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CuS heterostructure photoanode with highly PEC performance and excellent charge separation efficiency
Journal Pre-proof Facile fabrication of ZnO/CuS heterostructure photoanode with highly PEC performance and excellent charge separation efficiency Huixia Guo, Ce Su, Dongmei Yu, Liangliang Li, Ziye Liu, Zhengang Han, Xiaoquan Lu PII:
S1572-6657(19)30814-8
DOI:
https://doi.org/10.1016/j.jelechem.2019.113546
Reference:
JEAC 113546
To appear in:
Journal of Electroanalytical Chemistry
Received Date: 19 July 2019 Revised Date:
6 September 2019
Accepted Date: 1 October 2019
Please cite this article as: H. Guo, C. Su, D. Yu, L. Li, Z. Liu, Z. Han, X. Lu, Facile fabrication of ZnO/CuS heterostructure photoanode with highly PEC performance and excellent charge separation efficiency, Journal of Electroanalytical Chemistry (2019), doi: https://doi.org/10.1016/ j.jelechem.2019.113546. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
1
Facile
fabrication
of
ZnO/CuS
heterostructure
2
photoanode with highly PEC performance and
3
excellent charge separation efficiency
4
Huixia Guoa,∗, Ce Sua, Dongmei Yua, Liangliang Lia, Ziye Liua, Zhengang Hana,
5
Xiaoquan Lua
6
a
7
Province, College of Chemistry & Chemical Engineering, Northwest Normal
8
University, Lanzhou 730070, China
Key Laboratory of Bioelectrochemistry & Environmental Analysis of Gansu
9 10 11 12 13 14 15 16 17 18 19 20 21
∗
Corresponding author at: Key Laboratory of Bioelectrochemistry & Environmental Analysis of Gansu Province,
College of Chemistry & Chemical Engineering, Northwest Normal University, Lanzhou 730070, China (H. Guo). E-mail addresses
[email protected] (H. Guo)
22
Abstract
23
Photoelectrochemical (PEC) water splitting, as a promising route of STH(solar to
24
hydrogen) conversion, has attracted a lot attention recently. However, the rather high
25
recombination and the low transfer rate of photoinduced electron-hole pairs restrict
26
the applications. In this work, n-ZnO/p-CuS photoanodes were fabricated using
27
hydrothermal method and successive ionic layer adsorption and reaction (SILAR)
28
method. The PEC performance of the ZnO photoanode is significantly improved after
29
depositing CuS. The applied bias photon-to-current efficiency (ABPE) of ZnO/CuS 10
30
(the cycles of SILAR) photoanode attained 0.368% at a bias of 0.984 V vs SCE which
31
is 45 times than ZnO. The relatively high value of photocurrent density and ABPE are
32
due to the advantages of the ordered structure of ZnO nanorods arrays and the common
33
effect comes from the formation of heterojunction and faster transfer rate of
34
photogenerated electrons which further facilitate charge carrier separation and the light
35
absorption. This work represents a facile strategy using low cost CuS for PEC water
36
splitting that can be applied in photocatalysis.
37
Keywords: ZnO/CuS heterojunction; SILAR; Photoelectrochemical performance;
38
Photocatalysis
39 40
41
42
1. Introduction
43
With the gradually increasing demand of energy, how to utilize a large number of
44
solar energy becomes an intractable issue to solved[1]. An example of converting
45
sunlight into other kinds of energy method is photoelectrochemical (PEC) water
46
splitting which has attracted considerable attention in past decades. The strategy has
47
the most appealing merit of combining the conversion of solar to hydrogen (STH)
48
with water electrolysis. Besides, the PEC performance was also widely used in
49
biosensor field for detection[2-5]. Photoelectric conversion process has great
50
application potential. Therefore, many researchers made great efforts to use metal
51
oxide semiconductors as photoanode in PEC water splitting and biosensor for the
52
relatively low cost, simple preparation on semiconductor which could be recycled
53
with no pollution to the environment[6-14]. Nevertheless, the water splitting still has
54
lots of problems to be solved, such as: (1) rather low percentage of the light absorption
55
and capture; (2) photocorrosion restricts the application of catalysts; (3) low conversion
56
efficiency of solar to hydrogen energy etc[15]. Thus, fast electron transport, effective
57
charge carrier separation and the evolution of H2 are central points to the PEC
58
performance of semiconductor photoanode[16-18]. However, it is hard for us to find
59
appropriate semiconductors which could match the band structure successfully, only a
60
small part of semiconductor materials have shown superior photocurrent density and
61
PEC performance under AM 1.5G illumination after a large number of studies[19,20].
62
Among various metal oxide materials, the low cost and stable ZnO, highly ordered
63
one-dimensional arrays in particular, is one of the most commonly semiconductors. It
64
has some notable physicochemical characters, for example, high aspect ratio,
65
adjustable morphology and alignment, relatively high excitation binding energy of 60
66
meV at room temperature[21, 22]. These advantages make ZnO as a good candidate for
67
the photocatalysis. However, ZnO has a wide band gap (3.2 eV) which leads to the ZnO
68
can only be used in ultraviolet light[23]. Meanwhile, the relatively rapid recombination
69
rate of charge carrier hinders its performance and application in photocatalysis and
70
water splitting. Therefore, exploring new ways to improve the photocatalytic activity
71
and to further expand the region of the visible light absorption is still an issue to be
72
solved.
73
In the past few decades, a lot of efforts had been made on ZnO. In general, forming
74
heterostructure[24, 25], doping with metal or nonmetal atoms[26-29] and construction
75
of nanostructure[30] are profitable for the transfer of electrons and the separation of
76
charge carrier to solve the above problems. For instance, Kuang et al. constructed two
77
p-n heterojunctions in CdS/Cu2O/ZnO photoanode and reduced the recombination of
78
charge carrier, the ABPE of the sample was 261 times greater than that of the ZnO NRs
79
arrays[31]. Kar et al. doped C, N, and S in ZnO NRs arrays photoanodes which
80
exhibited enhanced efficiency of visible light absorption and charge carrier separation
81
significantly[32]. Hsu et al. synthesized ZnO/Fe2O3 core–shell NWs for PEC water
82
electrolysis, the photocurrent increased significantly compared to pristine ZnO
83
NWs[33].
84
Recently, copper sulfide (CuS) has attracted extensive interest attributed to its
85
application in photocatalysis, photoelectrochemical hydrogen production, solar cells
86
etc. CuS has a narrow band gap about 2.1 eV that matches the visible light
87
spectrum[34-38]. Meanwhile, it could be constructed with other n-type semiconductor
88
and form heterostructure that provides large surface area and more active sites to
89
facilitate light absorption and photoinduced electron supply[39,40]. Above all, it is a
90
good way to fabricate ZnO/CuS heterostructure and play different roles for improving
91
the mobility of electrons and holes and extending the diffusion length of carrier. In the
92
reported literatures, the composite of CuS/ZnO heterojunction was fabricated by
93
two-step wet-chemical method on stainless steel mesh and used to degrade the dyes
94
under visible light and ultrasonic irradiation due to the preferable photocatalytic
95
activity and visible light utilization[41]. CuS/ZnO nanocomposite was also reported to
96
be used in methylene blue degradation owing to the ZnO/CuS junctions improved the
97
photocatalytic activity of ZnO and the separation of photoinduced carriers by
98
mechanical lapping method[42] and a wet-chemical method at low temperature[43].
99
In this work, we used different ways, combined the spin-coating and hydrothermal
100
method to synthesize ZnO NRs arrays on FTO, and the CuS was constructed by
101
successive ionic layer absorption and reaction (SILAR). The similar approaches were
102
reported in the construction of ZnO@CdS heterostructure[44] and CuS/ZnO
103
heterostructure nanowire arrays[45], both of the heterostructures represented
104
preferable PEC performance by electrodeposition and SILAR which demonstrated
105
SILAR was a feasible low cost method. In contrast, we use physical coating and
106
one-pot reaction to substitude electrodeposition method mentioned above. Attributed
107
to appropriate band and ZnO/CuS heterojunction, the charge carrier acquired fast
108
generation, rapid transfer rate and avoided partial recombination, the ZnO acted as a
109
pathway and transferred the photoinduced electrons continuously. Simultaneously, our
110
work also proved some Cu2S was generated which could also integrated with ZnO and
111
formed ZnO/Cu2S heterojunction. Both of the hetrojunctions promoted charge
112
migration and separation of electrons and holes. Consequently, all of the results below
113
exhibited ZnO/CuS heterostructure photoanodes were constructed successfully and had
114
a better PEC performance than ZnO. Therefore, ZnO/CuS photoanode might have
115
potential application in the photo electrochemical water splitting.
116 117
2. Experimental Section
118
2.1 ZnO seed layer synthesis
119
Fig. 1 shows the synthesis process of the ZnO/CuS heterostructure photoanodes.
120
Prior to the synthesis, the FTO substrates were thoroughly cleaned by ultrasonication in
121
DI water, acetone, and isopropanol for 15 min, respectively. Later, after FTO substrates
122
were dried in air, the seed layer sol-gel of ZnO was distilled on the surface of FTO
123
substrate by spin coating (rpm 3000, for 30 s). The ZnO sol-gel was made through 0.75
124
M solution of zinc acetate mixed by 10 mL 2-methoxyethanol and the same amount of
125
substance ethanolamine followed by stirring at 60 °C for 2 h and the solution was kept
126
stirring continuously in air at 20 °C for 12 h to get ZnO sol-gel. The sol-gel was
127
distilled on the FTO substrate by 100 µL each sample. The spin coating process was
128
repeated five times to ensure the adequate thickness of ZnO. The samples were dried
129
at 200 °C for 5 min after each process. Finally, the samples were subjected to heat
130
treatment at 400 °C, 3 °C min-1 for 1 h.
131 132
2.2 ZnO NRs arrays growth
133
Afterwards, the ZnO NRs arrays were grown by hydrothermal method. The sample
134
with ZnO seed layer was put against the wall of the Teflon-lined stainless steel
135
autoclave (25 mL). It is worth noting that the conductive surface of FTO should be
136
facing down. The solution of 20 mM zinc acetate and 20 mM hexamethylenetetramine
137
mixed into 80 mL DI water was transferred into the autoclaves. Hydrothermal synthesis
138
was conducted at 90 °C for 4 h followed by cleaning with DI water and alcohol for
139
several times. Then, the samples were put in furnace at 400 °C, 2 °C min-1 for 2 h.
140 141
2.3 Synthesis of ZnO/CuS heterostructure
142
CuS nanoclusters were deposited on ZnO NRs arrays by SILAR method. 5 mM
143
CuCl2 aqueous solution and 5 mM Na2S aqueous solution were used in the deposition
144
on the as-prepared ZnO NRs arrays samples at room temperature. First, the ZnO NRs
145
arrays samples were immersed into CuCl2 aqueous solution for 60 s. Secondly, this
146
sample was immersed in the DI water for 60 s to prevent precipitation and remove
147
loosen ions. Thirdly, the sample was immersed in the Na2S aqueous solution for 60 s,
148
S2− reacted with the deposited Cu+ on the surface of ZnO NRs arrays. In the end, the
149
sample was immersed in the DI water for 60 s. This is the first cycle of CuS deposition,
150
the amount of CuS could be increased by repeating the SILAR cycles. The immersion
151
procedures were repeated for 6, 8, 10, 12 cycles and these samples were named in
152
sequence as ZnO/CuS X (X is the cycles of SILAR process). The following chemical
153
reactions could take place during the SILAR cycle:
154
Cu2+ + S2-→Cu2S (nanoclusters)
(1)
155 156
2.4 Structural and optical characterizations
157
The element species, chemical state and composition of photoanodes were measured
158
by X-ray photoelectron spectroscopy (XPS) with Kratos Axis Ultra DLD. The X-ray
159
diffraction (XRD) patterns were characterized with the diffraction angle 2θ = 20° to 80°
160
(Rigaku D/max 2400 diffraction meter with Cu target Kα, λ=1.54Å). Surface
161
morphology was characterized by scanning electron microscopy (SEM, ULTRA plus,
162
Zeiss). The microstructure was measured by Transmission electron microscope (TEM,
163
FEI Tecnai G2 F20). The light absorption of samples was measured by UV–Vis
164
spectroscopy (UV-2600, Shimadzu).
165 166
2.5. PEC measurements
167
The PEC performance of the photoanodes was measured using the electrochemical
168
workstation (CHI 660E, China Chenhua) with a three-electrode system using a
169
ZnO/CuS work electrode, an Ag/AgCl reference electrode and a Pt counter electrode in
170
0.5 M Na2SO4 aqueous solution (pH = 7.35) under AM 1.5G irradiation (100 mW
171
cm−2,94011A-ES, Newport Oriel). Photocurrent density curves were investigated at a
172
scanning rate of 0.01 V s−1 from −0.6 to 1.2 V vs Ag/AgCl. In Fig. 7a, the measured
173
potential is converted into the reversible hydrogen electrode (RHE) according to Nernst
174
equation:
175
ERHE=EAg/AgCl+0.1976V+0.059*pH.
(2)
176
IPCE was measured under a monochromator coupled with a 1000 W Xe lamp. The
177
electrochemical impedance spectroscopies (EIS) curves were measured with an AC
178
voltage of 5 mV amplitude in the frequency range from 0.01 Hz to 100 kHz. The
179
Mott-Schottky curves were carried out from −0.5 to 1.5 V vs Ag/AgCl. Both of EIS and
180
Mott-Schottky curves were obtained using the electrochemical workstation with the
181
same three-electrode system as LSV curves.
182
3. Results and discussions
183
Fig. 1 shows the synthesis process of the ZnO/CuS heterostructure photoanodes. The
184
XRD patterns of ZnO and different ZnO/CuS photoanodes were shown in Fig. 2. For all
185
samples, the diffraction peaks at 31.9°, 34.4°, 36.3°, 47.5° and 62.9° were assigned to
186
the (100), (002), (101), (102) and (103) of ZnO crystal (JCPDS Card No. 36-1451),
187
respectively. The peaks at 31.8°, 47.8° and 67.3° were attributed to the (103), (110) and
188
(118) of CuS cystal (JCPDS Card No. 06-0464). The ZnO (100) peak was so close to
189
the CuS (103) peak that they overlapped in the vicinity of 31.9°.
190
The SEM images were shown in Fig. 3a-b. It could be seen the ZnO showed the
191
nanorods arrays, all of them appeared hexagonal prism shape and directly growed on
192
FTO with average size of 100-280 nm. Moreover, it could be found that the surface of
193
ZnO nanorods arrays were smooth without other impurities, while in Fig. 3b, the
194
ZnO/CuS photoanode could be clearly observed almost 5~50 nm granule nanoclusters
195
were attached to the ZnO nanorods arrays. The TEM image of ZnO/CuS was shown in
196
Fig. 3c, ZnO NRs and the attached CuS nanoclusters were clearly obseverd which
197
indicated the diameter of the ZnO NRs was about 120 nm and the diameter of the CuS
198
nanoclusters was about 50 nm. Fig. 3d-e showed the HRTEM images, the CuS
199
nanoclusters on the surface of ZnO NRs indicated the lattice spacing was 0.190 nm,
200
corresponding to the (107) crystal plane of CuS. Fig. 3e showed the crystal structure of
201
ZnO NRs arrays which indicated the lattice spacing was 0.28 nm, corresponding to the
202
(100) crystal plane of ZnO.
203
To further explore chemical state and composition of the photoanodes, the XPS was
204
analyzed. Fig. 4a showed the XPS survey spectrum of the ZnO/CuS photoanode, the
205
peaks of S 2p, C 1s, O 1s, Cu 2p, Zn 2p were signed distinctly. The appearance of the
206
peak of C 1s might due to the photoanodes were placed outside in the air and absorbed
207
CO2. In Fig. 4b, two peaks at 932.2 eV and 952.1 eV indexed to Cu 2p3/2 and Cu
208
2p1/2[46]. The peaks clearly indicated the existance of Cu2+ and Cu+. During deposition,
209
the possible formation of Cu2S was also a kind of p-type semiconductor, it was the
210
same as CuS and had a narrow band gap about 1.2 eV which could partially constructed
211
with ZnO and formed another heterostructure. The ZnO/Cu2S heterojunction could
212
also be beneficial to the transfer and separation of charge carrier[36]. In Fig. 4c, the
213
peaks located at binding energy of 161.9 eV and 163.1 eV were attributed to S 2p3/2 and
214
S 2p1/2[47]. Meanwhile, Fig. 4d showed the peak at 531.9 eV indexed to O2- in
215
ZnO[48], demonstrating the ZnO was successfully synthesized. As shown in Fig. 4e,
216
two peaks positioned at 1044.9 eV and 1021.8 eV were attributed to Zn 2p1/2 and Zn
217
2p3/2 which had the same effect with O 1s[49]. All these results proved the successful
218
synthesis of ZnO/CuS photoanodes.
219
Fig. 4f showed the UV–Vis reflectance spectra of ZnO/CuS (6, 8, 10, 12 SILAR
220
cycles) photoanodes. The ZnO/CuS photoanodes absorption edges were around 380
221
nm. It is obviously that the optical absorption intensity of ZnO/CuS photoanodes
222
improved in the range of 350–400 nm with the increasing SILAR cycles compared
223
with ZnO photoanode.
224
In order to deeper explore the PEC properties of ZnO/CuS, the photoanodes were
225
investigated in three-electrode cell with 0.5 M Na2SO4 aqueous solution served as
226
electrolyte under AM 1.5G illumination (100mW cm−2). Fig. 5a showed the linear
227
sweep voltammetry (LSV) curves of ZnO/CuS photoanodes. The highest photocurrent
228
density of ZnO/CuS 10 photoanode achieved 1.55 mA cm−2 at 0.37 V vs Ag/AgCl,
229
meanwhile, the ZnO NRs arrays photoanode showed very low current density. In
230
addition, it could be found that the photocurrent density increased with increasing
231
SILAR cycles at first while had a marked decrease along with the SILAR process was
232
increased to 12 times. The phenomenon might be attributed to the overmuch content of
233
CuS could prevent the ZnO NRs arrays from absorbing the ultraviolet light. As shown
234
in Fig. 5b, under dark circumstance, all photoanodes represented relatively low current
235
density, while the ZnO/CuS current density was still larger than that of ZnO.
236
Fig. 5c showed the curves of transient photocurrent response for ZnO/CuS
237
photoanodes. The transient photocurrent response curves of these photoanodes
238
exhibited sharp and repeatable response when the light was turned on or off. The stable
239
photocurrent indicated the fast and steady response of these photoanodes which meant
240
the photoinduced electrons had the efficient separation. The open circuit potential
241
(OCP) curves of different ZnO/CuS samples were also carried out under interval light
242
on and off. A greater OCP indicated the much more band bending and the improvement
243
of the separation efficiency of photogenerated charge carrier which led to the enhanced
244
PEC performance[50]. As shown in Fig. 5d, the greater OCP of different ZnO/CuS
245
samples than ZnO could be observed. The formation of the ZnO/CuS heterostructure
246
may lead to the result that demonstrated the successful preparation and the effect of
247
the ZnO/CuS p-n heterojunction. The stability characterization of ZnO and ZnO/CuS
248
10 was shown in Figure S1.
249
In order to make a further inquiry of charge transport property of the ZnO/CuS
250
photoanodes, electrochemical impedance spectroscopy (EIS) [51] plots were measured
251
at 0.3 V vs Ag/AgCl (Fig. 6a). The radius of the ZnO/CuS photoanodes arcs gradually
252
decreased with the increased SILAR cycles and they were all smaller than that of ZnO,
253
indicating the heterojunction of ZnO/CuS could greatly improve the interfacial charge
254
mobility. The EIS plots indicated that the depositing of CuS nanoclusters could
255
significantly facilitate the separation and transfer of charge carrier, besides, it could
256
decrease the recombination of electrons and holes and enhance the electrical
257
conductivity which indicated the photoinduced electrons could be transferred quickly
258
to FTO via the ZnO NRs arrays. Due to the CuS nanocluster was deposited on the ZnO
259
NRs arrays by SILAR, the data of the Nyquist plots was also analysed by the
260
ZSimpWin to make an equivalent circuit which was shown in the inset of Fig. 6a. The
261
simulated parameters were shown in Table S1. From Table S1, it could be found that
262
the values of Q1 increased with SILAR cycles till 10 but the Q1 of ZnO/CuS 12. The
263
highest Q1 of ZnO/CuS 10 proved it has lower recombination rate of electrons and
264
holes[52,53]. Meanwhile, the values of R2 decreased with SILAR cycles till 10 but the
265
R2 of 12 cycles. The trend was consisted with the LSV characteristics. These results
266
demonstrated ZnO/CuS 10 exhibited better charge transfer performance.
267
Moreover, Mott−Schottky (M−S) plots were carried out to investigate the separation
268
of electrons and holes and discriminate types of semiconductors from −1.5 to 1.5 V vs
269
Ag/AgCl in 0.5 M Na2SO4 aqueous solution[54]. The slope of the Mott−Schottky plots
270
is usually used to distinguish the p-type or n-type semiconductor, p-type semiconductor
271
has a negative slope while n-type semiconductor has a positive slope. It could be seen
272
from the Fig. 6b that ZnO/CuS photoanodes expressed an obviously inverted “U”
273
shape, which indicated the characteristic of p−n heterojunction and demonstrated we
274
have successfully synthesized the ZnO/CuS heterostructure. Moreover, the density
275
values of charge carrier could be calculated according to the following equation
276
=
(3)
277
Where e is the elementary electron charge (1.602 × 10−19 C), ε is the dielectric
278
constant and ε0 is the permittivity in vacuum (8.854 × 10−12 F m−1), and k is the slope
279
of the linear part of M−S plots[13,55]. The flat band potential and carrier density of
280
different photoanodes were listed in Table S2. The calculation showed the carrier
281
density increased after depositing CuS a certain number of cycles till 10 but the
282
ZnO/CuS 12. The results were in agreement with LSV characteristics.
283 284 285
The ABPE is an important means of measuring photoelectrochemical properties. It could be calculated by the following equation[56]:
×.
286
ABPE =
287
Where " is current density, #$%&'( is the power density of the incident light, *+,, is
288
the applied external potential vs. RHE. Fig. 7a showed the calculation results of ABPE.
289
As shown in picture, the ZnO/CuS 10 photoanode attained the greatest ABPE of 0.368%
290
at 0.984 V vs RHE, which was 1.12 and 3.27 times higher than ZnO/CuS 8, ZnO/CuS 6
291
photoanodes, respectively. The increased ABPE results of ZnO/CuS 10 photoanode
292
was due to both of the effect of ZnO/CuS heterostructure and ordered structure of ZnO
293
NRs arrays, indicating the heterostructure led to more efficient charge carrier
294
separation and faster photoinduced electrons migration.
× 100%
(4)
295
Incident photon to current conversion efficiency (IPCE) is one of the most important
296
measurements to investigate the PEC performance. In this work, IPCE curves of
297
ZnO/CuS photoanodes were calculated at wavelengths in the range of 300-600 nm at
298
0.6 V vs Ag/AgCl. The IPCE could be estimated by following equation[57]: /0×
299
IPCE =
300
Where 3 is the wavelength of simulated light, 4$%&'( is the irradiance intensity for a
301
certain wavelength, 5 is the photocurrent density at 0.6 V vs Ag/AgCl for a specific
302
wavelength. Fig. 7b showed the improved IPCE with increasing SILAR cycles of the
1×2
× 100%
(5)
303
ZnO/CuS photoanodes. The ZnO/CuS 10 exhibited the highest IPCE improvement
304
among different samples at 380 nm. However, the IPCE of the ZnO/CuS 12 was lower
305
than ZnO/CuS 10 which consisted with the PEC measurements. The ZnO/CuS 10
306
achieved IPCE of 16.39% at 380 nm, which was 2.8 and 1.31 times higher than
307
ZnO/CuS 6 (IPCE380 nm = 5.85%) and ZnO/CuS 10 (IPCE380 nm = 12.50%). The IPCE
308
curves showed the ZnO/CuS heterostructure could promote transfer and separation
309
efficiency of charge carrier, meanwhile, the results of IPCE were closely related to the
310
SILAR cycles. It was presumably the content of the CuS influenced the charge
311
migration that excessive CuS assembling on the surface of ZnO facilitated
312
recombination, collected electrons and holes and further reduced the separation or the
313
excessive CuS might restrict the absorption of incident light and decrease the
314
generation rate of charge carrier. Thus, the proper CuS SILAR cycles and appropriate
315
content of CuS covering on ZnO/CuS could attain the highest PEC performance. The
316
PEC performance parameters for ZnO and different ZnO/CuS photoanodes were listed
317
in Table S3.
318
As mentioned above, the ZnO/CuS heterostructure photoanode was proved to be a
319
kind of good candidate for photoelectrochemical water splitting. Fig. 8 showed the
320
possible mechanism of the ZnO/CuS photoanodes PEC water oxidation. In photoanode,
321
the photoinduced electrons flowed from conduction band of CuS to ZnO, the ZnO
322
ordered NRs arrays acted as continuous passageway to transport electron to the FTO
323
substrate. Then, the electrons transferred to the Pt electrode through circuit and reacted
324
with the H+ in the electrolyte to generate H2. Meanwhile, the photogenerated holes
325
flowed to CuS valence band to react with water and generate O2. Furthermore, the
326
electrons from the valence band of CuS could react with the dissolved O2 from the air
327
and generate •O2- through photo reduction. According to other reports, the •O2- could
328
further facilitate the O2 generation and lead to the enhancement of PEC performance. In
329
some other kinds of heterojunctions, the similar possible mechanisms also have been
330
recorded[40]. Thus, CuS could be concluded as an oxygen evolution reaction catalyst
331
and promoted the charge separation effectively. With depositing the CuS on the ZnO
332
NRs arrays, positive charge accumulation was relieved and the recombination of
333
photoinduced electron-hole pairs was decreased obviously.
334 335
4. Conclutions
336
In summary, the ZnO/CuS photoanodes were successfully prepared by spin-coating,
337
hydrothermal, and SILAR methods. As shown in the PEC performance measurments,
338
the photocurrent density of ZnO/CuS photoanode was higher than ZnO and the ABPE
339
and IPCE were significantly improved. In all the samples, ZnO/CuS 10 photoanode
340
displayed the greatest photocurrent density of 1.55 mA cm−2 at 0.37 V vs Ag/AgCl and
341
attained the highest ABPE of 0.368% at 0.984 V vs SCE under AM 1.5G illumination
342
in 0.5 M Na2SO4 aqueous solution. All these improvements may be due to the formed
343
p-n heterojunction, depletion layer and the self–built electric field could improve the
344
transfer rate and the separation of charge carrier. The CuS also acted as a light absorber,
345
captured sunlight and generated photoinduced electrons. It not only facilitated the
346
transport of electrons but also improved the light absorption ability of photoanode. The
347
XRD patterns also proved the formed Cu2S and the ZnO/Cu2S heterojunction would
348
promote the PEC performance with ZnO/CuS heterojunction together. Thus, the
349
construction of p-n heterojunction is an effective way to enhance the PEC
350
performance and efficiency of charge separation. Our work demonstrates that the
351
ZnO/CuS photoanode has potential applications in PEC water splitting and
352
photocatalysis. It might be a good candidate for developing hydrogen energy.
353
Acknowledgements
354
This work was supported by Scientific Research Project of Colleges and Universities
355
in Gansu Province (2018D-03); Natural Science Foundation of China (Grant Nos.
356
21575115).
357 358
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552
Figure captions
553
Fig. 1. Synthesis process of the ZnO/CuS photoanodes.
554
Fig. 2. XRD patterns of ZnO and different ZnO/CuS photoanodes.
555
Fig. 3. SEM images for the (a) ZnO, (b) ZnO/CuS photoanode, respectively. (c) TEM,
556
(d), (e) HRTEM of the as prepared ZnO/CuS photoanode.
557
Fig. 4. XPS results of ZnO/CuS heterostructure photoanode: (a) Survey, (b) Cu 2p, (c)
558
S 2p, (d) O 1s, (e) Zn 2p. (f) UV–Vis spectra of the ZnO and different ZnO/CuS
559
photoanodes.
560
Fig. 5. (a) Linear sweep voltammetry (LSV) curves of the ZnO and different
561
ZnO/CuS photoanodes under AM 1.5 G illumination and (b) in the dark in 0.5 M
562
Na2SO4 aqueous solution with a scan rate of 10 mV s-1. (c) Transient photocurrent
563
response of the ZnO and different ZnO/CuS photoanodes under interval light at 0.6V
564
vs Ag/AgCl. (d) Open-circuit potential curves of the ZnO and different ZnO/CuS
565
photoanodes under interval light.
566
Fig. 6. (a) Electrochemical impedance spectroscopy (EIS) plots of the ZnO and
567
different ZnO/CuS photoanodes in the frequency range of 0.1 Hz–100 kHz under AM
568
1.5 G illumination in 0.5 M Na2SO4 electrolyte. (b) Mott-Schottky plots measured at a
569
frequency of 1 kHz
570
Fig. 7. (a) Applied bias photon to current efficiency (ABPE) of the ZnO and different
571
ZnO/CuS photoanodes. (b) Incident photon to current conversion efficiency (IPCE) of
572
different ZnO/CuS photoanodes.
573
Fig. 8. Schematic diagram of band gap structures and PEC reaction mechanism
574 575 576 577 578 579
580
581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616
Figure 1
Figure 2
20
618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634
30
40
50
60
2Theta (degree)
CuS ZnO FTO
(118)
(103)
(110)
ZnO ZnO/CuS 6 ZnO/CuS 8 ZnO/CuS 10 ZnO/CuS 12
(102)
(101)
(100)
Intensity (a.u.)
(103) (002)
617
70
80
635
636 637 638 639 640 641 642 643 644
Figure 3
Figure 4
0
200
400
600
800
Cu+
1000
930
Intensity (a.u.)
2p1/2 2p3/2
165
170
525
530
646 647 648 649 650 651
(f)
Zn 2p
Binding energy (eV)
540
ZnO ZnO/CuS 6 ZnO/CuS 8 ZnO/CuS 10 ZnO/CuS 12
Absorbance (a.u.)
2p1/2
1040
535
Binding energy (eV)
2p3/2
Intensity (a.u.)
(e)
1030
960
O 1s
Binding energy (eV)
1020
950
(d)
S 2p
Intensity (a.u.)
940
Cu2+
Binding energy (eV)
(c)
160
2p1/2
Cu+
Binding energy (eV)
155
Cu 2p
Intensity (a.u.)
O 1s
S 2p
C 1s
Intensity (a.u.)
(b)
2p3/2
Survey Cu 2p
(a)
Zn 2p
645
1050
400
500
Wavelength (nm)
600
1.5
(a)
Current Density (mA/cm2)
Figure 5 Current Density (mA/cm2)
652
ZnO ZnO/CuS 6 ZnO/CuS 8 ZnO/CuS 10 ZnO/CuS 12
1.0
0.5
0.0 -0.2
0.0
0.2
0.6
(b)
ZnO ZnO/CuS 6 ZnO/CuS 8 ZnO/CuS 10 ZnO/CuS 12
0.4
0.2
0.0 -0.2
0.4
0.0
(c)
Open Circuit Potential (V)
Current Density (mA/cm2)
0.25
ZnO ZnO/CuS 6 ZnO/CuS 8 ZnO/CuS 10 ZnO/CuS 12
0.20
0.2
0.4
Potential (V vs. Ag/AgCl)
Potential (V vs. Ag/AgCl)
0.15 0.10 0.05
(d)
ZnO ZnO/CuS 6 ZnO/CuS 8 ZnO/CuS 10 ZnO/CuS 12
0.1
0.0
-0.1
0.00 0
653 654 655 656 657 658 659 660 661 662 663
20
40
60
80
Time (s)
100
120
140
0
20
40
60
80
Time (s)
100
120
140
Figure 6 5000
(a)
4000
16
Q1
R
-Z'' (ohm)
R1 R2
3000
2000 ZnO ZnO/CuS 6 ZnO/CuS 8 ZnO/CuS 10 ZnO/CuS 12
1000
0 0
665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681
1000
2000
(b)
14
Q2
1/C2 (107 cm4/F2）
664
3000
Z' (ohm)
4000
ZnO/CuS 8 ZnO/CuS 10 ZnO/CuS 12
12 10 8 6 4 2
5000
0 0.2
0.4
0.6
0.8
Potential (V vs Ag/AgCl)
1.0
Figure 7 0.4
(a)
ABPE (%)
0.3
20 ZnO ZnO/CuS 6 ZnO/CuS 8 ZnO/CuS 10 ZnO/CuS 12
0.2
0.1
0.0 0.4
683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699
(b)
ZnO/CuS 6 ZnO/CuS 8 ZnO/CuS 10 ZnO/CuS 12
16
IPCE (%)
682
12 8 4 0
0.6
0.8
1.0
Potential (V vs. RHE)
1.2
400
450
500
Wavelength (nm)
550
600
700
701 702 703 704 705
Figure 8
Highlights 1. ZnO/CuS photoanode was successfully prepared by facile method. 2. ZnO/CuS photoanode showed highly PEC performance. 3. The migration of photoinduced electrons was improved. 4. ZnO/CuS and ZnO/Cu2S heterojunctions enhanced the separation of charge carrier.
S1572-6657(19)30814-8
DOI:
https://doi.org/10.1016/j.jelechem.2019.113546
Reference:
JEAC 113546
To appear in:
Journal of Electroanalytical Chemistry
Received Date: 19 July 2019 Revised Date:
6 September 2019
Accepted Date: 1 October 2019
Please cite this article as: H. Guo, C. Su, D. Yu, L. Li, Z. Liu, Z. Han, X. Lu, Facile fabrication of ZnO/CuS heterostructure photoanode with highly PEC performance and excellent charge separation efficiency, Journal of Electroanalytical Chemistry (2019), doi: https://doi.org/10.1016/ j.jelechem.2019.113546. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
1
Facile
fabrication
of
ZnO/CuS
heterostructure
2
photoanode with highly PEC performance and
3
excellent charge separation efficiency
4
Huixia Guoa,∗, Ce Sua, Dongmei Yua, Liangliang Lia, Ziye Liua, Zhengang Hana,
5
Xiaoquan Lua
6
a
7
Province, College of Chemistry & Chemical Engineering, Northwest Normal
8
University, Lanzhou 730070, China
Key Laboratory of Bioelectrochemistry & Environmental Analysis of Gansu
9 10 11 12 13 14 15 16 17 18 19 20 21
∗
Corresponding author at: Key Laboratory of Bioelectrochemistry & Environmental Analysis of Gansu Province,
College of Chemistry & Chemical Engineering, Northwest Normal University, Lanzhou 730070, China (H. Guo). E-mail addresses
[email protected] (H. Guo)
22
Abstract
23
Photoelectrochemical (PEC) water splitting, as a promising route of STH(solar to
24
hydrogen) conversion, has attracted a lot attention recently. However, the rather high
25
recombination and the low transfer rate of photoinduced electron-hole pairs restrict
26
the applications. In this work, n-ZnO/p-CuS photoanodes were fabricated using
27
hydrothermal method and successive ionic layer adsorption and reaction (SILAR)
28
method. The PEC performance of the ZnO photoanode is significantly improved after
29
depositing CuS. The applied bias photon-to-current efficiency (ABPE) of ZnO/CuS 10
30
(the cycles of SILAR) photoanode attained 0.368% at a bias of 0.984 V vs SCE which
31
is 45 times than ZnO. The relatively high value of photocurrent density and ABPE are
32
due to the advantages of the ordered structure of ZnO nanorods arrays and the common
33
effect comes from the formation of heterojunction and faster transfer rate of
34
photogenerated electrons which further facilitate charge carrier separation and the light
35
absorption. This work represents a facile strategy using low cost CuS for PEC water
36
splitting that can be applied in photocatalysis.
37
Keywords: ZnO/CuS heterojunction; SILAR; Photoelectrochemical performance;
38
Photocatalysis
39 40
41
42
1. Introduction
43
With the gradually increasing demand of energy, how to utilize a large number of
44
solar energy becomes an intractable issue to solved[1]. An example of converting
45
sunlight into other kinds of energy method is photoelectrochemical (PEC) water
46
splitting which has attracted considerable attention in past decades. The strategy has
47
the most appealing merit of combining the conversion of solar to hydrogen (STH)
48
with water electrolysis. Besides, the PEC performance was also widely used in
49
biosensor field for detection[2-5]. Photoelectric conversion process has great
50
application potential. Therefore, many researchers made great efforts to use metal
51
oxide semiconductors as photoanode in PEC water splitting and biosensor for the
52
relatively low cost, simple preparation on semiconductor which could be recycled
53
with no pollution to the environment[6-14]. Nevertheless, the water splitting still has
54
lots of problems to be solved, such as: (1) rather low percentage of the light absorption
55
and capture; (2) photocorrosion restricts the application of catalysts; (3) low conversion
56
efficiency of solar to hydrogen energy etc[15]. Thus, fast electron transport, effective
57
charge carrier separation and the evolution of H2 are central points to the PEC
58
performance of semiconductor photoanode[16-18]. However, it is hard for us to find
59
appropriate semiconductors which could match the band structure successfully, only a
60
small part of semiconductor materials have shown superior photocurrent density and
61
PEC performance under AM 1.5G illumination after a large number of studies[19,20].
62
Among various metal oxide materials, the low cost and stable ZnO, highly ordered
63
one-dimensional arrays in particular, is one of the most commonly semiconductors. It
64
has some notable physicochemical characters, for example, high aspect ratio,
65
adjustable morphology and alignment, relatively high excitation binding energy of 60
66
meV at room temperature[21, 22]. These advantages make ZnO as a good candidate for
67
the photocatalysis. However, ZnO has a wide band gap (3.2 eV) which leads to the ZnO
68
can only be used in ultraviolet light[23]. Meanwhile, the relatively rapid recombination
69
rate of charge carrier hinders its performance and application in photocatalysis and
70
water splitting. Therefore, exploring new ways to improve the photocatalytic activity
71
and to further expand the region of the visible light absorption is still an issue to be
72
solved.
73
In the past few decades, a lot of efforts had been made on ZnO. In general, forming
74
heterostructure[24, 25], doping with metal or nonmetal atoms[26-29] and construction
75
of nanostructure[30] are profitable for the transfer of electrons and the separation of
76
charge carrier to solve the above problems. For instance, Kuang et al. constructed two
77
p-n heterojunctions in CdS/Cu2O/ZnO photoanode and reduced the recombination of
78
charge carrier, the ABPE of the sample was 261 times greater than that of the ZnO NRs
79
arrays[31]. Kar et al. doped C, N, and S in ZnO NRs arrays photoanodes which
80
exhibited enhanced efficiency of visible light absorption and charge carrier separation
81
significantly[32]. Hsu et al. synthesized ZnO/Fe2O3 core–shell NWs for PEC water
82
electrolysis, the photocurrent increased significantly compared to pristine ZnO
83
NWs[33].
84
Recently, copper sulfide (CuS) has attracted extensive interest attributed to its
85
application in photocatalysis, photoelectrochemical hydrogen production, solar cells
86
etc. CuS has a narrow band gap about 2.1 eV that matches the visible light
87
spectrum[34-38]. Meanwhile, it could be constructed with other n-type semiconductor
88
and form heterostructure that provides large surface area and more active sites to
89
facilitate light absorption and photoinduced electron supply[39,40]. Above all, it is a
90
good way to fabricate ZnO/CuS heterostructure and play different roles for improving
91
the mobility of electrons and holes and extending the diffusion length of carrier. In the
92
reported literatures, the composite of CuS/ZnO heterojunction was fabricated by
93
two-step wet-chemical method on stainless steel mesh and used to degrade the dyes
94
under visible light and ultrasonic irradiation due to the preferable photocatalytic
95
activity and visible light utilization[41]. CuS/ZnO nanocomposite was also reported to
96
be used in methylene blue degradation owing to the ZnO/CuS junctions improved the
97
photocatalytic activity of ZnO and the separation of photoinduced carriers by
98
mechanical lapping method[42] and a wet-chemical method at low temperature[43].
99
In this work, we used different ways, combined the spin-coating and hydrothermal
100
method to synthesize ZnO NRs arrays on FTO, and the CuS was constructed by
101
successive ionic layer absorption and reaction (SILAR). The similar approaches were
102
reported in the construction of ZnO@CdS heterostructure[44] and CuS/ZnO
103
heterostructure nanowire arrays[45], both of the heterostructures represented
104
preferable PEC performance by electrodeposition and SILAR which demonstrated
105
SILAR was a feasible low cost method. In contrast, we use physical coating and
106
one-pot reaction to substitude electrodeposition method mentioned above. Attributed
107
to appropriate band and ZnO/CuS heterojunction, the charge carrier acquired fast
108
generation, rapid transfer rate and avoided partial recombination, the ZnO acted as a
109
pathway and transferred the photoinduced electrons continuously. Simultaneously, our
110
work also proved some Cu2S was generated which could also integrated with ZnO and
111
formed ZnO/Cu2S heterojunction. Both of the hetrojunctions promoted charge
112
migration and separation of electrons and holes. Consequently, all of the results below
113
exhibited ZnO/CuS heterostructure photoanodes were constructed successfully and had
114
a better PEC performance than ZnO. Therefore, ZnO/CuS photoanode might have
115
potential application in the photo electrochemical water splitting.
116 117
2. Experimental Section
118
2.1 ZnO seed layer synthesis
119
Fig. 1 shows the synthesis process of the ZnO/CuS heterostructure photoanodes.
120
Prior to the synthesis, the FTO substrates were thoroughly cleaned by ultrasonication in
121
DI water, acetone, and isopropanol for 15 min, respectively. Later, after FTO substrates
122
were dried in air, the seed layer sol-gel of ZnO was distilled on the surface of FTO
123
substrate by spin coating (rpm 3000, for 30 s). The ZnO sol-gel was made through 0.75
124
M solution of zinc acetate mixed by 10 mL 2-methoxyethanol and the same amount of
125
substance ethanolamine followed by stirring at 60 °C for 2 h and the solution was kept
126
stirring continuously in air at 20 °C for 12 h to get ZnO sol-gel. The sol-gel was
127
distilled on the FTO substrate by 100 µL each sample. The spin coating process was
128
repeated five times to ensure the adequate thickness of ZnO. The samples were dried
129
at 200 °C for 5 min after each process. Finally, the samples were subjected to heat
130
treatment at 400 °C, 3 °C min-1 for 1 h.
131 132
2.2 ZnO NRs arrays growth
133
Afterwards, the ZnO NRs arrays were grown by hydrothermal method. The sample
134
with ZnO seed layer was put against the wall of the Teflon-lined stainless steel
135
autoclave (25 mL). It is worth noting that the conductive surface of FTO should be
136
facing down. The solution of 20 mM zinc acetate and 20 mM hexamethylenetetramine
137
mixed into 80 mL DI water was transferred into the autoclaves. Hydrothermal synthesis
138
was conducted at 90 °C for 4 h followed by cleaning with DI water and alcohol for
139
several times. Then, the samples were put in furnace at 400 °C, 2 °C min-1 for 2 h.
140 141
2.3 Synthesis of ZnO/CuS heterostructure
142
CuS nanoclusters were deposited on ZnO NRs arrays by SILAR method. 5 mM
143
CuCl2 aqueous solution and 5 mM Na2S aqueous solution were used in the deposition
144
on the as-prepared ZnO NRs arrays samples at room temperature. First, the ZnO NRs
145
arrays samples were immersed into CuCl2 aqueous solution for 60 s. Secondly, this
146
sample was immersed in the DI water for 60 s to prevent precipitation and remove
147
loosen ions. Thirdly, the sample was immersed in the Na2S aqueous solution for 60 s,
148
S2− reacted with the deposited Cu+ on the surface of ZnO NRs arrays. In the end, the
149
sample was immersed in the DI water for 60 s. This is the first cycle of CuS deposition,
150
the amount of CuS could be increased by repeating the SILAR cycles. The immersion
151
procedures were repeated for 6, 8, 10, 12 cycles and these samples were named in
152
sequence as ZnO/CuS X (X is the cycles of SILAR process). The following chemical
153
reactions could take place during the SILAR cycle:
154
Cu2+ + S2-→Cu2S (nanoclusters)
(1)
155 156
2.4 Structural and optical characterizations
157
The element species, chemical state and composition of photoanodes were measured
158
by X-ray photoelectron spectroscopy (XPS) with Kratos Axis Ultra DLD. The X-ray
159
diffraction (XRD) patterns were characterized with the diffraction angle 2θ = 20° to 80°
160
(Rigaku D/max 2400 diffraction meter with Cu target Kα, λ=1.54Å). Surface
161
morphology was characterized by scanning electron microscopy (SEM, ULTRA plus,
162
Zeiss). The microstructure was measured by Transmission electron microscope (TEM,
163
FEI Tecnai G2 F20). The light absorption of samples was measured by UV–Vis
164
spectroscopy (UV-2600, Shimadzu).
165 166
2.5. PEC measurements
167
The PEC performance of the photoanodes was measured using the electrochemical
168
workstation (CHI 660E, China Chenhua) with a three-electrode system using a
169
ZnO/CuS work electrode, an Ag/AgCl reference electrode and a Pt counter electrode in
170
0.5 M Na2SO4 aqueous solution (pH = 7.35) under AM 1.5G irradiation (100 mW
171
cm−2,94011A-ES, Newport Oriel). Photocurrent density curves were investigated at a
172
scanning rate of 0.01 V s−1 from −0.6 to 1.2 V vs Ag/AgCl. In Fig. 7a, the measured
173
potential is converted into the reversible hydrogen electrode (RHE) according to Nernst
174
equation:
175
ERHE=EAg/AgCl+0.1976V+0.059*pH.
(2)
176
IPCE was measured under a monochromator coupled with a 1000 W Xe lamp. The
177
electrochemical impedance spectroscopies (EIS) curves were measured with an AC
178
voltage of 5 mV amplitude in the frequency range from 0.01 Hz to 100 kHz. The
179
Mott-Schottky curves were carried out from −0.5 to 1.5 V vs Ag/AgCl. Both of EIS and
180
Mott-Schottky curves were obtained using the electrochemical workstation with the
181
same three-electrode system as LSV curves.
182
3. Results and discussions
183
Fig. 1 shows the synthesis process of the ZnO/CuS heterostructure photoanodes. The
184
XRD patterns of ZnO and different ZnO/CuS photoanodes were shown in Fig. 2. For all
185
samples, the diffraction peaks at 31.9°, 34.4°, 36.3°, 47.5° and 62.9° were assigned to
186
the (100), (002), (101), (102) and (103) of ZnO crystal (JCPDS Card No. 36-1451),
187
respectively. The peaks at 31.8°, 47.8° and 67.3° were attributed to the (103), (110) and
188
(118) of CuS cystal (JCPDS Card No. 06-0464). The ZnO (100) peak was so close to
189
the CuS (103) peak that they overlapped in the vicinity of 31.9°.
190
The SEM images were shown in Fig. 3a-b. It could be seen the ZnO showed the
191
nanorods arrays, all of them appeared hexagonal prism shape and directly growed on
192
FTO with average size of 100-280 nm. Moreover, it could be found that the surface of
193
ZnO nanorods arrays were smooth without other impurities, while in Fig. 3b, the
194
ZnO/CuS photoanode could be clearly observed almost 5~50 nm granule nanoclusters
195
were attached to the ZnO nanorods arrays. The TEM image of ZnO/CuS was shown in
196
Fig. 3c, ZnO NRs and the attached CuS nanoclusters were clearly obseverd which
197
indicated the diameter of the ZnO NRs was about 120 nm and the diameter of the CuS
198
nanoclusters was about 50 nm. Fig. 3d-e showed the HRTEM images, the CuS
199
nanoclusters on the surface of ZnO NRs indicated the lattice spacing was 0.190 nm,
200
corresponding to the (107) crystal plane of CuS. Fig. 3e showed the crystal structure of
201
ZnO NRs arrays which indicated the lattice spacing was 0.28 nm, corresponding to the
202
(100) crystal plane of ZnO.
203
To further explore chemical state and composition of the photoanodes, the XPS was
204
analyzed. Fig. 4a showed the XPS survey spectrum of the ZnO/CuS photoanode, the
205
peaks of S 2p, C 1s, O 1s, Cu 2p, Zn 2p were signed distinctly. The appearance of the
206
peak of C 1s might due to the photoanodes were placed outside in the air and absorbed
207
CO2. In Fig. 4b, two peaks at 932.2 eV and 952.1 eV indexed to Cu 2p3/2 and Cu
208
2p1/2[46]. The peaks clearly indicated the existance of Cu2+ and Cu+. During deposition,
209
the possible formation of Cu2S was also a kind of p-type semiconductor, it was the
210
same as CuS and had a narrow band gap about 1.2 eV which could partially constructed
211
with ZnO and formed another heterostructure. The ZnO/Cu2S heterojunction could
212
also be beneficial to the transfer and separation of charge carrier[36]. In Fig. 4c, the
213
peaks located at binding energy of 161.9 eV and 163.1 eV were attributed to S 2p3/2 and
214
S 2p1/2[47]. Meanwhile, Fig. 4d showed the peak at 531.9 eV indexed to O2- in
215
ZnO[48], demonstrating the ZnO was successfully synthesized. As shown in Fig. 4e,
216
two peaks positioned at 1044.9 eV and 1021.8 eV were attributed to Zn 2p1/2 and Zn
217
2p3/2 which had the same effect with O 1s[49]. All these results proved the successful
218
synthesis of ZnO/CuS photoanodes.
219
Fig. 4f showed the UV–Vis reflectance spectra of ZnO/CuS (6, 8, 10, 12 SILAR
220
cycles) photoanodes. The ZnO/CuS photoanodes absorption edges were around 380
221
nm. It is obviously that the optical absorption intensity of ZnO/CuS photoanodes
222
improved in the range of 350–400 nm with the increasing SILAR cycles compared
223
with ZnO photoanode.
224
In order to deeper explore the PEC properties of ZnO/CuS, the photoanodes were
225
investigated in three-electrode cell with 0.5 M Na2SO4 aqueous solution served as
226
electrolyte under AM 1.5G illumination (100mW cm−2). Fig. 5a showed the linear
227
sweep voltammetry (LSV) curves of ZnO/CuS photoanodes. The highest photocurrent
228
density of ZnO/CuS 10 photoanode achieved 1.55 mA cm−2 at 0.37 V vs Ag/AgCl,
229
meanwhile, the ZnO NRs arrays photoanode showed very low current density. In
230
addition, it could be found that the photocurrent density increased with increasing
231
SILAR cycles at first while had a marked decrease along with the SILAR process was
232
increased to 12 times. The phenomenon might be attributed to the overmuch content of
233
CuS could prevent the ZnO NRs arrays from absorbing the ultraviolet light. As shown
234
in Fig. 5b, under dark circumstance, all photoanodes represented relatively low current
235
density, while the ZnO/CuS current density was still larger than that of ZnO.
236
Fig. 5c showed the curves of transient photocurrent response for ZnO/CuS
237
photoanodes. The transient photocurrent response curves of these photoanodes
238
exhibited sharp and repeatable response when the light was turned on or off. The stable
239
photocurrent indicated the fast and steady response of these photoanodes which meant
240
the photoinduced electrons had the efficient separation. The open circuit potential
241
(OCP) curves of different ZnO/CuS samples were also carried out under interval light
242
on and off. A greater OCP indicated the much more band bending and the improvement
243
of the separation efficiency of photogenerated charge carrier which led to the enhanced
244
PEC performance[50]. As shown in Fig. 5d, the greater OCP of different ZnO/CuS
245
samples than ZnO could be observed. The formation of the ZnO/CuS heterostructure
246
may lead to the result that demonstrated the successful preparation and the effect of
247
the ZnO/CuS p-n heterojunction. The stability characterization of ZnO and ZnO/CuS
248
10 was shown in Figure S1.
249
In order to make a further inquiry of charge transport property of the ZnO/CuS
250
photoanodes, electrochemical impedance spectroscopy (EIS) [51] plots were measured
251
at 0.3 V vs Ag/AgCl (Fig. 6a). The radius of the ZnO/CuS photoanodes arcs gradually
252
decreased with the increased SILAR cycles and they were all smaller than that of ZnO,
253
indicating the heterojunction of ZnO/CuS could greatly improve the interfacial charge
254
mobility. The EIS plots indicated that the depositing of CuS nanoclusters could
255
significantly facilitate the separation and transfer of charge carrier, besides, it could
256
decrease the recombination of electrons and holes and enhance the electrical
257
conductivity which indicated the photoinduced electrons could be transferred quickly
258
to FTO via the ZnO NRs arrays. Due to the CuS nanocluster was deposited on the ZnO
259
NRs arrays by SILAR, the data of the Nyquist plots was also analysed by the
260
ZSimpWin to make an equivalent circuit which was shown in the inset of Fig. 6a. The
261
simulated parameters were shown in Table S1. From Table S1, it could be found that
262
the values of Q1 increased with SILAR cycles till 10 but the Q1 of ZnO/CuS 12. The
263
highest Q1 of ZnO/CuS 10 proved it has lower recombination rate of electrons and
264
holes[52,53]. Meanwhile, the values of R2 decreased with SILAR cycles till 10 but the
265
R2 of 12 cycles. The trend was consisted with the LSV characteristics. These results
266
demonstrated ZnO/CuS 10 exhibited better charge transfer performance.
267
Moreover, Mott−Schottky (M−S) plots were carried out to investigate the separation
268
of electrons and holes and discriminate types of semiconductors from −1.5 to 1.5 V vs
269
Ag/AgCl in 0.5 M Na2SO4 aqueous solution[54]. The slope of the Mott−Schottky plots
270
is usually used to distinguish the p-type or n-type semiconductor, p-type semiconductor
271
has a negative slope while n-type semiconductor has a positive slope. It could be seen
272
from the Fig. 6b that ZnO/CuS photoanodes expressed an obviously inverted “U”
273
shape, which indicated the characteristic of p−n heterojunction and demonstrated we
274
have successfully synthesized the ZnO/CuS heterostructure. Moreover, the density
275
values of charge carrier could be calculated according to the following equation
276
=
(3)
277
Where e is the elementary electron charge (1.602 × 10−19 C), ε is the dielectric
278
constant and ε0 is the permittivity in vacuum (8.854 × 10−12 F m−1), and k is the slope
279
of the linear part of M−S plots[13,55]. The flat band potential and carrier density of
280
different photoanodes were listed in Table S2. The calculation showed the carrier
281
density increased after depositing CuS a certain number of cycles till 10 but the
282
ZnO/CuS 12. The results were in agreement with LSV characteristics.
283 284 285
The ABPE is an important means of measuring photoelectrochemical properties. It could be calculated by the following equation[56]:
×.
286
ABPE =
287
Where " is current density, #$%&'( is the power density of the incident light, *+,, is
288
the applied external potential vs. RHE. Fig. 7a showed the calculation results of ABPE.
289
As shown in picture, the ZnO/CuS 10 photoanode attained the greatest ABPE of 0.368%
290
at 0.984 V vs RHE, which was 1.12 and 3.27 times higher than ZnO/CuS 8, ZnO/CuS 6
291
photoanodes, respectively. The increased ABPE results of ZnO/CuS 10 photoanode
292
was due to both of the effect of ZnO/CuS heterostructure and ordered structure of ZnO
293
NRs arrays, indicating the heterostructure led to more efficient charge carrier
294
separation and faster photoinduced electrons migration.
× 100%
(4)
295
Incident photon to current conversion efficiency (IPCE) is one of the most important
296
measurements to investigate the PEC performance. In this work, IPCE curves of
297
ZnO/CuS photoanodes were calculated at wavelengths in the range of 300-600 nm at
298
0.6 V vs Ag/AgCl. The IPCE could be estimated by following equation[57]: /0×
299
IPCE =
300
Where 3 is the wavelength of simulated light, 4$%&'( is the irradiance intensity for a
301
certain wavelength, 5 is the photocurrent density at 0.6 V vs Ag/AgCl for a specific
302
wavelength. Fig. 7b showed the improved IPCE with increasing SILAR cycles of the
1×2
× 100%
(5)
303
ZnO/CuS photoanodes. The ZnO/CuS 10 exhibited the highest IPCE improvement
304
among different samples at 380 nm. However, the IPCE of the ZnO/CuS 12 was lower
305
than ZnO/CuS 10 which consisted with the PEC measurements. The ZnO/CuS 10
306
achieved IPCE of 16.39% at 380 nm, which was 2.8 and 1.31 times higher than
307
ZnO/CuS 6 (IPCE380 nm = 5.85%) and ZnO/CuS 10 (IPCE380 nm = 12.50%). The IPCE
308
curves showed the ZnO/CuS heterostructure could promote transfer and separation
309
efficiency of charge carrier, meanwhile, the results of IPCE were closely related to the
310
SILAR cycles. It was presumably the content of the CuS influenced the charge
311
migration that excessive CuS assembling on the surface of ZnO facilitated
312
recombination, collected electrons and holes and further reduced the separation or the
313
excessive CuS might restrict the absorption of incident light and decrease the
314
generation rate of charge carrier. Thus, the proper CuS SILAR cycles and appropriate
315
content of CuS covering on ZnO/CuS could attain the highest PEC performance. The
316
PEC performance parameters for ZnO and different ZnO/CuS photoanodes were listed
317
in Table S3.
318
As mentioned above, the ZnO/CuS heterostructure photoanode was proved to be a
319
kind of good candidate for photoelectrochemical water splitting. Fig. 8 showed the
320
possible mechanism of the ZnO/CuS photoanodes PEC water oxidation. In photoanode,
321
the photoinduced electrons flowed from conduction band of CuS to ZnO, the ZnO
322
ordered NRs arrays acted as continuous passageway to transport electron to the FTO
323
substrate. Then, the electrons transferred to the Pt electrode through circuit and reacted
324
with the H+ in the electrolyte to generate H2. Meanwhile, the photogenerated holes
325
flowed to CuS valence band to react with water and generate O2. Furthermore, the
326
electrons from the valence band of CuS could react with the dissolved O2 from the air
327
and generate •O2- through photo reduction. According to other reports, the •O2- could
328
further facilitate the O2 generation and lead to the enhancement of PEC performance. In
329
some other kinds of heterojunctions, the similar possible mechanisms also have been
330
recorded[40]. Thus, CuS could be concluded as an oxygen evolution reaction catalyst
331
and promoted the charge separation effectively. With depositing the CuS on the ZnO
332
NRs arrays, positive charge accumulation was relieved and the recombination of
333
photoinduced electron-hole pairs was decreased obviously.
334 335
4. Conclutions
336
In summary, the ZnO/CuS photoanodes were successfully prepared by spin-coating,
337
hydrothermal, and SILAR methods. As shown in the PEC performance measurments,
338
the photocurrent density of ZnO/CuS photoanode was higher than ZnO and the ABPE
339
and IPCE were significantly improved. In all the samples, ZnO/CuS 10 photoanode
340
displayed the greatest photocurrent density of 1.55 mA cm−2 at 0.37 V vs Ag/AgCl and
341
attained the highest ABPE of 0.368% at 0.984 V vs SCE under AM 1.5G illumination
342
in 0.5 M Na2SO4 aqueous solution. All these improvements may be due to the formed
343
p-n heterojunction, depletion layer and the self–built electric field could improve the
344
transfer rate and the separation of charge carrier. The CuS also acted as a light absorber,
345
captured sunlight and generated photoinduced electrons. It not only facilitated the
346
transport of electrons but also improved the light absorption ability of photoanode. The
347
XRD patterns also proved the formed Cu2S and the ZnO/Cu2S heterojunction would
348
promote the PEC performance with ZnO/CuS heterojunction together. Thus, the
349
construction of p-n heterojunction is an effective way to enhance the PEC
350
performance and efficiency of charge separation. Our work demonstrates that the
351
ZnO/CuS photoanode has potential applications in PEC water splitting and
352
photocatalysis. It might be a good candidate for developing hydrogen energy.
353
Acknowledgements
354
This work was supported by Scientific Research Project of Colleges and Universities
355
in Gansu Province (2018D-03); Natural Science Foundation of China (Grant Nos.
356
21575115).
357 358
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537 538 539 540 541 542 543 544 545 546 547 548 549 550 551
552
Figure captions
553
Fig. 1. Synthesis process of the ZnO/CuS photoanodes.
554
Fig. 2. XRD patterns of ZnO and different ZnO/CuS photoanodes.
555
Fig. 3. SEM images for the (a) ZnO, (b) ZnO/CuS photoanode, respectively. (c) TEM,
556
(d), (e) HRTEM of the as prepared ZnO/CuS photoanode.
557
Fig. 4. XPS results of ZnO/CuS heterostructure photoanode: (a) Survey, (b) Cu 2p, (c)
558
S 2p, (d) O 1s, (e) Zn 2p. (f) UV–Vis spectra of the ZnO and different ZnO/CuS
559
photoanodes.
560
Fig. 5. (a) Linear sweep voltammetry (LSV) curves of the ZnO and different
561
ZnO/CuS photoanodes under AM 1.5 G illumination and (b) in the dark in 0.5 M
562
Na2SO4 aqueous solution with a scan rate of 10 mV s-1. (c) Transient photocurrent
563
response of the ZnO and different ZnO/CuS photoanodes under interval light at 0.6V
564
vs Ag/AgCl. (d) Open-circuit potential curves of the ZnO and different ZnO/CuS
565
photoanodes under interval light.
566
Fig. 6. (a) Electrochemical impedance spectroscopy (EIS) plots of the ZnO and
567
different ZnO/CuS photoanodes in the frequency range of 0.1 Hz–100 kHz under AM
568
1.5 G illumination in 0.5 M Na2SO4 electrolyte. (b) Mott-Schottky plots measured at a
569
frequency of 1 kHz
570
Fig. 7. (a) Applied bias photon to current efficiency (ABPE) of the ZnO and different
571
ZnO/CuS photoanodes. (b) Incident photon to current conversion efficiency (IPCE) of
572
different ZnO/CuS photoanodes.
573
Fig. 8. Schematic diagram of band gap structures and PEC reaction mechanism
574 575 576 577 578 579
580
581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616
Figure 1
Figure 2
20
618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634
30
40
50
60
2Theta (degree)
CuS ZnO FTO
(118)
(103)
(110)
ZnO ZnO/CuS 6 ZnO/CuS 8 ZnO/CuS 10 ZnO/CuS 12
(102)
(101)
(100)
Intensity (a.u.)
(103) (002)
617
70
80
635
636 637 638 639 640 641 642 643 644
Figure 3
Figure 4
0
200
400
600
800
Cu+
1000
930
Intensity (a.u.)
2p1/2 2p3/2
165
170
525
530
646 647 648 649 650 651
(f)
Zn 2p
Binding energy (eV)
540
ZnO ZnO/CuS 6 ZnO/CuS 8 ZnO/CuS 10 ZnO/CuS 12
Absorbance (a.u.)
2p1/2
1040
535
Binding energy (eV)
2p3/2
Intensity (a.u.)
(e)
1030
960
O 1s
Binding energy (eV)
1020
950
(d)
S 2p
Intensity (a.u.)
940
Cu2+
Binding energy (eV)
(c)
160
2p1/2
Cu+
Binding energy (eV)
155
Cu 2p
Intensity (a.u.)
O 1s
S 2p
C 1s
Intensity (a.u.)
(b)
2p3/2
Survey Cu 2p
(a)
Zn 2p
645
1050
400
500
Wavelength (nm)
600
1.5
(a)
Current Density (mA/cm2)
Figure 5 Current Density (mA/cm2)
652
ZnO ZnO/CuS 6 ZnO/CuS 8 ZnO/CuS 10 ZnO/CuS 12
1.0
0.5
0.0 -0.2
0.0
0.2
0.6
(b)
ZnO ZnO/CuS 6 ZnO/CuS 8 ZnO/CuS 10 ZnO/CuS 12
0.4
0.2
0.0 -0.2
0.4
0.0
(c)
Open Circuit Potential (V)
Current Density (mA/cm2)
0.25
ZnO ZnO/CuS 6 ZnO/CuS 8 ZnO/CuS 10 ZnO/CuS 12
0.20
0.2
0.4
Potential (V vs. Ag/AgCl)
Potential (V vs. Ag/AgCl)
0.15 0.10 0.05
(d)
ZnO ZnO/CuS 6 ZnO/CuS 8 ZnO/CuS 10 ZnO/CuS 12
0.1
0.0
-0.1
0.00 0
653 654 655 656 657 658 659 660 661 662 663
20
40
60
80
Time (s)
100
120
140
0
20
40
60
80
Time (s)
100
120
140
Figure 6 5000
(a)
4000
16
Q1
R
-Z'' (ohm)
R1 R2
3000
2000 ZnO ZnO/CuS 6 ZnO/CuS 8 ZnO/CuS 10 ZnO/CuS 12
1000
0 0
665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681
1000
2000
(b)
14
Q2
1/C2 (107 cm4/F2）
664
3000
Z' (ohm)
4000
ZnO/CuS 8 ZnO/CuS 10 ZnO/CuS 12
12 10 8 6 4 2
5000
0 0.2
0.4
0.6
0.8
Potential (V vs Ag/AgCl)
1.0
Figure 7 0.4
(a)
ABPE (%)
0.3
20 ZnO ZnO/CuS 6 ZnO/CuS 8 ZnO/CuS 10 ZnO/CuS 12
0.2
0.1
0.0 0.4
683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699
(b)
ZnO/CuS 6 ZnO/CuS 8 ZnO/CuS 10 ZnO/CuS 12
16
IPCE (%)
682
12 8 4 0
0.6
0.8
1.0
Potential (V vs. RHE)
1.2
400
450
500
Wavelength (nm)
550
600
700
701 702 703 704 705
Figure 8
Highlights 1. ZnO/CuS photoanode was successfully prepared by facile method. 2. ZnO/CuS photoanode showed highly PEC performance. 3. The migration of photoinduced electrons was improved. 4. ZnO/CuS and ZnO/Cu2S heterojunctions enhanced the separation of charge carrier.