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 Solar-Hydrogen Production Saeid Masudy-Panah, Roozbeh Siavash Moakhar, Chin Sheng Chua, Ajay Kumar Kushwaha, and Goutam Kumar Dalapati ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b02685 • Publication Date (Web): 21 Jul 2017 Downloaded from http://pubs.acs.org on July 21, 2017

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ACS Applied Materials & Interfaces

Stable and Efficient CuO Based Photocathode through Oxygen-Rich Composition and Au-Pd Nanostructure Incorporation for Solar-Hydrogen Production

Saeid Masudy-Panah1,2,3#, Roozbeh Siavash Moakhar1,4#, Chin Sheng Chua1, Ajay Kushwaha1,5, and Goutam Kumar Dalapati1*

1

Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way; Innovis, #08-03, Singapore 138634

2

Low Energy Electronic System (LEES), Singapore-MIT Alliance for Research and

Technology (SMART), 1 CREATE Way, #09-01/02 CREATE Tower, Singapore 138602

3

4

Electrical and Computer Engineering, National University of Singapore, Singapore 119260

Department of Materials Science and Engineering, Sharif University of Technology, Tehran, 11155-9466, Iran 5

Discipline of Metallurgy Engineering and Materials Science, IIT Indore, Indore, MP, India 453552

#

Saeid Masudy-Panah and Roozbeh Siavash Moakhar contributed equally to this work. *Corresponding author E-mail: [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT:

Enhancing stability against photo-corrosion 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 0V 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 demonstrate highly stable photocathode with retained photocurrent of ~90% for 20 minutes. The influence of chemical composition on the photocathode performance and stability have been discussed in detail. In addition, O-rich CuO photocathodes deposited with Au-Pd nanostructures has 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.

Keyword: Oxygen-rich CuO, Stability, Photo-electrochemical 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 photo-electrochemical (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 inexpensive, non-toxic and easy to synthesis materials. It has suitable band gap (varying from 1.2 eV to 2.5 eV), and a potential candidate for visible light driven PEC water splitting. However, its photo-corrosion with time and low photocurrent are the key concern to implement it as photocathode in PEC water splitting. Towards this, more 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 generate electron−hole pairs, followed by chemical reaction at the electrodeelectrolyte interface to produce

hydrogen.5 Light absorption capability, photo-corrosion

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, non-toxicity 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 of the attractive features of CuO for PEC 3



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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 protecting layer.36,37 In our previous report, it was shown that by improving crystal quality of the CuO film, the photocurrent

and photocorrosion stability of the CuO photocathode can be improved

singnificantly.33,34 With the increase of sputtering power from 30 W to 300 W, photocurrent and stability of CuO photocathode 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 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 solardriven water splitting is of high importance which can provide new opportunities to realize a stable CuO based PEC cells. Stability of photocathode can be significantly controlled by tuning the elemental composition and stoichiometry of 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 FTO-coated glass substrate. PEC water splitting measurements show that the stability of CuO electrodes is considerably influenced by the amount of Cu or O elements in the films. Oxygen-rich CuO electrode shows enhanced photocurrent and stability against photo-corrosion. In addition, Au-Pd nanostructures are coated on oxygen-rich CuO film to harvest wider solar spectrum.39,40, Because of the interaction of plasmonic 4


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nanostructures with semiconductor, light trapping and electron−hole pair production in the nearby semiconductor enhanced, which results in improvement of the energy conversion efficiency.41-43 The present work demonstrates stable and efficient photocathode using sputter grown O-rich CuO thin film for PEC water splitting applications. The performance of the Orich CuO photocathode is further improved significantly through Au-Pd nanoparticles deposition on the CuO surface. 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 purging with nitrogen, substrates were loaded into the sputtering chamber. CuO thin film was deposited at 25oC using stoichiometric CuO target in argon (Ar) ambient at a sputtering power of 300 W. Sputtering was performed at different working pressure ranging from 3.3 mTorr to 40 mTorr by changing Ar gas flow to vary the chemical composition in the deposited film. The Cu-rich CuO film was synthesized with co-sputter of Cu. Sputtering power of Cu was varied from 0 W to 6 W. Rapid thermal 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 gold-palladium (Au-Pd) nanoparticles on O-rich CuO surface, bimetallic AuPd was sputtered from Au-Pd (60:40) target using a JEOL smart coater machine with the current of 30 mA.

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Table 1. List of the CuO films and corresponding sputtering power and working pressure Films

Sputtering power

Working pressure

CuO (W)

Cu (W)

mTorr

B1

300

0

3.3

B2

300

0

8

B3

300

0

15

B4

300

0

40

B5

300

3

40

B6

300

6

40

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 was measured by Shimadzu UV-3101PC scanning spectrophotometer. 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 GADDS XRD system using a CuKα (λ = 0.15418 nm) radiation. The photo-electrochemical characteristics of 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). Autolab 302N galvanostat/potentiostat with reference (Ag/AgCl), counter (Pt) and working electrode (CuO) 6


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was employed to measure the electrochemical characteristics. Active area of the CuO and Pt electrode is 1 cm2. 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 (B1B4) and films grown at different Cu co-sputtering power (B4-B6). Figures 1a,b show intensity of Cu and O which is considerably varying with working pressure. With the increase of working pressure, intensity of O increases, while corresponding intensity of Cu element decreases, indicating the formation of O-rich CuO thin films. Figure 1c,d show the distribution of O and Cu of B4-B6 samples. With the increase of sputtering power of Cu from 0 to 6 W, intensity of Cu enhances while intensity of O reduces which suggests the formation of O-poor (Cu-rich) CuO thin film.

7



Copper distribution

2.0 1.5

B1 @ 3.3 mTorr (O/Cu: 0.80) B2 @ 8.0 mTorr (O/Cu: 0.96) B3 @ 15 mTorr (O/Cu: 1.12) B4 @ 40 mTorr (O/Cu: 1.27)

1.0 0.5 0.0

4 Intensity (10 counts/s)


Oxygen distribution

(a)

0

40

80 120 Depth (nm)

2.0 1.5

0.5 0.0

160

B1 @ 3.3 mTorr (O/Cu: 0.80) B2 @ 8.0 mTorr (O/Cu: 0.96) B3 @ 15 mTorr (O/Cu: 1.12) B4 @ 40 mTorr (O/Cu: 1.27)

1.0

(b)

0

40

Oxygen distribution

2.0



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B4 @ 40 mTorr (O/Cu: 1.27) B5 @ 40 mTorr (O/Cu: 1.04) B6 @ 40 mTorr (O/Cu: 0.89)

1.5 1.0 0.5 0.0

160

Copper distribution

2.0 1.5 B4 @ 40 mTorr (O/Cu: 1.27) B5 @ 40 mTorr (O/Cu: 1.04) B6 @ 40 mTorr (O/Cu: 0.89)

1.0 0.5

(c)

0

80 120 Depth (nm)

(d)

40

80

120

160

0.0

Depth (nm)

0

40

80 120 Depth (nm)

160

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

Chemical composition of the CuO thin films was investigated by XPS analysis. The deconvolution of O1s spectra of the sample B1 is also presented in this figure. The O1s 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. Peak I and 8


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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 “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. On the other hand, by increasing the Cu content in the film through the increase of co-sputtering power of Cu from 0 W to 6 W, intensity of “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 excessive oxygen existence on the surface, most likely in the O2(ads) form. 46

Table 2. The value of Cu and O percentage in the films B1-B6 calculated using XPS analysis.

B1

B2

B3

B4

B5

B6

Cu(%)

55

51

47

44

49

53

O(%)

45

49

53

56

51

47

O/Cu

0.80

0.96

1.12

1.27

1.04

0.89

9



(I)

O1s

O1s B1 B2 B3 B4

Intensity (a.u.)

Intensity (a.u.)

(a)

(II)

(b)

529 530 531 532 533 534 Binding Energy (eV)

525

530 535 Binding Energy (eV)

540

B4 B5 B6

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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O1s (c)

525

530 535 Binding Energy (eV)

540

Figure 2. XPS analysis of CuO thin film grown at different working pressure. (a) deconvolution of O1s XPS spectra of B1 sample, (b) O1s XPS spectra of B1-B4 samples and (c) O1s XPS spectra of B4-B6 samples.

10


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(002) (111)

(002)

B6

(110)

B3 B2

30

B5 (O/Cu: 1.04)

Intensity (a.u.)

Intensity (a.u.)

B4

B1

(111)

B6 (O/Cu: 0.89)

(020)

B5

B4 (O/Cu: 1.27) B3 (O/Cu: 1.12) B2 (O/Cu: 0.96) B1 (O/Cu: 0.80)

(a)

40 50 2 Theta (2θ)

60

34

35

36 37 38 2 Theta (2θ)

39

40

(002) (111)

(002) (111) B1 B2

0.6

B3 B4

B1 B2

0.5

B4

FWHM

0.6

B3

0.5

B4

B4

0.4 0

10

20

30

B5

B6

B5

B6

(d)

(c)

0.4

(b)

0.7

0.7

FWHM

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40

Working pressure (mTorr)

0 2 4 6 Cu co-sputtering power (W)

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

Structural properties of the sputter deposited O-rich and O-poor 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 (002), CuO (111) and CuO (020), respectively (JCPDS# 05-0661). For better illustration, the magnified views of 11



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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 the films B1-B6 are illustrated in Figures 3c,d. All films have the CuO dominant phase while no trace of Cu2O phases is observed. When working pressure is increased from 3.3 mTorr to 40 mTorr, FWHM value is also similar, which indicates the crystal quality of the deposited films is comparable. It is worth to note that the crystal quality of the sputter grown CuO film increases with sputtering power and annealing temperature.33,34 In the present work, CuO film was deposited at a sputtering power of 300 W and annealed at 550oC in nitrogen ambient. 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 to investigate the crystallinity of the prepared samples. Figure 4 shows the HRTEM images of the samples. As shown in Figure 4a-d, the 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 can be seen that the lattice fringes look similar in all three films. Indeed, the co-sputtering of Cu does not significantly affects 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 were examined by UV-VIS-NIR scanning spectrophotometer. Transmittance spectra of the samples were measured at normal incidence (0°). Figures S1 of supporting information illustrates optical transmittance and the Tauc curves of (αhν)0.5 versus photon energy (hν). As presented, optical property of O-rich and Opoor CuO thin film is comparable. 12


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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. HRTEM of CuO thin films grown at 40 mTorr at Cu co-sputtering power of (e) 3 W and (f) 6 W.

13



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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, thickness of CuO is ~150 nm, measured by using cross-sectional TEM as presented in Figure S2 in the supporting information. Sample B1 with O/Cu ratio of 0.80 produces a photocurrent density of -0.92 mA/cm2 at 0V 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 in supporting information), it can be concluded that the compositional change of the CuO films significantly affects the PCE performance. Indeed, 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 photo-induced electron-hole pairs.47 The thickness of 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 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 14


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it is found to be the highest reported photocurrent using plain CuO based thin film photocathode with the thickness of ~500 nm (Table 3).

Sputter grown CuO: 150 nm Light off

Sputter grown CuO at 40 mTorr 0.0

2

0.0

Current density (mA/cm )

2 Current density (mA/cm )

-0.5

B1: CuO (O/cu: 0.80) B2: CuO (O/cu: 0.96) B3: CuO (O/Cu: 1.12) B4: CuO (O/Cu: 1.27)

-1.0

-1.5

(a)

Light on

0.0

0.1

0.2

0.3

0.4

Light off -0.5

-1.0

B4: CuO (O/Cu: 1.27) B5: CuO (O/Cu: 1.04) B6: CuO (O/Cu: 0.89) Light on

-1.5

0.5

0.0

0.5 2

0.0

0.1

(b) 0.2

0.3

0.4

0.5

Potential (V vs. RHE)


Current density (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CuO at 40 mTorr (B4) Light off

-0.5 -1.0 -1.5 -2.0

Thickness: 150 nm Thickness: 500 nm

-2.5 -3.0 0.0

(c)

Light on 0.1

0.2

0.3

0.4

0.5


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 co-sputter), B5 (deposited at 40 mTorr with Cu co-sputtering power of 3W), and B6 (deposited at 40 mTorr with Cu co-sputtering power of 6W). (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 mVs-1 and 0.2 V vs RHE, respectively. 15



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Table 3. Photocurrents of CuO electrodes fabricated by different synthesis approach for PEC water splitting

Sn.

Type of photocathode

Fabrication process

Photocurrent

stability

Thickness

Ref.

of CuO

1

CuO thin film

Sputter deposition

-1.8 mA/cm2 @−0.55 V (Ag/AgCl)

Not

520 nm

8

8% in 20

Not

9

min

provided

90% in 5

600 nm

25

850 nm

26

1000 nm

27

500 nm

28

1340 nm

30

500 nm

32

~10µm

34

387 nm

35

1700 nm

46

provided

2

CuO thin film

Electrolysis deposition-

-1.8 mA/cm 2 @ 0V (RHE)

Thermal oxidation

3

CuO thin films

Sol-gel Process

-0.55 mA/cm2@ 0.05V( RHE)

min

4

Ni doped CuO

Flame spray pyrolysis

-1.07 mA/[email protected] (Ag/AgCl)

nanoparticles

5

CuO nanowire

Not provided

Electrochemical deposition

-0.25mA/cm2 @ -0.5V (Ag/AgCl)

95% in 2 min

6

Ti-alloyed CuO thin film

Radiofrequency (RF)

-0.09mA/cm2 @-0.5V (Ag/AgCl)

magnetron sputter

7

CuO nanoparticles films

Solution process

Not provided

-1.2 mA/cm2 @ -0.7V (Ag/AgCl)

Not provided

8

CuO thin film

RF-Magnetron sputtering

-2.5 mA/cm 2 @ 0V (RHE)

68% in 20 min

9

10

Heterojunction CuO

Thermal oxidation and

-0.65 mA/cm 2 @ -0.45V

~75% in

nanowire

hydrothermal growth

(Ag/AgCl)

3 min

CuO nanoparticle



Not provided

11

Li doped CuO



16


Not

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nanoparticles

12

CuO thin film

provided


-3.1 mA/cm 2 @ 0V (RHE)

89% in

500 nm

20 min

13

Au-Pd decorated CuO


-3.88 mA/cm 2 @ 0V (RHE)

91% in 20 min

thin film

Present work

500 nm

Present work

The realization of the CuO-based photocathode for PEC water splitting is hindered due to the poor stability against photo-corrosion. 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 seconds at a potential of 0.125 V vs. RHE. Photocathodic current overshoots immediately upon illumination with visible light is mainly originated from sudden generation of photo charge carriers and electrons accumulation at the surface of photocathode. Current overshoot decays rapidly and reaches 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. The O-rich CuO photocathode (sample B4) is more stable in the photo-oxidation process. The comparative study of the Cu-rich and Orich CuO photocathodes shows that higher O/Cu ratio results in better photo-corrosion stability. The thicker CuO photocathode (500 nm) also exhibits stable photocurrent and retained ~90% of the initial photocurrent after 20 minutes, 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. 17



-0.2

-0.4

-0.6

Page 18 of 44

0.0

0.0

B1: CuO (150 nm) at 3.3 mTorr (a)

0

400

800

-0.2

-0.4 B3: CuO (150 nm) at 15 mTorr (c)

-0.6

1200

0

400 800 Time (s)

0.0

-0.2

-0.4 B4: CuO (150 nm) at 40 mTorr

-0.6


Time (s)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



0.0 -0.2 -0.4 -0.6 -0.8 -1.0 B4: CuO at 40 mTorr (150 nm) B4: CuO at 40 mTorr (500 nm) (d)

-1.2

(d)

0

400

800

0

1200

1200

400

800

1200

Time (s)

Time (s)

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 nm and 500 nm.

18


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CuO grown at 40 mTorr

0.0 2 Current density (mA/cm )


0.0

-0.2

-0.4 B4: CuO (150 nm)

-0.6

(a)

0

-0.2

-0.4

-0.6

400

800

1200


B5: CuO (150 nm) + Cu (3 W) (b)

0

Time (s)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0

400 800 Time (s)

1200


-0.2

-0.4

-0.6

B6: CuO (150 nm) + Cu (6 W) (c)

0

400

800

1200

Time (s)

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 co-sputtering power of (b) 3 W (sample B5) and (c) 6 W (sample B6), respectively.

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 remain the key factors to prepare a stable and efficient photocathode. The semiconducting properties and charge transport behavior of prepared CuO photo-electrodes were analyzed by 19



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Page 20 of 44

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 following M-S equation.16 ଵ

஼మ

=

ଶ

ேಲ ௘ఌఌబ

ቂሺܸௌ − ܸி஻ ሻ −

௞் ௘

ቃ

(1)

where NA, k, e, C, ɛ0, ɛ, 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 (ɛ of CuO is 10.26); temperature and applied potential. Since flat electrode model was used to derive M-S, it just provided a qualitative comparison of the different samples with similar material morphology, composition, and device geometry. M-S plot of the samples B1-B6 as 1/C2 vs potential is presented in Figure 8. The linear part of MS curves have the negative slope, indicating that sputter grown CuO thin films are p-type. The x-intercept of the extrapolated linear part of MS 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 valence band. The estimated value of NA of the prepared samples can also be determined from the slope of extrapolated linear part of MS 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 increases. 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 increases.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 20


Page 21 of 44

2 11 2 (1/Cp )× 10 ((1/F ))

0.10 0.08 0.06 0.04 0.02 0.00

B1: CuO grown at 3.3 mTorr B2: CuO grown at 8.0 mTorr B3: CuO grown at 15 mTorr B4: CuO grown at 40 mTorr

(a)

0.25 0.30 0.35 0.40 0.45 0.50 Potential (V vs. RHE)

0.10 2 11 2 (1/Cp )× 10 ((1/F ))

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



0.08 0.06 0.04 0.02

B4: CuO without co-sputter Cu B5: CuO with co-sputter Cu at 3 W B6: CuO with co-sputter Cu at 6 W

(b)

0.00

0.25 0.30 0.35 0.40 0.45 0.50 Potential (V vs. RHE)

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

Table 4. Charge carrier density (NA) and flatland potential (VFB) of of CuO film B1-B6 calculated from M-S plots.

B1

B2

B3

B4

B5

B6

NA

7.41 × 1019

2.05 × 1020

8. 2 × 1020

1.57 × 1021

8.42 × 1020

4.87 × 1020

VFB

0.75

0.8

0.83

0.94

0.86

0.82

21



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 light on 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 the films with higher O/Cu ratio facilitates photo-induced 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 photo-induced charges under light on condition 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.

1.0

6

B1: CuO @ 3.3 mTorr B2: CuO @ 8.0 mTorr B3: CuO @ 15 mTorr B4: CuO @ 40 mTorr

Sputter grown CuO @ 40 mTorr

0.8

Z'' (kΩ )

4 Z'' (kΩ )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 44

2

0.6 0.4 (b)

0.2 (a)

0 0

3

6 Z' (kΩ )

9

B4: CuO B5: CuO + Cu (3 W) B6: CuO + Cu (6 W)

0.0 12

0

3

6 Z' (kΩ)

9

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

22


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The O-rich CuO photocathode shows higher stability against photo corrosion. 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 CuO photocathode, XPS, XRD and SEM analysis were performed before and after PEC tests. Cu2p XPS spectra of sample the B1-B6 were measured before and after amperometry measurement and compared in Figure 10. The observed peaks at 933.8 eV and 954 eV correspond to Cu2p3/2 and Cu2p1/2 peak of Cu2+, respectively, indicating the formation of CuO. While, the peaks at 952.8 eV and 932.7eV, which are observed after amperometry measurement are ascribed to the Cu2p1/2 and Cu2p3/2 characteristic peaks of Cu+, respectively, representing the formation of Cu2O. The existence of shake up peaks at higher binding energies of around 940.8 eV, 943.8 eV 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 CuO dominant phase in all of these samples. Before amperometry measurement, the spin-orbit splitting separation between Cu2p3/2 and Cu2p1/2 is also around 19 eV which further confirms the formation of 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 Cu2p1/2 and Cu2p3/2 shifts from 954 eV and 933.8 eV to 952.8 eV 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.

23



Cu2p3/2

B6 (O/Cu: 0.89)

Cu2p1/2

B6

Cu2p3/2

Cu2p1/2

B5 B4

Intensity (a.u.)

B5 (O/Cu: 1.04)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 44

B4 (O/Cu: 1.27) B3 (O/Cu: 1.12) B2 (O/Cu: 0.96)

B3 B2 B1

B1 (O/Cu: 0.80)

(b)

(a)

930

930

940 950 960 Binding Energy (eV)

940

950

960

Binding Energy (eV)

Figure 10. Cu2p XPS spectra of B1-B6 samples (a) before and (b) after the amperometry measurement.

To investigate further of photo-corrosion stability, XRD spectra of samples B1-B6 were measured after PEC test. The results are presented in Figure S3 of supporting information. As shown in the figure, XRD peak at 36.45o is ascribed to the Cu2O (111) phase (JCPDS# 05-0667). With the increase of Cu/O ratio, 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 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 24


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samples after amperometry measurement reveals 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 photo corrosion stability of O-rich CuO photocathode is improved. 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+. 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 aggregates 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 fit 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 play 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

25



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induction time and slows down the phase reduction. As a result O-rich samples are more stable.

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

26


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Long-term photocorrosion stability test of the sample B4 (O-rich sample) and sample B1 (O-poor sample) are compared in Figure S4 of supporting information. As shown in the figure, O-rich sample is more stable than O-poor sample. This is mainly due to the faster reduction process of CuO in O-poor sample. XPS spectra of the O-rich B4 sample and Opoor B1 sample before and after stability test for 20 min 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 O-rich B4 sample was CuO. These results are also supported by XRD analysis (data was presented in supporting information). 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) is 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 co-sputtering the Cu at sputtering power of 60 W. Figure S6 of supporting information shows the J-V characteristics and photocorrosion stability of CuO and Cu2O photocathodes. 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 al14 and Lim et al26.

27



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Figure 12. Top view SEM images of O-rich CuO photocathode decorated with Au-Pd nanoparticles. The Au-Pd were deposited on O-rich CuO film for (a) 30 sec, (b) 60 sec, (c) 120 sec, and (d) 180 sec. (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).

28


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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 was sputtered from Au-Pd (60-40) target on the O-rich CuO photocathode for 30 sec (sample C1), 60 sec (sample C2), 120 sec (sample C3), and 180 sec (sample C4). Top view SEM images of the samples C1-C4 are presented in Figure 12. It can be seen that by increasing the deposition time from 30 sec to 180 sec, the distribution of Au-Pd nanoparticles is significantly influenced. 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 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 of supporting file 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 29



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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 supporting information (Figure S8). The J-V characteristics, photo-corrosion stability and Nyquist characterization of Orich CuO photocathode 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 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 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.

30


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Page 31 of 44

Light off


0 -1 -2

(a)

Au-Pd 30 sec Au-Pd 60 sec Au-Pd 120 sec Au-Pd 180 sec control B4

-3 -4

Light on

0.0 0.1 0.2 0.3 0.4 0.5 0.6 Potential (V vs RHE)

0 0.3

-1 (c)

0.2 Z'' (kΩ )


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-2

-3 (b)

-4

0

300

0.1

Control (B4) Au-Pd 30 secd Au-Pd 60 sec Au-Pd 120 sec Au-Pd 180 sec

600

900

1200

0.0

B4 AuPd 30 sec AuPd 60 sec AuPd 120 sec AuPd 180 sec

0

Time (s)

1

2

3

Z' (kΩ)

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

31



B1 (O/Cu: 0.80) B2 (O/Cu: 0.96) B3 (O/Cu: 1.12) B4 (O/Cu: 1.27) B5 (O/Cu: 1.04) B6 (O/Cu: 0.89)

Hydrogen evolution (µmol)

12

8

4 (a)

0 0

2

4

6

Irradiation time (hr)

16

Hydrogen evolution (µ mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

O-rich CuO without Au-Pd O-rich CuO with Au-Pd

12

8

4 (b)

0

0

2

4

6

Irradiation time (hr)

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 sec) (sample C3).

From the measured photocurrent and the amount of gas evolved, the Faradaic efficiency of the reaction is determined from the following relation.18,61 32


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

(2)

where Q, ݊‫ܪ‬2, 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 hour illumination period for the Orich CuO photocathode with optimized Au-Pd nanostructures is 15.2 µmol at an average photocurrent of 0.16 mA. Therefore, ݊‫ܪ‬2= 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 study shows the importance of composition engineering on the development of the stable and efficient photocathode for visible light driven water splitting applications. Incorporation of metal nanostructure enhanced the optical absorption, charge separation, and thus overall performance of the photocathode increased.

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 details. 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 0V versus RHE for 500 nm thick sputtered photocathode with 90% retention of photocurrent after 20 minutes. It is explicated that the formation of unwanted Cu2O phase which significantly influences the photocorrosion stability of photoelectrode can 33



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Page 34 of 44

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 µmolh-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.

ACKNOWLEDGMENT 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. Ajay Kushwaha gratefully acknowledge the A*STAR-MND funded green building project (IMRE/2C-0109) for financial support.

ASSOCIATED CONTENT Supporting information available: [Thickness of CuO, Optical properties of O-rich and Curich 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 can be available in supporting information.] This material is available free of charge via the internet at http://pubs.acs.org.

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References (1)

Brillet, J.; Yum, J-H.; Cornuz, M.; Hisatomi, T.; Solarska, R.; Augustynski, J.; Graetzel, M.; Sivula, K. Highly Efficient Water Splitting by a Dual-absorber Tandem Cell. Nat. Photonics. 2012, 6, 824-828.

(2)

Dias, P.; Vilanova, A.; Lopes, T.; Andrade, L.; Mendes, A. Extremely Stable bare Hematite Photoanode for Solar Water Splitting. Nano Energy 2016, 23, 70-79.

(3)

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TOC

2

0.0 O/Cu: 0.80 O/Cu: 1.27

-0.2

-0.4

-0.6

0

300 600 900 1200

Hydrogen evolution (µ mol)

O-rich CuO Photocathode for Solar-Hydrogen Current density (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

16

CuO (O/Cu: 1.27) with Au-pd CuO (O/Cu: 1.27)

12

Time (S)

8 4 0

CuO (O/Cu: 0.80)

0

2 4 6 Irradiation time (hr)

44


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Stable and Efficient CuO Based Photocathode through Oxygen-Rich

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