Nanocrystals on Si Nanowires for Efficient Photoelectrochemical

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Orthorhombic NiSe Nanocrystals on Si Nanowires for Efficient Photoelectrochemical Water Splitting Suyoung Lee, SeungHwan Cha, Yoon Myung, Kidong Park, In Hye Kwak, Ik Seon Kwon, Jaemin Seo, Soo A Lim, Eun Hee Cha, and Jeunghee Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10425 • Publication Date (Web): 06 Sep 2018 Downloaded from http://pubs.acs.org on September 6, 2018

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ACS Applied Materials & Interfaces

Orthorhombic NiSe2 Nanocrystals on Si Nanowires for Efficient Photoelectrochemical Water Splitting Suyoung Lee,† Seunghwan Cha,† Yoon Myung,‡ Kidong Park,§ In Hye Kwak,§ Ik Seon Kwon,§ Jaemin Seo,§ Soo A Lim,† Eun Hee Cha,*,† and Jeunghee Park*,§ †

Department of Pharmaceutical Engineering, Hoseo University, Asan 336-795, Republic of Korea

‡

Department of Nanotechnology and Advanced Engineering, Sejong University, Seoul 05006, Republic of Korea §

Department of Chemistry, Korea University, Sejong 339-700, Republic of Korea

*Corresponding authors. KEYWORDS: Nickel selenide, orthorhombic phase, silicon nanowires, photoelectrochemical cells.

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ABSTRACT Photocatalytic water splitting is a vital technology for clean renewable energy. Despite enormous progress, the search for earth-abundant photocatalysts with long-term stability and high catalytic activity is still an important issue. We report three possible polymorphs of nickel selenide (orthorhombic phase NiSe2, cubic phase NiSe2, and hexagonal phase NiSe) as bifunctional catalysts for water-splitting photoelectrochemical (PEC) cells. Photocathodes or photoanodes were fabricated by depositing the nickel selenide nanocrystals onto p- or n-type Si nanowire arrays. Detailed structure analysis reveals that compared to the other two types, the orthorhombic NiSe2 nanocrystals are more metallic and form less surface oxides. As a result, the orthorhombic NiSe2 nanocrystals significantly enhanced the performance of water-splitting PEC cells by increasing the photocurrents and shifting the onset potentials. The high photocurrent is ascribed to the excellent catalytic activity toward water splitting, resulting in a low charge transfer resistance. The onset potential shift can be determined by the shift of the flat-band potential. A large band bending occurs at the electrolyte interface, so that photoelectrons or photoholes are efficiently generated to accelerate the photocatalytic reaction at the active sites of orthorhombic NiSe2. The remarkable bifunctional photocatalytic activity of orthorhombic NiSe2 promises efficient PEC water-splitting.


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1. INTRODUCTION To meet the demand for clean, renewable, and economical energy sources, hydrogen generation via water splitting is a hot research topic. Since the first use of TiO2 photocatalysts under UV irradiation in 1972, much attention has been focused on photoelectrochemical (PEC) water splitting.1–6 Until now, TiO2 has remained the most prevalent photocatalyst because of its high stability as well as excellent catalytic activity.1,3,6 However, it is not suitable for solar-driven PEC cells due to its large band gap ( NS-2 > NS-1. The low oxidation level of NS-1 implies that the NCs afford good protection to Si NWs due to their high density and strong adhesion, as shown in the HRTEM images. According to the XPS analysis, NS-1 is more metallic and contains less surface oxide than the others, implying that o-NiSe2 is the most metallic phase and robust toward surface oxidation. Furthermore, the o-NiSe2 NCs adhere strongly via lattice matching with the Si surface and are deposited densely, thereby protecting the Si NWs from oxidation. Our data also show that c7 ACS Paragon Plus Environment


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NiSe2 (NS-2) is more metallic than h-NiSe (NS-3), which is consistent with previous calculations showing that h-NiSe is a semiconductor and c-NiSe2 is a metal.34,42 The Zhang group reported mixed o- and c-phase CoSe2 NCs to be a highly active electrocatalyst toward HER.43 Xie and coworkers found that the metallic o-phase CoSe2 atomic layers exhibit excellent catalytic activity for OER.44 Hou and co-workers observed that o-phase CoSe2 nanotubes exhibit a high cell efficiency for dye-sensitized solar cells and good electrocatalytic performance toward HER.45 The Li group demonstrated o-phase Zn0.1Co0.9Se2 as an excellent bifunctional electrocatalyst toward HER and OER.46 All of these works on o-CoSe2 supported that the o-NiSe2 NCs have the most metallic properties. Recently, metallic rhombohedral phase Ni3Se2 also attracted attention as excellent HER and OER catalysts.29,31,34 In contrast, there are no corresponding studies on oNiSe2. Therefore, next we discuss its photocatalytic activity toward the HER and OER. We measured the I-V curves for the water-splitting PEC cells that consisted of bare p-Si NWs, NS-1, NS-2, or NS-3 as photocathodes under solar irradiation (AM1.5G, 100 mW cm–2). The current density (mA cm–2) versus applied potential (V vs. RHE) was plotted in Figure 3a. The quick response of photocurrents was observed during light irradiation in 2 s on/off cycles. For bare p-Si NWs, the onset potential was estimated to be around 0 V. In NS-1, the onset potential (about 0.2 V) was shifted anodically and the photocurrent was much enhanced (6.7 mA cm–2 at 0 V). NS-2 produced lower photocurrents than NS-1, while NS-3 showed negligible enhancements. Their onset potentials were about 0 V. The highest PEC efficiency for NS-1 could be attributed to the higher HER catalytic activity of o-NiSe2 than that of c-NiSe2 and h-NiSe. We examined the electrocatalytic HER activity by linear sweep voltammetry measurements using the samples as working electrodes. The onset overpotential and current density follow the same order as that of PEC cells (Supporting 8 ACS Paragon Plus Environment

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Information, Figure S5). The densely deposited NCs increased the surface area toward the HER as well. In order to confirm water splitting in the PEC cell, we monitored the evolution of hydrogen (H2) and oxygen (O2) gases at 0 V (vs. RHE). Figure 3b displays the photocurrent (at 0 V) and gas evolution data (in µmol), showing an overall photocurrent degradation of 3% over 120 min. The molar ratio of [H2]/[O2] is 2.2±0.1. The slight excess of H2 is probably due to the slow kinetics of O2 evolution at the Pt counter electrode. The Faradaic efficiency for H2 generation, O2 generation, and water splitting was 73%, 67%, and ca.70%, respectively. Figure 4a displays the Nyquist plots obtained by EIS measurements under dark condition, namely the negative imaginary part (–Z′′) versus real part (Z′) of the impedance. A semicircle in the high frequency range represent the charge transfer process, and its diameter reflects the charge-transfer resistance (Rct). An equivalent circuit is shown in the inset, where the fitting parameters Re and CPE represent respectively the internal resistance of electrolyte and the constant-phase element. Simulating EIS spectra with the equivalent circuit, as shown by the fitting curve, yields the Rct value of 6.5, 1.6, 3.5, and 4 kΩ for p-Si, NS-1, NS-2, and NS-3, respectively. The Re value is 400–500 Ω. Under solar irradiation (AM1.5G, 100 mW cm-2), two semicircles appear at high frequencies with smaller diameters than that under dark (Figure 4b). p-Si also shows a second semicircle that is above the range of this plot. As shown in the inset, the equivalent circuit consisted of Rct1 and Rct2, and their corresponding CPE1 and CPE2. From the first semicircle, Rct1 = 1.3, 0.26, 0.30, and 0.30 kΩ for p-Si, NS-1, NS-2, and NS-3, while the Rct2 value from the second semicircle is 3, 0.05, 0.2, and 2 kΩ, respectively. The Rct1 and Rct2 of the NS samples may be related to the charge transfer of NCs and Si NWs, respectively, but the details remain to be clarified in future



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studies. The sum of Rct1 and Rct2 results in Rct = 4.3, 0.31, 0.5, and 2.3 kΩ, respectively, following the same sequence as that obtained in dark condition. The largest Rct of bare p-Si NWs indicates their sluggish charge transfer kinetics. In contrast, NS-1 exhibits the smallest Rct, implying that the deposition of NiSe2 NCs induces the most efficient charge transfer. Therefore, we conclude that large photocurrents obtained for NS-1 is primarily ascribed to the reduced charge transfer resistance. The o-NiSe2 NCs act as active catalytic sites for HER, allowing efficient extraction of photogenerated electrons from the p-Si NWs and improving the electron-transfer kinetics by bridging the Si NWs and the electrolyte. Now we estimate the flat-band potential (Efb) using Mott–Schottky (MS) plots measured at 0.5, 1, and 2 kHz (Figure 5). For p-Si NWs, NS-1, NS-2, and NS-3, the Efb was determined as 0.05, 0.30, 0.27, and 0.15 V (vs. RHE), respectively, using the linear intercepts of the data points with the x-axis. The Efb of NS-1 is most positive, which is similar to the onset potential of the PEC cell. Therefore, the onset potential of PEC cell could be determined by the Efb. The magnitude of Efb determines the extent of band bending (Eb) in the surface space region (SCR) near the interface with electrolyte, according to Eb = E - Efb, where E is the applied potential.47,48 As Efb shifts positively upon the NC deposition, Eb becomes more negative under the negative E. Therefore, a larger downward band bending occurs for NS-1 due to the presence of o-NiSe2 NCs, promoting faster separation of generated photoelectrons and photoholes under the applied potential. The possibility of charge recombination or surface trapping at interfacial defects may also be diminished. The strong driving force for electron transfer to the surface benefits the following HER. The large downward band bending prevents hole-induced selfoxidation. Moreover, the reciprocal of the slope in the MS plots can indicate the carrier concentrations in the samples, which are much larger in NS-1 by ~103 times than in Si NWs. 10 ACS Paragon Plus Environment

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Hence, the metallic o-NiSe2 NCs result in significantly higher carrier concentrations to induce the larger band bending. Finally, we tested the n-type Si NWs to demonstrate that NS-1 can also be used as photoanode for water-splitting PEC cells. The current density (mA cm-2) versus applied potential (V vs. RHE) curves is displayed in Figure 6a. For bare n-Si NWs, the onset potential is about 0.6 V, and the photocurrent at 1.23 V (water oxidation potential) is 1.5 mA cm-2. For NS-1 on n-Si NWs, the corresponding values are 0.4 V and 5.6 mA cm-2. For comparison, the photocurrents at 1.23 V in NS-2 and NS-3 are 5.5 and 2.6 mA cm–2, respectively (Supporting Information, Figure S6). Again, NS-3 shows the least enhancement. Figure 6b displays the photocurrent and gas evolution data for NS-1. The degradation of photocurrent over 80 min is 5%. The molar ratio of [H2]/[O2] is 2.0±0.1, indicating that the O2 evolution becomes more efficient compared to the case of photocathode (2.2±0.1). The closeness of this ratio to the 2:1 stoichiometric value confirms the excellent photocatalytic activity with negligible self-oxidation. The Faradaic efficiency for water splitting was 60%. The EIS data for the photoanodes are shown in Supporting Information, Figure S7. The Rct value under light irradiation is 7.5, 0.6, 4.4, and 5.5 kΩ for bare n-Si NWs, NS-1, NS-2, and NS3, respectively. The much smaller Rct value of NS-1 is consistent with that of the photocathode, showing very efficient charge transfer between the photoelectrode and electrolyte, owing to the excellent OER catalytic activity of o-NiSe2 NCs. The Efb values obtained using the MS plots were 0.88 and 0.42 V (vs. RHE), respectively, for bare n-Si NWs and NS-1 (see Supporting Information, Figure S8). The negative Efb shift of NS-1 indicates the presence of upward band bending. This upward band bending protects the photoelectrons generated in the SCR from direct transfer toward the electrolyte, while it propels the photoholes toward the interface and thus 11 ACS Paragon Plus Environment


enhances the catalytic activity toward OER. The photoelectrons participate in cathodic HER by drifting toward the counter electrode. The surface of NiSe2 NCs is oxidized under the positive potential, as shown in our previous work.32 The surface oxide layers are actual catalytic sites for OER, while the metallic o-NiSe2 underneath functions as highly conductive layers between the active sites and Si NWs. The excellent photocatalytic activity of o-NiSe2 retards the hole-induced self-oxidation of the NCs and Si NWs. Nevertheless, some self-oxidation is inevitable during the water splitting reaction, and it is probably more serious for the OER than the HER. Hence, the PEC performance is less stable than the case of photocathodes.

3. CONCLUSIONS We synthesized o-NiSe2, c-NiSe2, and h-NiSe NCs deposited on Si (p- or n-type) NW array. The NS-1, NS-2, and NS-3 samples, consisting of mainly o-NiSe2, c-NiSe2, and h-NiSe, were selectively synthesized using different growth temperatures (270, 350, and 400 °C, respectively). NS-1 has the smallest NC size and the highest deposition density. XPS analysis also revealed that it is more metallic than the others and has much less surface oxide due to the nature of oNiSe2 NCs. NS-1 with p-Si NWs as the photocathode exhibits large photocurrents (6.7 mA cm–2 at 0 V) with positive onset potential (0.2 V) for water-splitting PEC cells. The Faradaic efficiency for water splitting was 70%. The EIS measurements revealed that the excellent catalytic activity of o-NiSe2 NCs toward the HER increases the photocurrents. The MS plots showed positive shift of the flat-band potential, which caused an anodic shift of the onset potential. The metallic o-NiSe2 NCs induced large downward band bending that drove the HER by moving the photoelectrons to the catalytically active sites. Photoanodes of NS-1 on n-Si NWs also exhibited enhanced photocurrents with a cathodically shifted onset potential. In this case, the large upward band bending propelled the photoholes toward the electrolyte interface region, 12 ACS Paragon Plus Environment

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and thus enhanced the catalytic activity toward OER. In short, the o-NiSe2 nanocrystals acts as excellent bifunctional HER and OER photocatalysts for the water-splitting PEC cells.

ACKNOWLEDGEMENTS

This study was supported by 2014R1A6A1030732, 2017K000494, and 2018R1A2B6003624, funded by the Ministry of Science and ICT. The HVEM measurements were supported by the KBSI under the R&D program (D38700). The experiments at the PLS were partially supported by MOST and POSTECH.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. ORCID Suyoung Lee (0000-0003-3589-9768) Seunghwan Cha (0000-0001-5860-733X) Yoon Myung (0000-0002-5774-6183) Kidong Park (0000-0003-1273-2076) In Hye Kwak (0000-0003-4697-0566) Ik Seon Kwon (0000-0003-0611-2276) Jaemin Seo (0000-0002-1499-622X) Soo A Lim (0000-0003-0020-8250) Eun Hee Cha (0000-0002-0847-5655) Jeunghee Park (0000-0002-6913-5569) ASSOCIATED CONTENT

Supporting Information. Experimental details and Figures S1-S8. This material is available free of charge via the Internet at http://pubs.acs.org.



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(44) Liang, L.; Cheng, H.; Lei, F.; Han, J.; Gao, S.; Wang, C.; Sun, Y.; Qamar, S.; Wei, S.; Xie, Y. Metallic Single-Unit-Cell Orthorhombic Cobalt Diselenide Atomic Layers: Robust WaterElectrolysis Catalysts. Angew. Chem. Int. Ed. 2015, 54, 12004-12008. (45) Li, H.; Qian, X.; Zhu, C.; Jiang, X.; Shao, L.; Hou, L. Template Synthesis of CoSe2/Co3Se4 Nanotubes: Tuning of Their Crystal Structures for Photovoltaics and Hydrogen Evolution in Alkaline Medium. J. Mater. Chem. A, 2017, 5, 4513-4514. (46) Wang, X.; Li, F.; Li, W.; Gao, W.; Tang, Y.; Li, R. Hollow Bimetallic Cobalt-Based Selenide Polyhedrons Derived from Metal–Organic Framework: An Efficient Bifunctional Electrocatalyst for Overall Water Splitting. J. Mater. Chem. A, 2017, 5, 17982-17989. (47) Lewis, N. S. Quantitative Investigation of the Open-Circuit Photovoltage at the Semiconductor/Liquid Interface. J. Electrochem. Soc. 1984, 131, 2496-2503. (48)

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Page 19 of 25 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


Figure 1. (a) SEM and HRTEM images of Si NW array deposited with NiSe2 NCs (NS-1) and _

(b) lattice-resolved TEM and corresponding FFT images of (i) o-NiSe2 (zone axis = [101]; d010 = 6.0 Å) and (ii) Si NW (zone axis = [011]) for NS-1. HRTEM images and corresponding FFT ED _

pattern of (c) c-NiSe2 (zone axis = [223]; d110 = 4.2 Å) for NS-2, and (d) h-NiSe (zone axis = [21 _

10]; d001 = 5.3 Å) for NS-3. (e) HAADF STEM image, EDX elemental mapping of Si, O, Ni, and Se, and EDX spectrum of NiSe2 (NS-1). (f) HAADF STEM image and EDX spectrum of NiSe (NS-3).



(a) Ni 2p NS-3

Intensity (arb. units)

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

Ni

0

N1 N2 N3

0

Ni

(b) Se 3d NS-3

Se

S1 S2

0

Page 20 of 25

(c) Si 2p

Si1 Si2

0

Si

NS-3

NS-2 NS-2

NS-2 NS-1

NS-1

880

NS-1 870

860

850 60

Binding Energy (eV)

58

p-Si 56

54

52

Binding Energy (eV)

108 106 104 102 100

98

Binding Energy (eV)

Figure 2. Fine-scanned XPS data of (a) Ni 2p, (b) Se 3d, and (c) Si 2p peaks of three NS samples (NS-1, NS-2, and NS-3) and p-Si NWs. The data points (open circles) are fitted by Voigt functions, and the sum of the resolved bands is represented by black lines. The position of the neutral peak (Ni0, Se0, and Si0) is marked by a dotted line to delineate the blueshift or redshift.


Page 21 of 25

(a)

(b)

Current Density -2 (mA cm )

0 -2 -4 -6 NS-1 -8 200

0 -2 -4 -6

p-Si NS-1 NS-2 NS-3

-8 -10 -0.4

-0.2

0.0

0.2

Gas Evolution (µmol)

-2

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


H2

150 100 50

O2

0 0

Potential (V vs. RHE)

20

40

60

80 100 120

Time (min)

Figure 3. (a) Current density (mA cm-2) vs. potential (V vs. RHE) for bare p-Si NWs and NS samples as photocathodes, measured in 0.5 M H2SO4 electrolyte under AM1.5G irradiation (100 mW cm–2) and with on/off cycles. The scan rate is 20 mV s–1. (b) Stability of photocurrent and H2 and O2 evolution vs. time (min) for NS-1 under an applied potential of 0 V vs. RHE.



0.0

(a)

5

1.0

p-Si NS-1 NS-2 NS-3

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 25

3

0.8 0.6

2

0.4

1

0.2

0

0.0 7 0.0

0

1

2

3

4

5

6

(b)

0.00

p-Si NS-1 NS-2 NS-3

0.3

0.6

0.9

1.2

Z' (kΩ)

Z' (kΩ)

Figure 4. Nyquist plots of p-Si and NS samples for EIS experiments in the frequency range from 1 MHz to 0.1 Hz at 0 V (vs. RHE) under (a) dark and (b) AM1.5G irradiation (100 mW cm–2). The equivalent circuit is shown in the inset, and the fitting curves are represented by the solid lines.


Page 23 of 25

(a) p-Si

0.05 V

0.30 V

-2

4

1

-2

-2

1

2

2 kHz 1 kHz 0.5 kHz

15

18

-2

2

C (10 F cm )

(b) NS-1 2 kHz 1 kHz 0.5 kHz

4

C (10 F cm )

3

0

0 0.0

0.1

0.2

0.3

0.4

0.0


0.3

0.4

0.27 V

2

0.15 V

4 -2

2

2 kHz 1 kHz 0.5 kHz

4

-2 16

4

C (10 F cm )

4 -2 15 -2

0.2

(d) NS-3 2 kHz 1 kHz 0.5 kHz

6

0.1


(c) NS-2

8

C (10 F 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


0

0 0.0

0.1

0.2

0.3


0.4

0.0

0.1

0.2

0.3

0.4


Figure 5. Mott-Schottky plots at 0.5, 1, and 2 kHz for (a) Si, (b) NS-1, (c) NS-2, and (d) NS-3. The flat-band potentials are obtained from the intercepts of the extrapolated lines


(b)

n-Si 6

NS-1

4 2 0 0.2

Page 24 of 25

(a)

8

Gas Evolution Current Density -2 (µmol) (mA cm )

-2

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

Current Density (mA cm )


0.4

0.6

0.8

1.0

1.2

6

NS-1

4 2 0 80

H2

60 40 20 0

1.4

O2 0


20

40

60

80

Time (min)

Figure 6. (a) Current density (mA cm-2) vs. potential (vs. RHE) for bare n-Si NW and NS-1 as photoanodes, measured in 1 M KOH electrolyte and under AM1.5G irradiation (100 mW cm–2) with on/off cycles. The scan rate is 20 mV s–1. (b) Stability of photocurrent and H2 and O2 evolution for NS-1 under an applied potential of 1.23 V (vs. RHE).


Page 25 of 25 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


TOC Figure


Nanocrystals on Si Nanowires for Efficient Photoelectrochemical

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