ZnS Thin Films for Visible-Light Active Photoelectrodes: Effect of Film

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ZnS Thin Films for Visible-Light Active Photoelectrodes: Effect of Film Morphology and Crystal Structure Fran Kurnia, Yun Hau Ng, Yiming Tang, Rose Amal, Valanoor Nagarajan, and Judy Nancy Hart Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b01590 • Publication Date (Web): 31 Mar 2016 Downloaded from http://pubs.acs.org on April 3, 2016

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Crystal Growth & Design

1

ZnS Thin Films for Visible-Light Active

2

Photoelectrodes: Effect of Film Morphology and

3

Crystal Structure

4

Fran Kurnia1, Yun Hau Ng2, Yiming Tang2, Rose Amal2, Nagarajan Valanoor1, and Judy N.

5

Hart1

6

1

7

2

8

2052, Australia

School of Materials Science and Engineering, UNSW Australia, UNSW Sydney 2052, Australia

Particles and Catalysis Research Group, School of Chemical Engineering, UNSW Australia, UNSW Sydney

9 10

KEYWORDS. ZnS, surface morphology, thin film, crystal structure, photoelectrochemistry

11 12

ABSTRACT. The photoelectrochemical (PEC) performance of zinc sulfide (ZnS) thin films

13

under visible light has been studied as a function of thin film fabrication conditions. In

14

particular, N2 background gas during film deposition was exploited to tune the film crystal

15

structure and surface roughness. The photocurrent was highest for films with relatively large

16

crystallite size in the wurtzite structure and high surface roughness. Films with such

17

optimized characteristics showed a maximum photocurrent density of 1.6 mA/cm2 under

18

visible-light irradiation. Hence, this work provides a guide for how ZnS thin films with high

19

PEC performance, particularly under visible light, can be realized.

20

ACS Paragon Plus Environment

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Zinc sulfide (ZnS) is an important optoelectronic device material for use at UV

2

wavelengths owing to its wide band gap (∼3.66 eV) and low exciton binding energy of

3

40 meV.1,2 In addition, ZnS is a well-known photocatalyst for hydrogen evolution as a result

4

of the rapid generation of electron-hole pairs by photoexcitation and the presence of highly

5

active surface sites for H2 production, such that noble metal co-catalysts are not required.3,4

6

However, due to its large band gap, pure ZnS is not active under visible-light irradiation.

7

Recently, Huang et al.5 investigated the photoelectrochemical (PEC) properties of Ni-

8

doped ZnS thin films. The films were produced by chemical bath deposition. To measure the

9

photocurrent density, they used a three-electrode PEC cell equipped with computer-

10

controlled potentiostat in 0.5 M K2SO4 electrolyte solution under a light intensity of

11

100 mW/cm2. The observed photocurrent density for undoped ZnS films reached

12

0.25 mA/cm2 at an applied voltage of 1.0 V; Ni doping was used to achieve higher

13

photocurrents. Zhou et al.6 studied the PEC properties of ZnS with and without nitrogen

14

doping. The ZnS sample was prepared by a solid-state reaction. The photocurrent density was

15

measured in a standard PEC cell in 0.5 M Na2SO4 electrolyte solution. The photocurrent of

16

the undoped ZnS could only reach a maximum of 0.1 mA/cm2 at 1.0 V external bias. In PEC

17

applications, achieving a high photocurrent from the active material is important as a high

18

current output results in high efficiency for conversion of the incident light to chemical

19

energy.

20

It is well known that the performance of a material in PEC and other optoelectronic

21

applications is affected by its structure and morphology, including particle size, crystal

22

structure, surface morphology and both surface and bulk defects.7−11 Thus, it is possible to

23

improve performance by modifying these characteristics. For example, a higher surface area

24

provides more active sites at which electrochemical reactions can take place. In TiO2

25

photocatalysts, engineered surfaces with oxygen defects have been found to play an


2

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1

important role in determining the photocatalytic efficiency.7 The surface defects may serve as

2

charge carrier traps as well as adsorption sites where the photogenerated charge carriers can

3

be transferred to the adsorbed species, thus preventing electron-hole recombination and

4

resulting in efficient charge separation.

5

In addition, for ZnS, it has been predicted from first-principles calculations that the

6

wurtzite phase should have a higher reducing ability than the zinc blende phase, due in part to

7

the higher conduction band energy.12 Meng et al. have reported that the surface of wurtzite

8

ZnS has lower energy barriers for removing H from water (0.94 and 2.24 eV for the first and

9

second H removal, respectively) than zinc blende ZnS (0.98 and 2.35 eV) on the Zn-

10

terminated faces.12 In addition, Hong et al. reported that ZnS(w) material gives a higher

11

photocatalytic efficiency than ZnS(zb) due to the inter-polar electric field promoting the

12

separation of photo-excited electron-hole pairs.13 Hence, the phase of ZnS has an important

13

influence on PEC performance.

14

In this work, we investigate the effect of morphology and crystal structure on the optical

15

properties and PEC performance of ZnS thin films, with the aim of finding the film

16

deposition conditions that can optimize the performance under visible light. The samples

17

were prepared using pulsed laser deposition as described in the experimental section

18

(Supporting Information). ZnS thin films were grown under two different chamber pressures:

19

vacuum (10-6 mTorr) and 100 mTorr of N2. For each pressure, films were grown with three

20

different numbers of laser pulses (3000, 6000 and 10000) resulting in three different

21

thicknesses (0.61 µm, 1.23 µm, and 2.05 µm for films grown under vacuum; 0.35 µm,

22

0.70 µm, and 1.17 µm for films grown under 100 mTorr N2 gas pressure). Film thicknesses

23

were determined by SEM imaging of the film cross-sections (Figure S1, Supporting

24

Information). Hereafter, we label the samples grown under vacuum as ZnS(zb)-3k, ZnS(zb)-


3


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1

6k, and ZnS(zb)-10k and the samples grown under 100 mTorr N2 pressure as ZnS(w)-3k,

2

ZnS(w)-6k, and ZnS(w)-10k.

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3

The ZnS crystal structures and x-ray diffraction (XRD) patterns of the films are shown in

4

Figure 1. For films grown under vacuum, the ZnS crystallizes as the zinc blende (zb) phase in

5

which both zinc and sulphur atoms are tetrahedrally coordinated and stacked in an ABCABC

6

arrangement as illustrated in Figure 1a. For the films grown under 100 mTorr N2 gas, ZnS

7

forms the meta-stable wurtzite (w) structure. In this case, the layers are stacked in an ABAB

8

arrangement, resulting in a hexagonal close-packed structure with tetrahedral bonding as

9

shown in Figure 1b. Figure 1c shows the XRD θ−2θ scan of ZnS films grown under the

10

vacuum, a peak at d ≈ 3.154 Å is observed, assigned to the (111) lattice plane of the zinc

11

blende (zb) ZnS crystal structure.14 For films grown in a 100 mTorr N2 atmosphere (Figure

12

1d), peaks at d ≈ 3.107 Å and d ≈ 1.626 Å were assigned to the (002) and (112) lattice planes

13

of the ZnS wurtzite (w) structure.14 Additional shoulder peaks at d100 ≈ (3.107 + 0.182) Å and

14

d101 ≈ (3.107 − 0.122) Å, just above and below the (002) peak, were also assigned to the

15

wurtzite phase of ZnS. TEM images also confirm the appearance of the wurtzite phase for

16

samples deposited in a 100 mTorr N2 atmosphere (Figure S2 in Supporting Information).

17

Thus, it can be seen that the use of N2 pressure during deposition changes the crystal structure

18

of the ZnS films.

19 20 21 22 23 24 25


4

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1

(a)

(b)

(111)-zb

(001)-w A

A

2

B

B

3

C

4

7 8

(c)

(111)

Si (004)

6

(111)

Intensity (a.u.)

5

ZnS(zb)-10k ZnS(zb)-6k ZnS(zb)-3k

20

30

50

9

60

70

80 27

30

33

11 12

20

13

(112)

(002)

(101)

ZnS(w)-10k

(100)

Si (004)

(d)

(112)

10

(002)

2θ (deg.)

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


ZnS(w)-6k ZnS(w)-3k

30

50

60

70

80 27 30

54

57

60

2θ (deg.)

14

Figure 1. The crystal structure of (a) zinc blende ZnS, (b) wurtzite ZnS. XRD patterns of ZnS thin films

15

deposited for different number of laser pulses, i.e. different thicknesses, under (c) vacuum condition, showing

16

peaks corresponding to the zinc blende phase, and (d) 100 mTorr N2 gas pressure , showing peaks corresponding

17

to the wurtzite phase . The right panels in (c) and (d) show enlarged views of the main diffraction peak(s) of the

18

ZnS thin films.

19 20

The full width at half-maximum (FWHM) of the XRD peaks were taken as a measure of

21

crystallinity (e.g. crystal size and disorder). For ZnS(zb), the FWHM of the (111) reflection

22

slightly decreases with increasing thickness (0.988o, 0.975o, 0.960o for ZnS(zb)-3k, ZnS(zb)-

23

6k, and ZnS(zb)-10k, respectively). For ZnS(w), the FWHM of the (002) reflection also

24

decreases with increasing film thickness (0.975o, 0.923o, and 0.637o for ZnS(w)-3k, ZnS(w)-

25

6k, and ZnS(w)-10k, respectively). Overall, these results suggest that, in both the zinc blende

26

and wurtzite films, the ZnS crystallinity improves with increasing film thickness. In


5


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1

particular, the small FWHM value for ZnS(w)-10k indicates good crystallinity and a

2

relatively large crystal size in this film.

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3

Atomic force microscopy (AFM) and scanning electron microscopy (SEM) images of the

4

ZnS thin films with different thicknesses are shown in Figure 2.The root mean square (RMS)

5

roughness of the films was calculated from the AFM images. (Due to the presence of

6

relatively large nanoparticles on the sample surface that made AFM imaging impossible, the

7

AFM images for samples ZnS(w)-10k and ZnS(w)-6k were obtained at different areas of the

8

sample surface from the SEM images. This does not affect the qualitative comparison of the

9

roughness values. For all other samples, both SEM and AFM images were taken from

10

equivalent areas that are representative of the whole film surface.) A larger surface roughness

11

was found for the thicker films, consistent with the increased grain size that can be observed

12

in the SEM images and was also evident in the XRD results.

13

The ZnS(w) films grown under 100 mTorr N2 pressure have a larger surface roughness

14

and decreased thickness compared with the ZnS(zb) films grown under vacuum. The change

15

in roughness indicates that the growth mode undergoes a transition from layer-by-layer

16

growth under vacuum to island (Stranski-Krastanov) growth under 100 mTorr N2 pressure.

17

Film growth by PLD in the presence of a background gas is known to result in relatively slow

18

growth, as the ablated material undergoes more collisions as it is transported to the

19

substrate,15,16 hence producing thinner films. This slow film growth rate allows more time for

20

the growth of nanostructures and thermalization of the ablated material, resulting in rough,

21

nanostructured films. In contrast, for vacuum deposition, the high nucleation density on the

22

substrate and high kinetic energy and hence surface mobility of the deposited species tends to

23

produce smoother films. The effect observed here of deposition of ZnS in the presence of N2

24

background gas giving nanostructured films is consistent with previous reports of the effect

25

on film morphology of a background gas during PLD growth of ZnO and Nb2O5.17,18


6

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1 (a)

(b)

(c)

2 3 500 nm

4

20 nm

5 6

RMS = 1.52 nm

200 nm

RMS = 1.49 nm

RMS = 1.28 nm

(e)

(f)

7 (d)

8 9 10

0

11 12

RMS = 2.24 nm

RMS = 2.04 nm

RMS = 1.39 nm

13 14

Figure 2. Surface morphologies of ZnS thin films deposited under (a-c) vacuum condition: ZnS(zb)-10k,

15

ZnS(zb)-6k, and ZnS(zb)-3k, respectively, and (d-f) 100 mTorr N2 gas pressure: ZnS(w)-10k, ZnS(w)-6k, and

16

ZnS(w)-3k, respectively. The RMS roughness value for each AFM image is measured for 1 µm × 1 µm image.

17

SEM images are shown in the insets.

18 19

To investigate the effect of surface morphology on the photoelectrochemical (PEC)

20

properties of the ZnS films, we measured linear sweep voltammograms (LSV) under 300 W

21

Xe lamp illumination with an intermittent on-off cycle from −0.5 V to 1.0 V. Measurements

22

were taken under visible-light illumination (λ ≥ 435 nm); the results are shown in Figure 3a

23

for films deposited under vacuum and Figure 3b for films deposited under 100 mTorr N2

24

background pressure. The photocurrent response observed in the LSV curves reflects the

25

number of charge carriers produced from the incident light and their subsequent involvement

26

in redox reactions at the surface.

27 28

The ZnS films deposited by PLD produce a photocurrent under visible light, even though pure ZnS should only be active under UV light; the origin of this visible-light activity is


7


1

discussed below. The photocurrent density increases with ZnS film thickness (for the same

2

deposition pressure) and with increased N2 pressure used during the deposition. The highest

3

photocurrent (∼ 1.6 mA/cm2) was measured for the ZnS(w)-10k film, which has the highest

4

surface roughness (Figure 2d). On the other hand, the highest photocurrent achieved for

5

deposition under vacuum was ∼ 0.7 mA/cm2 for the ZnS(zb)-10k sample. There is almost no

6

observable photocurrent for the thinnest ZnS(zb) film. These photocurrents are significantly

7

higher than those reported previous for ZnS photoelectrodes.19,20

8 (a)

10

1.0 2

11 12 13 14 15 16

0.5

(c)

Zn S(w) -1 0k

λ ≥ 435 n m

d ri ection

ln D

1.5

(b)

ZnS(z b)- 10 k

on o ff

0.0 ZnS( zb) -6 k 1.5

ZnS( w)- 6k

1.0

0.0 ZnS( zb) -3 k 0.4

ZnS( w)- 3k

0.2

18

-0.2 -0.5

ZnS(w)-10 k ZnS(w)-6k ZnS(w)-3k ZnS(zb)-10 k ZnS(zb)-6k

0

2

4

0.0

0. 5

-0.5

0. 0

0.5

Poten tial (V vs. Ag /AgC l)

1.0

@1 V

6

Time (s)

8

10

ZnS(w)-10k ZnS(w)-6k ZnS(w)-3k ZnS(zb)-10k ZnS(zb)-6k ZnS(zb)-3k

8

0.0

17

0 -1 -2 -3 -4 -5 -6

(d)

0.5

IPCE (%)

9 Current density (mA/cm )

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Page 8 of 19

6 4 2

0 200 300 400 500 600 700

Wavelength (nm)

19 20

Figure 3. Linear sweep voltammogramms (LSV) measured under visible light (λ ≥ 435 nm) for ZnS films

21

deposited under (a) vacuum, (b) 100 mTorr N2 pressure for different number of laser pulses, i.e. different

22

thicknesses. Note the reduced y-axis scale for the 3k pulses samples. (c) Normalized plot of the current-time

23

dependence measured at 1 V (vs. Ag/AgCl) bias under visible light; data is not included for ZnS (zb)-3k, since

24

the photocurrent for this sample under visible-light was very small. (d) The incident photon-to-current-

25

conversion efficiency (IPCE) of the ZnS thin films. Black lines are linear fits of the data in the inital period.

26 27 28

Recombination processes have an important influence on PEC performance, so it is useful to determine the differences in the rate of these processes for the ZnS films deposited under


8

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1

different conditions. The recombination rate was evaluated by plotting the natural logarithm

2

of the transient decay, D, versus time for the photocurrent transient response based on the

3

following photocurrent relaxation equation:21,22

4 5

D=

I t − I st I in − I st

(1)

6

where It is the current at time t, Iin is the current at t = 0, and Ist is the steady-state current. The

7

normalized plots of ln D versus t for the photoanodic current transient response are shown in

8

Figure 3c. There is no significant photoactivity for ZnS(zb)-3k under visible-light irradiation,

9

so data for this sample is not included. The non-linearity of the relationship between ln D and

10 11

t indicates that the both bulk and surface recombination occur.21,22 A quantitative understanding of the effect of different film morphology can be gained by

12

calculating the transient time constant, τ, defined as the time at which ln D = −1; τ is a

13

measure of the recombination rate, with higher values of τ indicating slower recombination.

14

For ZnS(w)-10k, τ = 9.32 s, which is the longest transient time obtained in this work.

15

Consistent with the trends in the photocurrent measurements, the value of τ decreases with

16

decreasing ZnS film thickness for the same deposition pressure, reaching 0.98 s for ZnS(w)-

17

3k, and is also lower for the films deposited under vacuum than 100 mTorr of N2 for the same

18

deposition time (for ZnS(zb)-10k, τ = 6.01 s). These results indicate that the rate of

19

recombination losses decreases as film thickness increases, despite a potentially longer

20

transport distance for the photogenerated charges, and with increased N2 pressure used during

21

film deposition. The possible causes of these effects are discussed below.

22

The incident photon-to-current-conversion efficiency (IPCE) of the ZnS thin films

23

measured at 1.0 V versus Ag/AgCl is presented in Figure 3d. IPCE is the ratio of the

24

photocurrent (converted to an electron transfer rate) to the number of incident photons

25

(calculated from the incident light energy and intensity) as a function of wavelength. Our ZnS


9


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1

samples show a photocurrent onset at a wavelength of ∼ 465 nm corresponding to a band gap

2

of ∼ 2.7 eV, which is significantly less than the band gap of pure bulk ZnS. The trends in the

3

IPCE results are consistent with the measured photocurrents, with the IPCE values being

4

higher for samples deposition under an N2 atmosphere than vacuum, and values increasing

5

with increasing film thickness. The highest IPCE was found for the ZnS(w)-10k sample

6

(∼ 4.3 % at 435 nm); this indicates that the incident photons can be converted to electrons in

7

the external circuit with relatively high efficiency in these ZnS thin films even under visible

8

light. For the ZnS(zb)-10k sample, the IPCE reaches only ∼ 2.8 % at 435 nm.

9

To understand the origin of the visible-light activity of the ZnS films and the differences

10

between samples deposited under different conditions, we conducted various spectroscopic

11

analyses. Unfortunately, UV-vis spectra (Figure S3 in Supporting Information) were not

12

unequivocal due to interference effects. (The UV-Vis spectra with the interference effects

13

subtracted using some approximations,23 including the Kubelka-Munk function to calculate

14

the band gap of each film, are included in Figure S4, Supporting Information. It is clear that

15

there are some defect states that exist in the electronic structure of ZnS that can narrow the

16

band gap and increase the observed photocurrent under visible light. The visible-light PEC

17

activity is also supported by the wavelength-dependent activity seen in the action spectra,

18

presented in Figure S5, Supporting Information, which show significant photocurrents at

19

visible-light wavelengths.)

20

On the other hand, photoluminescence (PL) spectra of the ZnS films were found to be

21

insightful. The spectra for all samples display a broad band between 400 nm and 900 nm

22

(Figure 4). The observation of luminescence peaks in the wavelength range of visible-light

23

may be attributed to the presence of defects in ZnS, such as vacancies, interstitial atoms,

24

surfaces and grain boundaries, as reported previously.24 The wavelengths and photon energies

25

for the observed PL peaks with their FWHM are shown in Table S1 Supporting Information.


10

Page 11 of 19

1

The observed weak emission peak at ∼ 450 nm for all samples can be attributed to zinc

2

vacancies.24,25 Both the ZnS(w) and ZnS(zb) samples also show green (510 nm ≤ λ ≤ 590 nm)

3

luminescence, which may be related to deep defect energy levels resulting from both zinc

4

interstitials and pairs of zinc and sulfur vacancies.24 In the visible light to IR wavelength

5

region (600 to 800 nm), two emission peaks centered at ~ 670 nm and ~ 760 nm are observed

6

for the ZnS(w) samples, while the ZnS(zb) samples show a broad emission band centered at

7

670 ≤ λ ≤ 720 nm. Strong emission continues to longer wavelengths for the ZnS(w) films

8

than the ZnS(zb). (a)

9

(b)

CB

10 UV 3.6 eV

12

Visible-light 1.6 − 2.7 eV

Visible-light 1.8 − 2.7 eV

11

IZn

VZn,VS

VZn,V

VZn

VZn

13 VB

14

400

15

(c) ZnS(zb)-10k

400

200

16 Intensity (cps)

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


17 18 19

0 400

0 400

21

200

22

0

23

200 0 400

ZnS(zb)-6k

200

20

(d) ZnS(w)-10k

ZnS(w)-6k

200 0 400

ZnS(zb)-3k

ZnS(w)-3k

200

400

600

800

0

400

600

800

Wavelength (nm)

24

Figure 4. Proposed electronic transitions that can allow visible-light activity for ZnS (UV absorption ≈ 3.6 eV)

25

for (a) zinc blende ZnS, (b) wurtzite ZnS. The photoluminescence (PL) spectra of the ZnS thin films deposited

26

under (c) vacuum condition and (d) 100 mTorr N2 gas pressure for different number of laser pulses, i.e. different

27

thicknesses.


11


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Page 12 of 19

1

It is clear that the PEC performance of the ZnS films depends strongly on the deposition

2

conditions. The photocurrents produced by the ZnS thin films deposited under both vacuum

3

and 100 mTorr N2 pressure increased as the thickness increased, while the transient time

4

constants increased, suggesting a lower recombination rate. Since the PL spectra do not

5

change significantly with thickness for the same deposition pressure, we can assume that the

6

defect states are similar, so these differences in PEC properties with increasing thickness may

7

be due to (1) the increased surface roughness, which gives a larger surface area at which

8

redox reactions can take place, resulting in a larger photocurrent and reduced recombination

9

losses, (2) an increased amount of material available to absorb light, leading to increased

10

generation of photoexcited charge carriers and hence increased photocurrent, and (3)

11

improved crystallinity and increased crystal size, giving improved charge transport and

12

reduced recombination losses.26,27 It was observed from the transient decay results that both

13

bulk and surface recombination occur in the films; both may conceivably be affected by the

14

changes in the film structure and morphology that take place with increasing thickness. For

15

samples ZnS(zb)-10k and ZnS(zb)-6k, the roughness values are not significantly different

16

(Figure 2), so we can speculate that the latter two of the three effects mentioned above,

17

related more to bulk than surface properties, may dominate in that case.

18

The ZnS thin films deposited under a N2 pressure of 100 mTorr generally showed higher

19

photocurrents than the films deposited under vacuum. As above, this is consistent with the

20

higher surface roughness. Also, the PL spectra of the ZnS(w) films, deposited in 100 mTorr

21

background pressure, showed stronger PL emission at long wavelengths (> 700 nm) than the

22

ZnS(zb) films, which may correlate with stronger visible-light absorption and hence higher

23

photocurrents. In addition, as discussed earlier, the wurtzite phase of ZnS is known to have

24

better PEC properties than the zinc blende phase. These three effects appear to compensate


12

Page 13 of 19

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1

for the films being thinner when deposited in an N2 atmosphere and hence there being less

2

material available to absorb light.

3

The transient time constant is lower for the films deposited under vacuum than 100 mTorr

4

of N2 for the same deposition time, indicating a slower recombination rate. There are several

5

factors that may contribute to this difference, including the higher surface roughness and

6

decreased thickness of the films deposited in an N2 atmosphere, allowing more rapid

7

transport of charges to the surface, as well as differences in the crystallinity and crystal size,

8

differences in the defect states as observed in the PL spectra, and intrinsic differences

9

between the properties of the wurtzite and zinc blende phases.

10

In summary, we have observed an obvious dependence of PEC activity on the deposition

11

conditions of ZnS thin films. Firstly, compared to films deposited under vacuum, the

12

presence of a background gas (N2) during deposition results in higher surface roughness, a

13

change in the crystal structure and a change in the defect states that allow visible-light

14

activity. Secondly, increasing the number of laser pulses results in thicker films with

15

increased surface roughness and crystallite size. These factors ultimately lead to higher

16

photocurrents and a slower recombination rate, thus markedly improving PEC performance.

17

Our results provide a key step towards developing ZnS thin films with high PEC activity,

18

particularly under visible light, and will open up new opportunities for applications of non-

19

oxide photocatalysts and photoelectrodes.

20

ASSOCIATED CONTENT

21

Experimental method; Cross-section SEM images, TEM images, UV-vis spectra, action

22

spectra, wavelengths and photon energies of the photoluminescence peaks. This material is

23

available free of charge via the Internet at http://pubs.acs.org.”

24


13


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

1

AUTHOR INFORMATION

2

Corresponding Author

3

*Email: [email protected]

4

Author Contributions

5

The manuscript was written through contributions of all authors. All authors have given

6

approval to the final version of the manuscript.

7

ACKNOWLEDGMENT

8

We thank the Australian Microscopy and Microanalysis Research Facility (AMMRF,

9

UNSW), and Electron Microscope Unit (EMU, UNSW) for technical assistance.

10

REFERENCES

11

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Page 19 of 19

1

For Table of Contents Use Only

2 3 4

ZnS Thin Films for Visible-Light Active Photoelectrodes: Effect of Film Morphology and Crystal Structure

5 6

Fran Kurnia, Yun Hau Ng, Yiming Tang, Rose Amal, Nagarajan Valanoor, and Judy N. Hart

7 Zinc blende ZnS

Wurtzite ZnS

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


8

2.0 1.5 1.0 0.5 0.0 -0.5

direction

0.0

on off

0.5

-0.5

0.0

0.5

1.0

Potential (V vs. Ag/AgCl)


19

ZnS Thin Films for Visible-Light Active Photoelectrodes: Effect of Film

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