Nickel Coated Silicon Photocathode for Water Splitting in Alkaline … · 2014-11-19 · Nickel Coated Silicon Photocathode for Water Splitting in Alkaline Electrolytes Ju Feng†1,

Nano Res

1

Nickel Coated Silicon Photocathode for Water Splitting

in Alkaline Electrolytes

Ju Feng†1, Ming Gong†1, Michael J. Kenney†1, Justin Z. Wu1, Bo Zhang1, Yanguang Li2 and Hongjie

Dai1()

Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0643-4

http://www.thenanoresearch.com on November 19 2014

© Tsinghua University Press 2014

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Nano Research

DOI 10.1007/s12274-014-0643-4

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Nickel Coated Silicon Photocathode for

Water Splitting in Alkaline Electrolytes

Ju Feng†1

, Ming Gong†1

, Michael J. Kenney†1

,

Justin Z. Wu1, Bo Zhang

1, Yanguang Li

2 and

Hongjie Dai*1

1Department of Chemistry, Stanford University,

Stanford, California 94305, USA.

2Institute of Functional Nano & Soft Materials,

Soochow University, Suzhou 215123, China.

† These authors contributed equally.

We designed and fabricated a simple p-type Si based photocathode with

high activity and good stability in potassium borate buffer solutions. Ni

acts as both a protecting layer and hydrogen evolution reaction (HER)

catalyst, while the low work function of Ti is necessary to afford a high

photovoltage.

20

nm

Pd

P-S

i

15

nm

Ti

5 nm Ni

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6-30

-25

-20

-15

-10

-5

0

Ni/Ti/p-Si in KBi

Cu

rre

nt

de

ns

ity

(mA

/cm

2)

Potential (V vs RHE)

dark

light

light after12h

-0.6 -0.4 -0.2 0.0 0.2 0.4

-30

-25

-20

-15

-10

-5

0

5

Cu

rre

nt

de

ns

ity

(mA

/cm

2)

Potential(V vs RHE)

KBi

dark current

Pt NPs on Ni/Ti/p-Si

Nickel Coated Silicon Photocathode for Water Splitting

in Alkaline Electrolytes

Ju Feng†1

, Ming Gong†1


, Justin Z. Wu1, Bo Zhang

1, Yanguang Li

2 and Hongjie

Dai1()

1Department of Chemistry, Stanford University, Stanford, California 94305, USA.

2Institute of Functional Nano & Soft Materials, Soochow University, Suzhou 215123, China.

Received: day month year

Revised: day month year

Accepted: day month year

(automatically inserted by

the publisher)

© Tsinghua University Press

and Springer-Verlag Berlin

Heidelberg 2014

KEYWORDS

Photoelectrochemical

water splitting, Silicon

photocathode, Nickel

ABSTRACT

Photoelectrochemical (PEC) water splitting is a promising approach to harvest

and store solar energy.[1] Silicon is widely investigated for PEC

photo-electrodes due to its suitable band gap (1.12 eV) matching the solar

spectrum.[2] Here we investigated employing nickel both as a catalyst and

protecting layer for p-type silicon photocathode for photoelectrochemical

hydrogen evolution in basic electrolytes for the first time. The silicon

photocathode was made by depositing 15 nm Ti on a p-type silicon wafer

followed by 5 nm Ni. The photocathode afforded an onset potential of ~ 0.3 V

vs. RHE in alkaline solution (1 M KOH). The stability of the Ni/Ti/p-Si

photocathode showed a 100 mV decay over 12 h in KOH, but the stability was

significantly improved when the photocathode was operated in potassium

borate buffer solution (pH 9.5). The electrode surface was found intact after

12 h of continuous operation at a constant current density of 10 mA/cm2 in

potassium borate buffer, suggesting better protection of Ni in borate buffers for

Si based photocathodes.

Instructions for using the template

Introduction

Much effort has been put forth in the search for

renewable and environmentally friendly energy

sources due to the growing demand for energy

supplies and the decreasing fossil fuel reserves. Solar

energy is among the most promising candidates to

serve as an energy source at the lowest cost of

environment.[3] Since sunlight is intermittent, a

major challenge is to harvest and store solar energy

efficiently and cost-effectively.[4] Photoelectrolysis of

water using semiconductors as both the light

absorbers and energy convertors is becoming

increasingly attractive and is considered by many to

be "holy grail" of solar energy conversion and

storage.[5]

In principle, it is possible to complete overall water

splitting with a single semiconductor but it has been

challenging to find a material that is stable under

both cathodic and anodic condition and has a

suitable band alignment for water splitting. In

addition, calculations have shown that a band gap

greater than 3 eV is needed for a single

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DOI (automatically inserted by the publisher)

Address correspondence to Hongjie Dai, [email protected]

Research Article

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2 Nano Res.

semiconductor to drive water splitting due to a series

of large energy loss processes involved which limit

the solar to hydrogen efficiency significantly.[6] An

alternative approach is to use a combination of a

photocathode and a photoanode to drive the overall

water splitting. Silicon is known to have a suitable

band gap that is well matched to the solar spectrum.

Up to now, silicon is still the dominant material used

in photovoltaic systems.[7] The success of silicon

photovoltaic systems has motivated wide exploration

of silicon photoanodes and photocathodes.

A major challenge with silicon PEC devices is that

silicon is easily oxidized and corroded under PEC

conditions in solution.[8] Under acidic condition,

p-type silicon photocathodes were reported to be

stable for one or two hours.[8-12] As for long-term

stability, however, the activity of p-type silicon

photocathode would decay to less than 10% of its

original value after 5-12 h operation.[13-14] Thick

TiO2 films have recently been demonstrated to

successfully protect underlying silicon electrode for

12 h (80 nm TiO2)[13] and 72 h (100 nm TiO2)[14]

under hydrogen evolution condition in acidic

electrolytes. Thus far, p-type silicon based

photocathodes have only been explored under acidic

conditions and not in basic solutions.[8-9, 14-19] It is

desirable to pair high performance photocathodes

and photoanodes for water splitting in basic

solutions, since it would allow the utilization of low

cost non-noble metal based oxygen evolution

catalysts with high electrocatalytic activities (higher

activity than in acids)[20] integrated into

photoanodes. We recently reported a highly active

and stable Ni/n-Si photoanode in basic electrolytes,

using Ni as the Si protection layer and an

electrocatalyst for oxygen evolution reaction

(OER).[21] Here, we explored the possibility of

employing Ni as a coating layer of p-type silicon

photocathode alkaline solutions.

Results and discussion

We deposited 15 nm Ti via electron beam (e-beam)

evaporation on a p-type silicon wafer followed by 5

nm Ni. Ohmic contact was made to the backside of

the silicon wafer by electron beam deposition of 20

nm Pd (Figure 1(a)).

Cyclic-voltammograms (CV) of the Ni/Ti/p-Si

cathode in 1 M KOH under illumination from a 150

W Xe lamp (~ 225mW/cm2) showed high activity for

photoelectrochemical water reduction (Figure 1(c)).

The Ni/Ti/p-Si photocathode gave an hydrogen

evolution reaction (HER) onset potential of ~0.3 V vs.

RHE with a saturation current of 25-30 mA/cm2

(Figure S1), while bare p-type silicon by itself, Ti/p-Si

and Ni/p-Si all afforded much lower HER activity in

terms of onset potential (Figure 1(c)). We investigated

the stability of our Ni/Ti/p-Si photocathode in 1 M

KOH. The electrode was continuously operated at a

constant cathodic current density of 10 mA/cm2 and

CV curves were taken before and after 12 h of

operation. As shown in Figure 2(a), there was about

100 mV photovoltage loss after 12 h from CV

characterization. The loss of activity was also

observed in the chronopotentiometry curve (Figure

2(b)). Figure 2(b) indicated that most of the activity

decay occurred during the first 2-3 hours of

operation. The Ni/Ti/p-Si electrode was characterized

by scanning electron microscopy (SEM) and auger

photoelectron spectroscopy (AES) chemical mapping

after the 12 h stability test. SEM image (Figure 2(c))

showed that some plate-like structures were formed

and AES mapping indicated that the surface was

corroded with gradual loss of Ni (Figure 2(d)).

Recently, we found that Ni coating on n-type silicon

photoanodes afforded higher stability in potassium

borate buffer than in KOH under

photoelectrochemical anodization conditions.[21]

This motivated us to test our Ni/Ti/p-Si photocathode

in potassium borate buffer. The electrode afforded an

HER onset potential in potassium borate buffer at

0.25 V vs. RHE (Figure 3(a)), which was about 50 mV

worse than that in 1 M KOH. The Tafel slope in the

potassium borate buffer electrolyte was also larger,

mostly due to the higher impedance (note: all the

electrochemical data in this work was recorded and

presented without iR compensation). Though the

photocathode activity was slightly lowered by using

potassium borate buffer, we observed significantly

improved stability. The electrode was operated at a

constant current density of 10 mA/cm2 for 12 h in

potassium borate buffer and CV curves were taken

before and after the long time operation. The

photocatalytic activity was found to be retained after

12 h of continuous operation. (Figure 3(a, b))

Importantly, SEM (Figure 3(c)) showed that the

electrode surface was intact after the

photoelectrochemical measurement. AES mapping

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

3 Nano Res.

(Figure 3(d)) also revealed that the electrode surface

was still completely covered by Ni. Consistently, AES

spectrum (Figure 3(e)) showed uniform Ni signals on

the surface with negligible Ni etching or any

observable silicon signals present, indicating that the

underlying silicon was still under protection by the

Ni film.

Ni played two roles in our Ni/Ti/p-Si electrode. First,

Ni expedited HER kinetics that are known to be very

sluggish on bare silicon [17]. Both bare p-type silicon

and Ti coated p-type silicon showed more than 400

mV higher HER overpotential compared to

Ni/Ti/p-Si (Figure 1(c)). The electrochemical HER

activity of Ni deposited on conducting

fluorine-doped tin oxide was characterized, showing

an HER onset potential of about -0.15 V vs. RHE

(Figure S2) in 1M KOH and confirming the HER

catalytic activity of Ni in basic solutions.

Ni served as a much better protection layer to the

underlying silicon in the borate electrolyte than in

KOH. Even after PEC in 1 M KOH, although SEM

and AES mapping revealed surface corrosion of

Ni/Ti/p-Si after 12 h PEC operations for HER (Figure

2(c, d)), Ni did help to slow down the Si corrosion

evidenced by the fact that when Ti/p-Si was prepared

and measured under same PEC condition for only

3h(Figure S3). Ti was previously explored as a

protection layer for p-type silicon photocathodes in

acidic solutions[9] and found relatively stable during

a short~ 1 h test. However, longer time test led to Ti

corrosion[14]. Our result here revealed that Ti was

not an effective protection layer in alkaline solutions

either. In potassium borate buffer, Ni/Ti/p-Si was

found to be much more stable through spectroscopic

and microscopic surface characterization, which was

consistent with our previous observation in the

photoanode case.[21]

Since Pt is known to be the best HER catalyst with

zero overpotential, we deposited Pt nanoparticles on

our Ni/Ti/p-Si photocathode to glean the difference

between Pt and Ni for HER catalysis in our system.

As shown in Figure 4, Pt did improve the onset

potential of our p-type silicon photocathode by

80-100mV in both 1 M KOH and potassium borate

buffer and also afforded steeper Tafel slopes. This

result indicated that we could combine our current

electrode design with better HER catalyst to further

boost the performance of silicon photocathode in

basic solutions.

Though Ni film provide a certain protection to the

underlying p-type silicon in both KOH and

potassium borate buffer, we did notice that the

protection effect of Ni in Ni/Ti/p-Si photocathode

was poorer than its superior protection for n-type

silicon in the photoanode. This observation was

initially surprising to us since we expected that the

reducing potentials involved might be a less

corrosive condition than in the highly oxidizing

photoanode case. To better understand the

similarities and differences of Ni protection between

our p-type silicon photocathode and n-type silicon

photoanode, X-ray photoelectron spectroscopy (XPS)

depth profiling experiments were carried out by slow

Ar ion milling of a 5 nm Ni /15 nm Ti/p-Si sample

after 5 h PEC experiment in KOH (Figure 5). Similar

to the Ni/n-Si photoanode case[21], the Ni and O

signal intensities peaked at the surface and decreased

as depth increased (Figures 5a). High-resolution Ni

spectra at different milling times (Figure 5b) also

revealed that there was a thin oxidized Ni layer

followed by mainly metallic Ni. However, we found

that the oxidation state of surface oxidized Ni layer

on our photocathode was mainly Ni (II), while a

higher oxidized phase of Ni (III) was formed under

OER conditions[21]. In our photoanode work, we

found that adding lithium to basic solutions

including 1 M KOH and 1 M potassium borate

electrolytes greatly improved the electrode’s stability

by slowing down Ni corrosion[21]. However, the

same Li ion stabilization effect was not observed for

Ni under cathodic conditions with the Ni/Ti/p-Si

photocathode. We propose that in the photoanode

case the Ni film is overcharged to form a gamma

phase nickel oxyhydroxide (γ-NiOOH) under OER

conditions and lithium can hinder the formation of

the less dense and stable γ-NiOOH by incorporation

of Li into β-NiOOH[22-23], resulting in a more stable

film. Such an effect is not present under HER

conditions when the Ni species on the surface is in a

low oxidation state with oxidation number of ~ 2.

This suggested that the high stability of Ni against

silicon photoanode corrosion was owed to the higher

oxidation state of Ni, which was further stabilized by

Li incorporation to impede γ-phase formation. Thus,

the current results also helped to shed light into the

high stability of Ni/n-Si photoanodes.


4 Nano Res.

The use of Ti under Ni was important to maximize

photovoltage due to the low work function of Ti

affording a high Schottky barrier to the p-type silicon

valence band (Figure 1(b)) (The Schottky contact was

confirmed by I-V curve in Figure S5(b)). The

resulting larger ban bending (Figure 1(b)) favors the

transportation of photo-generated energetic electrons

to electrode surface, while holes going into the bulk

p-Si. Therefore, Ni/Ti/p-Si afforded a much more

positive onset potential for hydrogen evolution than

Ni/p-Si electrode without the intermediate Ti layer as

shown in Figure 1(c). Our photocathode was able to

provide us with a photovoltage reaching an onset

potential of HER at 0.3 V vs. RHE (Figure 1(c)).

Conclusions

In summary, we designed and fabricated a simple

p-type Si based photocathode with high activity and

good stability in potassium borate buffer solutions.

Ni acts as both a protecting layer and HER catalyst,

while the low work function of Ti is necessary to

afford a high photovoltage. This approach involving

Ni or other cheap metal/metal oxides could be

applied to other p-type semiconductors which are

unstable in alkaline solution under HER conditions,

or/and have very sluggish HER kinetics.

Acknowledgements

This work was supported by a grant from Stanford

GCEP, Precourt Institute of Energy and by the U.S.

Department of Energy, Office of Basic Energy

Sciences, Division of Materials Sciences and

Engineering under Award # DOE DE-SC0008684 (for

the microscopy and spectroscopy characterization

part of this work). M.J.K. acknowledges support from

an NSF Graduate Fellowship.

Electronic Supplementary Material: Supplementary

material is available in the online version of this

article at http://dx.doi.org/10.1007/s12274-***-****-*

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FIGURES.


6 Nano Res.

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

-40

-30

-20

-10

0

Cu

rre

nt

de

nsity(m

A/c

m2)


dark current

p-Si

15 nm Ti /p-Si

5 nm Ni /p-Si

5 nm Ni /15 nm Ti /p-Si

20

nm

Pd

P-S

i

15

nm

Ti

5 nm Ni(a) (b)

P-Si Ti

Ni

H+/H2

solutionelectrode

(c)

Figure 1: (a) Structure of Ni and Ti coated p-type silicon

photocathode. (b) A band diagram for the Ni/Ti/p-Si

photocathode. (c) Cyclic voltammograms (CV) of p-Si,

Ti/p-Si, Ni/p-Si, Ni/Ti/p-Si cathodes in 1 M KOH under

illumination with a 150 W Xe lamp (~ 225mW/cm2) and CV

curve of Ni/Ti/p-Si in 1M KOH in the dark. Data in this work

are all raw data without iR compensation applied.

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6-30

-25

-20

-15

-10

-5

0

55nm Ni/15 nm Ti/ p-Si

1M KOH

Cu

rren

t de

nsity (m

A/c

m2)


dark

light

after 12 h

0 2 4 6 8 10 12

-0.6

-0.3

0.0

0.3

0.6

Pote

ntia

l (V

vs R

HE

)

Time (h)

@ 10 mA/cm2

1 M KOH

5 nm Ni/ 15nm Ti/ p-Si

(a) (b)

(c) (d)

500 nm

SEM after 12 h PEC in KOH

400 nm

Ni map

Figure 2: (a) Cyclic voltammograms of Ni/Ti/p-Si in 1M KOH

(without iR compensation). CV data was taken before and after

the test in (b). (b) Potential vs. time data under a constant

current of 10 mA/cm2 of Ni/Ti/p-Si photocathode under

illumination in 1 M KOH for 12 h. (c) An SEM image of

Ni/Ti/p-Si electrode after 12 h operation in KOH. (d) An Auger

electron spectroscopic (AES) chemical mapping of Ni of

Ni/Ti/p-Si electrode after 12 h operation in KOH.


Nano Res.

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6-30

-25

-20

-15

-10

-5

0

Ni/Ti/p-Si in KBi

Cu

rre

nt

de

nsity(m

A/c

m2)


dark

light

light after12h

0 2 4 6 8 10 12

-0.9

-0.6

-0.3

0.0

0.3

Pote

ntial (V

vs R

HE

)Time(h)

KBi ≈ 9.5

5 nm Ni/ 15nm Ti/ p-Si

@ 10 mA/cm2

(a) (b)

(d)

0 500 1000 1500 2000-50000

-40000

-30000

-20000

-10000

0

10000

20000

30000

dN

(E)/

dE

Kinetic Energy (eV)

Ni / Ti / p-Si after 12h in KBi

(e) ONi

Si

(c)

500 nm400 nm

Ni mapSEM after 12 h PEC in KBi

Figure 3: (a) Cyclic voltammograms of a Ni/Ti/p-Si in

potassium borate (KBi) (without iR compensation). CV data

was taken before and after the operation in (b). (b) Potential vs.

time data under constant current of 10 mA/cm2 of Ni/Ti/p-Si

photocathode in KBi for 12 h under illumination. (c) An SEM

image of a Ni/Ti/p-Si electrode after 12 h operation in KBi. (d)

An AES chemical mapping of Ni and (e) a spectrum of

Ni/Ti/p-Si electrode after 12 h operation in KBi (O: 500 eV;

Ni: 600 - 900 eV; Si: 1600 eV) .

Figure 4: Cyclic voltammograms of Pt nanoparticle coated

Ni/Ti/p-Si in 1 M KOH (a) and in KBi (b) (see SI for

experimental details).


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890 880 870 860 850 840

Inte

ns

ity

(a

. u

.)

Binding Energy (eV)

3rd cycle

10th cycle

0 5 10 15 20

0.0

0.2

0.4

0.6

0.8

1.0

In

ten

sit

y r

ati

o

Milling time (min)

Ni

O

Si

Ti

Ni2+

Ni0

(a)

(b)

Figure 5: X-ray photoelectron spectroscopy depth profiling of

Ni/Ti/p-Si photoanode. (a) Elemental depth profile of 5 nm Ni

/15 nm Ti /p-Si photocathode after 5 h of continuous PEC

operation (10 mA/cm2) in 1M KOH under illumination. (b) Ni

2p spectra for the sample in (a) taken after two different

ion-milling times into the Si substrate. 3rd cycle corresponds

to 0.8 min milling time, and 10th cycle corresponds to 3.6 min

milling time. The intensity of the 3rd cycle Ni peak was

enlarged 3 times to be observable when compared to the

strong Ni peak at 10th cycle.


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Electronic Supplementary Material

Nickel Protected Silicon Photocathode for Water

Splitting in Alkaline Electrolytes

Type author names here. The font is "Helvetica 10". Please spell out first names and surnames. Do not

include professional or official titles or academic degrees. Place an “()” by the corresponding author(s).

For example, First A. Firstauthor1,2

(), Second B. Secondauthor2,†

, and Third C. Thirdauthor1 ()

Ju Feng†1

, Ming Gong†1


, Justin Z. Wu1, Bo Zhang

1, Yanguang Li

2 and Hongjie

Dai1()

1Department of Chemistry, Stanford University, Stanford, California 94305, USA. 2Inst itute of Funct ional Nano & Soft Materials, Soochow University, Suzhou 215123, China.

Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)

Experimental details Electrode preparation:

Ni/Ti/p-Si photocathode: The p-type silicon was used as received. 15 nm Ti was deposited on an as received

[100] p-type silicon wafer (0.1 – 0.9ohm·cm) via e-beam evaporation followed by 5 nm Ni at a deposition rate of

0.2–0.3 Å /s. Ni/p-Si and Ti/p-Si electrodes were also prepared by depositing15 nm Ti or 5 nm Ni on p-type

silicon wafer.

Pt NPs/Ni/Ti/p-Si photocathode: Pt nanoparticles (Pt NPs) synthesis: 0.1 ml 0.1 M H2PtCl6 and 0.25 mL 0.1 M

sodium acetate were added into 10mL ethylene glycol (EG). The mixed solution was refluxed at 160°C in oil

bath for 3h. The resulting Pt NPs were washed with ethanol for 3 times. Pt NPs were then dispersed into

ethanol. 0.1mL Pt NPs solution (contain ~0.1mg Pt) was then dropped onto a Ni/Ti/p-Si photocathode. The

electrode was dried at room temperature.

Back contact: Ohmic contact of all the electrodes was made to the backside of the wafer by e-beam deposition

of palladium (20 nm). Copper tape was used to contact the palladium on the backside for electrochemical

experiments. The Ohmic contact between palladium and p-type silicon was confirmed by I-V curves (Figure

S5(a)).

Electrochemical characterization:

Electrodes were analyzed in a homemade square cell with a circular 0.38 cm2 aperture sealed by the electrode.

A 150 W Xenon lamp from Newport Corporation was used as the light source. The power density of the light

irradiating the sample was measured with a Thorlabs PMT50 power meter to be ~ 225mW/cm2. The prepared

electrode was also characterized under 1 Sun, that is 100 mW/cm2, with an AM 1.5 filter involved (Figure S4).

All data presented in main text was obtained under ~ 225mW/cm2. We chose to run our test under this harsher

condition to investigate the stability of our electrode. Electrochemical experiments were carried out in a

three-electrode system controlled by a CHI 760D potentiostat. A standard calomel electrode (SCE) was used as

reference electrode and a stainless steel electrode was used as counter electrode. SCE was converted to RHE

using the following equation.

E(RHE) = E(SCE) + 0.244 V + 0.059*pH


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The electrolytes used were 1 M KOH and potassium borate buffer made by mixing 2 M boric acid and 1 M

KOH aqueous solution.

All CVs were taken at 100 mV/s. All electrochemical measurements were done without iR correction and all

data presented in this work was not iR corrected either.

Materials Characterization:

Prior to any physical characterization, the samples were washed with water, toluene and ethanol to remove

any organic contamination.

Scanning electron microscopy (SEM): SEM images was taken by an FEI XL30 Sirion scanning electron

microscope. Auger electron spectra and element mapping were taken by PHI 700 Scanning Auger Nanoprobe

operating at 10 kV and 10 nA.

X-ray photoelectron spectroscopy (XPS): XPS spectra and depth profiles were collected on a PHI VersaProbe

Scanning XPS Microprobe. The Ar milling was done at 5 kV and 1 μA with spot size of 1 mm×1 mm.

Materials:

Chemicals:

Potassium hydroxide: >=85% KOH basis; supplier: SIGMA-ALDRICH

Boric acid:ACS reagent, >=99.5%; supplier: SIGMA-ALDRICH

Chloroplatinic acid hexahydrate: ACS reagent, ≥37.50% Pt basis; supplier: SIGMA-ALDRICH

Sodium acetate:>= 99.0 %; supplier: J.T. Baker

Ethylene glycol:>= 99 %;supplier: SIGMA-ALDRICH

Metal targets:

Nickel pellets: 4N; supplier: ESPI Metals

Titanium: 99.99s%; supplier: Plasmaterials, Inc.

Palladium: 99.99%; supplier: Plasmaterials, Inc.


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Supplementary Figures

Figure S1: Zoom-in of cyclic voltammograms (CV) of Ni/Ti/p-Si cathodes, which is shown in Figure 1(c)

0.0 0.2 0.4 0.6

-10

0

Curr

ent density(m

A/c

m2)


dark current

5 nm Ni /15 nm Ti /p-Si

0.3 V vs. RHE


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Figure S2: Cyclic voltammograms of 5nm Ni/FTO in KBi and KOH.

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4-160

-140

-120

-100

-80

-60

-40

-20

0

20

Cu

rre

nt

(mA

)

Potential(V vs RHE)

1 M KOH

KBi


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Figure S3: SEM images Ti/p-Si electrode after 3 h operation in KOH showing significant etching.

200µm 10µm

(a) (b)


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Figure S4: Cyclic voltammograms of 5nm Ni/ 15 nm Ti/ p-Si in 1 M KOH under 1 Sun (100 mW/cm2), with an

AM 1.5 filter involved.


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Figure S5: I-V curves of (a) Pd/p-Si/Pd and (b) Pd/p-Si/Ti/Ni electrode.

Address correspondence to Hongjie Dai, [email protected]

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3-10

-8

-6

-4

-2

0

2

4

6

8

10

Voltage (V)

Cu

rre

nt (m

A)

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

0.0

0.5

1.0

Cu

rre

nt (m

A)

Voltage (V)

(a) (b)

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Nickel Coated Silicon Photocathode for Water Splitting in Alkaline … · 2014-11-19 · Nickel Coated Silicon Photocathode for Water Splitting in Alkaline Electrolytes Ju Feng†1,