changed from CuO to the mixed CuO−Cu2O. Thus, by tuning the chemical composition of the CuO film, photocorrosion stability can be enhanced significantly. Poor performance of the mixed phase CuO−Cu2O samples compared to the CuO dominant phase sample is mainly ascribed to the lower optical absorption of the Cu2O than CuO thin film with the same thickness. The surface morphology of the CuO photocathodes (samples B1−B6) before and after PEC test was analyzed using SEM (Figure 11). Cubicle structures on the surface of the samples after amperometry measurement reveal the formation of Cu2O during charge transfer.57 Density and size of the cubicle structures are significantly influenced by the O/Cu ratio. Indeed, by increasing the O/Cu ratio the formation of unwanted Cu2O is considerably reduced, and hence the photocorrosion stability of the O-rich CuO photocathode is improved.

Figure 11. Top-view SEM images of the B1−B6 samples (a−f) before and (g−l) after the amperometry measurement. 27602


Research Article

ACS Applied Materials & Interfaces was sputtered from Au−Pd (60−40) target on the O-rich CuO photocathode for 30 s (sample C1), 60 s (sample C2), 120 s (sample C3), and 180 s (sample C4). Top view SEM images of the samples C1−C4 are presented in Figure 12. It can be seen

Figure 12. Top view SEM images of O-rich CuO photocathode decorated with Au−Pd nanoparticles. The Au−Pd was deposited on O-rich CuO film for (a) 30 s, (b) 60 s, (c) 120 s, and (d) 180 s. (e) TEM and (f) HR-TEM images of Au−Pd nanoparticles of sample C3. (g) XRD spectra of O-rich CuO decorated with Au−Pd nanostructure (sample C3).

Figure 13. (a) Photocurrent density-applied bias characteristics, (b) photocorrosion stability, and (c) Nyquist characterization of O-rich CuO photocathode decorated with Au−Pd nanoparticles.

absorption of CuO through concentrating the incident field in the CuO.53 However, the improvement of the performance of the decorated samples is significantly influenced by the surface coverage of the photocathodes with Au−Pd plasmonic nanoparticles. The sample C3 has the highest photocurrent, which is possibly due to the lower surface plasmon losses. The CuO based photocathodes were used for hydrogen evolution test. Figure 14a shows hydrogen evolution results for the CuO photocathodes with varying O/Cu composition. The amount of hydrogen evolution significantly depends on the O/Cu ratio. It can be seen that by increasing the O/Cu ratio the rate of the H2 evolution increases. The H2 evolution rate of the sample B4 is ∼2 μmol·h−1, which is determined from the slope of the H2 evolution curve. Figure 14b shows the H2 gas evolution rate of the O-rich CuO photocathode with and without optimized Au−Pd nanostructures. The H2 evolution rate after incorporating the plasmonic nanoparticles is increased from ∼2 to 2.6 μmol h−1. From the measured photocurrent and the amount of gas evolved, the Faradaic efficiency of the reaction is determined from the following relation.18,61

that by increasing the deposition time from 30 to 180 s the distribution of Au−Pd nanoparticles is significantly influenced. The inset of Figure 12 shows the magnified portion of the SEM images. As shown in these figures, the size of Au−Pd nanoparticles of all the samples is around 40−70 nm. For further investigation, TEM and HR-TEM images of Au−Pd nanoparticles of sample C3 are presented in Figure 12f,g. It can be seen that the size of the Au−Pd nanoparticles is around 40 nm. Figure 12g shows XRD spectra of O-rich CuO with the Au−Pd nanostructure (sample C3). XRD peaks at 40.9° and 44.5° are corresponding to Pd and Au, respectively. The absorption spectrum is one of the most important characteristics which is influenced by the distribution of plasmonic nanoparticles. Optical absorption of the O-rich CuO thin film decorated with Au−Pd nanoparticles is presented in Figure S7 to investigate the effects of distribution of Au−Pd nanoparticles. The optical absorption of CuO thin films decorated with Au−Pd nanostructures is significantly enhanced for the samples with more Au−Pd nanoparticles. The enhancement of optical absorption is mainly ascribed to the light scattering from Au−Pd nanostructures and partial contributions from absorption in the Au−Pd nanostructures themselves. Chemical properties of the Au−Pd coated CuO thin films was then investigated by XPS analysis and presented in Figure S8. The J−V characteristics, photocorrosion stability, and Nyquist characterization of O-rich CuO photocathodes decorated with Au−Pd nanoparticles are shown in Figure 13. As shown in these figures, the incorporation of Au−Pd nanostructures significantly improves the performance of the CuO photocathode. The significant improvement of the photocurrent is mainly ascribed to the following factors: (i) SPR induced energy transformation from Au−Pd nanostructures to the CuO film, which leads to increase in the production of electron−hole pair (e−h-pair) in CuO,52 (ii) increasing light scattering, and (iii) increasing the optical

faradaic efficiency = (nH2)/(Q /zF )

(2)

where Q, nH2, F, and z are the total amount of charge passed through the cell, the number of moles of obtained hydrogen gas, the number of electrons transferred per hydrogen molecule (which is equal to 2), and Faraday’s constant (which is equal to 96 485 C/mol), respectively. The total number of moles of hydrogen obtained from a 6 h illumination period for the O-rich CuO photocathode with optimized Au−Pd nanostructures is 15.2 μmol at an average photocurrent of 0.16 mA. Therefore, nH2 = 1.52 × 10−5 mol and Q = 0.15 mA × 6 × 3600 s = 3.24 C. The obtained Faradaic efficiency is 90.5% which is slightly lower than 100% efficiency. Lower Faradaic efficiency is mainly attributed to the back reaction at the Pt electrode.62 The present 27603


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Thickness of CuO, optical properties of O-rich and Cu-rich CuO thin film, and Au−Pd nanoparticles deposited CuO, structural and chemical properties of CuO before and after PEC performance, and long-term stability of the CuO photocathode (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (G.K.D.). ORCID

Saeid Masudy-Panah: 0000-0002-2064-1783 Goutam Kumar Dalapati: 0000-0001-5011-1436 Author Contributions #

S.M.-P. and R.S.M. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Ren Yi, Vignesh Suresh, Ten It Wong, Siarhei Zhuk, Mohit Sharma, and Hui Ru Tan from Institute of Materials Research and Engineering for the fruitful discussion. A.K. gratefully acknowledges the A*STAR-MND funded green building project (IMRE/2C-0109) for financial support.

Figure 14. (a) H2 gas evolution using CuO-based photocathodes with varying O/Cu composition (B1−B6). (b) Hydrogen evolution using O-rich CuO photocathode (B4: O/Cu = 1.27) with Au−Pd coating (Au−Pd coating for 120 s) (sample C3).

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study shows the importance of composition engineering on the development of the stable and efficient photocathode for visiblelight-driven water splitting applications. Incorporation of metal nanostructure enhanced the optical absorption and charge separation, and thus the overall performance of the photocathode increased.

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CONCLUSION O-rich CuO thin film with Au−Pd deposited nanoparticles for efficient and stable photocathode has been developed. The impact of oxygen and copper content on the instability issue during PEC water splitting have been addressed in detail. The compositional changes of CuO from Cu-rich CuO to O-rich CuO significantly influence the stability and performance of CuO based electrodes. O-rich CuO films show remarkably high photocurrent of 3.1 mA/cm2 at 0 V versus RHE for 500 nm thick sputtered photocathode with 90% retention of photocurrent after 20 min. It is explicated that the formation of an unwanted Cu2O phase which significantly influences the photocorrosion stability of the photoelectrode can be considerably reduced through in situ materials engineering using O-rich CuO thin film. The O-rich CuO photocathode shows hydrogen evolution rate of ∼2 μmol h−1. The photocurrent and stability of the prepared photocathode are further improved by coating with Au−Pd nanostructures on the surface of O-rich CuO thin film. The nanostructures act as the plasmonic centers to trap the light and enhance the optical absorption and thus, hydrogen evolution amount increases compared with bare CuO photocathode. This work recommends potential opportunities to develop the PECs using sputter grown O-rich CuO photocathode for visible-light-driven water splitting applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02685. 27604


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Stable and Efficient CuO Based Photocathode through Oxygen-Rich

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