Accepted Manuscript MOF-derived C-doped ZnO composites for enhanced photocatalytic performance under visible light Ying Zhang, Jiabin Zhou, Xin Chen, Qinqin Feng, Weiquan Cai PII:
S0925-8388(18)34086-6
DOI:
https://doi.org/10.1016/j.jallcom.2018.10.383
Reference:
JALCOM 48201
To appear in:
Journal of Alloys and Compounds
Received Date: 22 June 2018 Revised Date:
28 October 2018
Accepted Date: 29 October 2018
Please cite this article as: Y. Zhang, J. Zhou, X. Chen, Q. Feng, W. Cai, MOF-derived C-doped ZnO composites for enhanced photocatalytic performance under visible light, Journal of Alloys and Compounds (2018), doi: https://doi.org/10.1016/j.jallcom.2018.10.383. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
ACCEPTED MANUSCRIPT 1
MOF-derived C-doped ZnO composites for enhanced photocatalytic
2
performance under visible light
3
5 a
6
School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, China
School of Resources and Environmental Engineering, Wuhan University of
M AN U
b
SC
7 8
Technology, Wuhan 430070, China
9 10
RI PT
Ying Zhang a, b, Jiabin Zhou a,b,*, Xin Chen b, Qinqin Feng b, Weiquan Cai c, *
4
c
School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006, PR China
11
TE D
12
*Corresponding author. Tel: +86-28-83037306
14
E-mail:
[email protected] (J. Zhou);
[email protected] (W. Cai)
16
AC C
15
EP
13
1
ACCEPTED MANUSCRIPT ABSTRACT
18
Carbon-doped zinc oxide (ZnO) with porous structure was synthesized by pyrolysis of
19
a zinc-based metal organic framework (MOFs). Photocatalytic effect was measured
20
by the photodegradation of Rhodamine B (RhB) under visible light irradiation. The
21
morphology, structure, and porous properties of the as-synthesized composites were
22
characterized by using field emission scanning electron microscopy (SEM), X-ray
23
diffraction (XRD), X-ray photoelectron spectroscopy (XPS), the thermo gravimetric
24
and differential scanning calorimetry analysis (TG-DSC), diffuse reflectance UV-vis
25
spectroscopy (UV-vis DRS), photoluminescence (PL) and N2 sorption-desorption
26
isotherms (BET). Compared with other conventional C-doping methods, MOF
27
sacrificial template method not only retains the porous structure with interconnected
28
ZnO nanoparticles but also introduces carbon doping evenly in ZnO lattice which
29
reduces the band gap of ZnO and thus improves the charge-separation efficiency. The
30
trapping experiment results showed that superoxide radicals (•O2-) and photoexcited
31
hole (h+) are the main and minor oxidative species in the photodegradation of RhB
32
respectively. And the enhanced photocatalytic mechanism was also proposed.
33
Keywords: Photocatalysis; ZnO; MOF-5; Carbon doping; RhB
AC C
EP
TE D
M AN U
SC
RI PT
17
34
2
ACCEPTED MANUSCRIPT 35
1. Introduction With the increase of serious water pollution issues around worldwide over the
37
past decades; photocatalytic technology based on semiconductor has attracted more
38
and more attention because of its high efficiency and low toxicity [1-3]. Up to date, a
39
large variety of semiconductor photocatalysts have been reported for solar energy
40
conversion and a treatment for organic pollutants (e.g., TiO2 [4], g-C3N4 [5], ZnO [6]
41
and BiOCl [7]). Among these semiconductors, ZnO have received considerable
42
scientific interest as an alternative to TiO2 due to its environmentally friendly, stability
43
and high catalytic efficiency [8]. However, ZnO still has many shortcomings in
44
practical applications, such as low visible light utilization (due to the wide band gap
45
3.27 eV), rapid recombination of photogenerated electron-holes and low specific
46
surface area. Many efforts have been made to overcome these weaknesses, such as
47
semiconductor coupling [9], metal or non-metal doping [10, 11], self-assembly [12]
48
and template method [13]. Among these methods, the template method of metal
49
organic frameworks (MOFs) derived C-doping ZnO can not only solve the problem of
50
low visible light utilization and rapid charge recombination but also the problem of
51
low specific surface area. C doping can introduce oxygen vacancy into the band gap
52
and increase the electron density of Fermi level leading to the efficient separation and
53
transportation of charges [12]. The porous structure with high specific surface area
54
can not only facilitate more pollutant molecules absorbed on the active sites of
55
photocatalysts and increase the light transmittance, but also can enable rapid transfer
56
of photogenerated charge carriers onto the surface of photocatalysts, promoting bulk
AC C
EP
TE D
M AN U
SC
RI PT
36
3
ACCEPTED MANUSCRIPT 57
charge separation [14]. Metal organic frameworks (MOFs) as hybrid organic-inorganic compounds, are
59
high porous materials synthesized through the coordination of metallic ions and
60
organic ligands [15]. Due to the remarkable characteristics of porous structure and
61
tunable pore size and shape, MOFs have received extensive attention in recent years
62
as a new type of high surface area and porous materials in catalysis [16], sensing [17],
63
storage/separation [18] and supercapacitor [19]. As built from metal ions and organic
64
ligands, MOFs have been used as templates for synthesis of carbon and metal oxides
65
materials [20, 21]. Using MOFs as precursors has many advantages over traditional
66
C-doping methods [22]. The MOFs sacrifice themselves to form the uniform elements
67
distribution and the porous structure of MOFs can be remained under certain
68
conditions. As one of the most robust porosity MOF structure, MOF-5 which uses
69
terephthalic acid and Zn ions as the organic ligands and the metallic sites can be
70
employed as carbon and zinc sources to synthesize C doped ZnO without extra
71
functional precursors or post-synthesis treatment. Song, et. al. fabricated hollow
72
porous ZnO/C nanocages by a one-step pyrolysis of hollow MOF-5 at 500 °C in N2
73
atmosphere to increase lithium ion batteries’ storage performance and rate capability
74
[13]. However, to the best of our knowledge, there is few report about the C doped
75
ZnO by pyrolysis of MOF-5 for visible light photodegradation of organic pollutant in
76
water.
AC C
EP
TE D
M AN U
SC
RI PT
58
77
Herein, a well-defined octahedral carbon doped ZnO hybrids with high specific
78
surface areas (>800 m2/g) using MOF-5 as the template precursor were synthesized 4
ACCEPTED MANUSCRIPT and exhibited high photocatalytic degradation of Rhodamine B (RhB) under visible
80
light irradiation. Subsequently, the C@ZnO shows excellent porous structure, optical
81
absorption, charge separation and mass transfer, and thus significantly high activity in
82
photocatalytic degradation of RhB under visible light irradiation.
RI PT
79
83
2. Experimental
85
2.1 Chemicals
SC
84
Zinc nitrate hexahydrate (Zn(NO3)2·6H2O), terephthalic acid (H2BDC), N, N
87
dimethyl-formamide (DMF), ethylene glycol, ethanol, methanol, isopropanol,
88
benzoquinone, rhodamine B (RhB), commercial ZnO were purchased from
89
Sinopharm Co. Ltd. All chemicals and reagents used were in analytical grade without
90
any further purification.
91
2.2 Sample preparation
TE D
92
M AN U
86
2.2.1 Preparation of MOF-5
MOF-5 were fabricated by a solvothermal method. The preparation processes
94
were as followed: 0.4 g Zn(NO3)2·6H2O and 0.2 g terephthalic acid (H2BDC) were
95
dissolved in 20 mL ethylene glycol and then 32 mL N, N-dimethyl formamide (DMF)
96
were added into the mixture. After that the mixture were stirred for 1 h, transferred to
97
a Teflon-lined stainless steel reactor and heated at 150 °C for 6 h. The products were
98
washed with DMF and methanol for several times respectively. Finally, the products
99
were dried in oven at 80 °C overnight. The resulted sample is MOF-5.
100
AC C
EP
93
2.2.2 Preparation of C@ZnO hybrids 5
ACCEPTED MANUSCRIPT The above-prepared MOF-5 was taken into a muffle furnace and was heated at
102
350 to 500 °C with a heating rate of 5 °C/min and held for 1 to 4 h under air. The
103
C@ZnO hybrids prepared by aforesaid method were named as 350-3h, 400-3h,
104
450-1h, 450-2h, 450-3h, 450-4h, and 500-3h. The numbers of 350, 400, 450, 500 and
105
1h, 2h, 3h, 4h were denoted the heating temperature and heating time respectively.
RI PT
101
named as ZnO.
108
2.3 Characterization
M AN U
107
SC
For comparison with C-doped ZnO, commercial ZnO without C doping was
106
X-ray diffraction patterns (XRD) characterizations were carried out using a
110
Bruker powder X-ray diffraction D 8 Advance diffractometer with Cu-Kα irradiation.
111
The surface composition and chemical environment were analysed with a VG
112
ESCA-LAB-210 X-ray photoelectron spectroscopy measurement (XPS). The
113
morphologies of the samples were investigated by Hitachi S-4800 field emission
114
scanning electron microscopy (SEM). The specific surface area (SBET) and pore size
115
distribution were calculated based on N2 adsorption/desorption isotherms recorded on
116
a Micromeritics ASAP 2020 nitrogen adsorption apparatus. The pore-size distribution
117
was
118
thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were
119
determined by SDT Q600 V5.0 Build 63 (TGA-DSC). The UV-vis absorbance and
120
diffuse-reflectance spectra (UV-vis DRS) were performed by PE Lambda 750 S with
121
an integrating sphere diffuse reflectance attachment. Photoluminescence (PL) spectra
122
were tested by a Gangdong F-380 fluorescence spectrometer with the excitation light
AC C
EP
TE D
109
calculated
using
the
Barret-Joyner-Halender
6
(BJH)
method.
The
ACCEPTED MANUSCRIPT at 325 nm. Electron spin resonance (ESR) signals were measured on a Bruker A 300
124
spectrometer (USA) under visible light using 5,5-dimethyl-l-pyrroline N-oxide
125
(DMPO) as spin-trapped paramagnetic species.
126
2.4 Adsorption and photocatalytic degradation experiments
RI PT
123
The adsorption and photocatalytic performance of MOF-5 and C@ZnO hybrids
128
were investigated by the photodegradation of RhB and phenol. The RhB and phenol
129
solution with a concentration of 1 mg/L was prepared by dissolving the dye in
130
distilled water. For reaction, 100 mg as-prepared samples were added into 100 mL
131
RhB aqueous solution in a 250 mL beaker with a water jacket to keep the temperature
132
of the beaker maintain at 25°C. The solution was kept in dark for 60 min under
133
magnetic stirring to reach the adsorption/desorption equilibrium. Afterwards, a 350 W
134
Xenon lamp with a 420 nm UV-cutoff filter was used (420 nm-780 nm). During the
135
reaction 3 mL of samples were withdrawn at every 30 min.
M AN U
TE D
136
SC
127
3. Results and discussion
138
3.1 Structure characterization
AC C
139
EP
137
The X-ray diffraction patterns of all the above-prepared samples showed the
140
changes in the phase structure from MOF-5 to C@ZnO under different calcination
141
temperature and time in Fig. 1. X-ray diffraction (XRD) analysis shows that the
142
diffraction of pristine MOF-5 sample fits well with the diffraction pattern in
143
references [23, 24] which confirms that the sample is successfully synthesized.
144
Interestingly, for the calcined composites, when the annealing temperature is below 7
ACCEPTED MANUSCRIPT 450°C, the main peak from MOF-5 is preserved. However, when the annealing
146
temperature and time is above 450°C and 1 h, most peaks from the component of
147
MOF are disappeared except that only some intensity peaks from MOF-5 are detected
148
at approximately 8.9 , 9.9 , 14.3 , 15.8 ºand 17.7 ºin C@ZnO 450-2h and 450-3h
149
hybrids. For 450-4h and 500-3h the characteristic diffraction peaks at 31.8 , 34.4 ,
150
36.3 , 47.5 , 56.6 , 62.8 , 66.4 , 67.9 , 69.1 , 72.6
151
corresponding to the (100), (002), (101), (102), (110), (103), (200), (112), (201), (004)
152
and (202) crystalline planes of ZnO (JCPDS No. 36-1451), respectively. Interestingly,
153
the above characteristic diffraction peaks also can be found in the C@ZnO 450-2h
154
and 450-3h hybrids but the peaks are less sharp than the peaks in 450-4h and 500-3h.
155
This phenomenon indicates that when the annealing temperature and time are 450°C
156
and 2h the ZnO structure could be just produced. Therefore the crystallinity of ZnO
157
are relatively low. While with the increasing annealing temperature and time (450-3h,
158
450-4h and 500-3h), the characteristic peaks of ZnO become sharp and intense,
159
suggesting the increase of ZnO crystallinity. The crystallite sizes of pure ZnO and
160
C@ZnO 450-2h were calculated to be 27.6 and 34.2 nm by application of the Scherrer
161
equation. The calculated crystallite size of C@ZnO was larger than pure ZnO due to
162
the C-doping as the carbon anion radius (69-76 pm) is greater than oxygen (57-66 pm)
163
[25].
RI PT
145
AC C
EP
TE D
M AN U
SC
and 76.9 ºare observed,
164
The elemental composition and electronic structure of MOF-5 and C@ZnO
165
450-2h hybrid were further analyzed by X-ray photoelectron spectroscopy (XPS).
166
Figure 2 shows the XPS survey spectrum and high-resolution XPS spectra of MOF-5 8
ACCEPTED MANUSCRIPT and C@ZnO 450-2h hybrids. It can be seen from Fig. 2a the C@ZnO 450-2h hybrid
168
mainly consists of Zn, C and O elements. The Zn 2p is shown in Fig. 2b. The Zn 2p3/2
169
and Zn 2p1/2 peaks of MOF-5 are located at 1022.4 eV and 1045.4 eV respectively.
170
The peak located at 1022.4 eV corresponding to Zn-O bonds. The binding energy (BE)
171
distance of the two peaks is 23 eV, which is within the standard reference value of
172
ZnO and indicates that the Zn ions in the composites are +2 states [26]. However, the
173
Zn 2p3/2 and Zn 2p1/2 peaks of C@ZnO 450-2h hybrid are located at 1020.8 eV and
174
1043.8 eV respectively. Obviously, there is a negative shift (1.6 eV) in the binding
175
energy of Zn 2p in C@ZnO hybrid compared to MOF-5. Theoretically, the shifts of
176
binding energy in XPS spectra might be caused by the strong interaction (electron
177
transfer) between nanocrystals [27] and in our experiment, this phenomenon is due to
178
the formation of ZnO particles. The peak located at 1020.8 eV can be identified as
179
Zn-C bonds (C-doping) [28, 29]. The C1s shows two carbon species for MOF-5 and
180
three carbon species for C@ZnO in Fig. 2c. The two peaks are observed in the C1s
181
XPS spectra for MOF-5 with the BE distance of 4 eV. The major C1s XPS spectra for
182
MOF-5 is located at 284.6 eV and the satellite peak at higher BE region is located at
183
288.7 eV. C 1s spectra of C@ZnO divided into three peaks at 284.6 eV, 285.4 eV and
184
288.5 eV. The peak at 284.6 eV is attributed to adventitious carbon contamination [25,
185
30, 31]. The peak at 285.4 eV is attributed to the band of C-O [32]. The peak at 288.5
186
eV is due to surface loosely bound carbonate species such as C=O [25, 29] and the
187
carbon may be incorporating into the interstitial positions of the ZnO lattice [31, 32].
188
Obviously, the C1s peaks of C@ZnO 450-2h have a red-shift relative to that in
AC C
EP
TE D
M AN U
SC
RI PT
167
9
ACCEPTED MANUSCRIPT MOF-5. Fig. 2d presents the O1s peaks of MOF-5 and C@ZnO 450-2h. For MOF-5
190
there are two peaks located at 531.7 eV and 532.4 eV which are attributed to the
191
oxygen atoms on the carboxylate groups of the H2BDC linkers and Zn-O bonds of
192
MOF-5 respectively. However, there are three peaks for C@ZnO 450-2h located at
193
529.8 eV, 531.2 eV and 531.9 eV respectively. The latter two peaks are at the same
194
BE as MOF-5 and the former one at 529.8 eV is assigned to O2- ions of Zn-O bonds in
195
Wurtzite structure with Zn2+ in hexagonal coordination [25, 29] indicating the
196
existence of ZnO structure.
197
3.2 Morphologic characterization
M AN U
SC
RI PT
189
The morphology and structure of MOF-5 and C@ZnO hybrids (350-3h, 450-2h
199
and 500-3h) are revealed by the SEM (Fig. 3a-b, 3c, 3d-e and 3f). As shown in Fig. 3a
200
the as-synthesized MOF-5 have a well-defined octahedral morphology. The
201
high-magnification SEM (Fig. 3b) shows that there are many textures on the surface
202
of the octahedron. All the products of thermal decomposition of MOF-5 under air
203
condition maintained the octahedral morphology (Fig. 3c-f). However, the calcined
204
temperature has influence on the morphology and structure of C@ZnO. When the
205
annealing temperature was 350°C, the surface of the C@ZnO was a little rougher than
206
MOF-5 and the size of the particles had almost no change (Fig. 3c). But when the
207
annealing temperature was risen to 450°C, the surface of the C@ZnO turned to
208
porous particles and the enlarged views (Fig. 3e) clearly show that the surface of the
209
octahedral is reorganized into aggregates of ZnO nanoparticles while the 3D
210
octahedral structure is still retained with a little shrunken. And with the annealing
AC C
EP
TE D
198
10
ACCEPTED MANUSCRIPT temperature increasing to 500°C (Fig. 3f), the above phenomenon is more obvious,
212
and the octahedral structure began to have large cracks leading to the collapse of the
213
octahedral structure. Furthermore the electronic image (Fig. 4b), EDS data (Fig. 4f)
214
and element mapping images of Zn, O, and C (Fig. 4c-d) for C@ZnO 450-2h hybrids
215
can clearly reveal that C, Zn and O elements are homogeneously distributed in
216
octahedron and prove successful preparation of C-doping ZnO.
RI PT
211
The thermo gravimetric and differential scanning calorimetry (TG-DSC) curves
218
are shown in Fig. 5. There are two main weight loss events observed in TG-DSC
219
curves in Fig. 5. The continuous weight loss under 450 °C were mainly attributed to
220
the solvent liberation or the loss of guest molecules, and the second weight loss
221
occurred from 450 to 510°C and a mass loss of 48.34% is observed which can be
222
ascribed to the decomposition of organic components in air. Obviously the direct
223
annealing MOF-5 in air can thermal decompose the organic components and remain
224
the exclusively ZnO with wurtzite structure (JCPDS 36-1451) as shown in XRD data
225
(Fig. 1).
EP
TE D
M AN U
SC
217
The adsorption-desorption N2 isotherms of MOF-5 and C@ZnO 450-2h hybrid
227
are shown in Fig. 6 (a) and (b). The parameters of MOF-5, C@ZnO 450-2h hybrid
228
and ZnO were calculated from the isotherms showing in Table 1. The calculated BET
229
surface area of MOF-5, C@ZnO (450-2h) and ZnO are 1101 m2/g, 833 m2/g and 7.82
230
m2/g, respectively. And the total pore volumes of MOF-5, C@ZnO (450-2h) and ZnO
231
are 0.40 cm3/g; 0.32 cm3/g; 0.06 cm3/g, respectively. From the above data, it can be
232
seen that the specific surface area and pore volumes of carbon doped ZnO hybrids
AC C
226
11
ACCEPTED MANUSCRIPT using MOF-5 as the template precursor are much larger than pure ZnO. As shown in
234
Fig. 6 (a) and (b), MOF-5 and C@ZnO 450-2h mainly follow the typical type- º
235
isotherm, according to the classification of Brunauer Deming, Deming and Teller
236
(BDDT). An obvious H2 type hysteresis loop is observed at P/P0 between 0.45 and
237
1.0 for MOF-5 according to IUPAC, which are associated with capillary condensation
238
taking place in mesopores. Correspondingly, the pore size distribution curve of the
239
inset image of Fig. 6a are revealed that the presence of a combination of micropores
240
(<2 nm) and mesopores (2-5 nm) in the samples. In contrast, a H3 type hysteresis loop
241
is observed at P/P0 between 0.5 and 1.0 for C@ZnO 450-2h hybrid indicating the
242
existence of narrow slit-shaped pores and the mesopores (3-5 nm) with a combination
243
of micropores (<2 nm). This phenomenon is consistent with the SEM results (Fig. 3).
244
The surface of MOF-5 is uniform porous structure with a H2 type hysteresis loop.
245
When the annealing temperature risen to 450°C, the organic framework of MOF-5
246
release the volatile gases such as CO2 and H2O, resulting in the uniform porous
247
structure. Meanwhile the MOF-5 was reorganized into aggregates of ZnO
248
nanoparticles leading to the uniform pore turned to mesoporous or macroporous with
249
slits and the type of hysteresis loop changed from H2 to H3.
250
3.3 The optical properties
SC
M AN U
TE D
EP
AC C
251
RI PT
233
UV-vis diffuse reflectance spectroscopy (DRS) of MOF-5, ZnO and C@ZnO
252
hybrids were carried out and shown in Fig. 7 (a). Consistent with the reference,
253
MOF-5 and ZnO have an absorption edge at about 330 nm and 400 nm out of the
254
visible light absorption region. In comparison with MOF-5 and ZnO, C-doped ZnO 12
ACCEPTED MANUSCRIPT 255
(C@ZnO) displays significant red shift of optical bandgap absorption edge into
256
visible-light region due to the electronic transitions from the valence to the conduction
257
band (O2p
258
The color of the samples also changed from white (MOF-5) to beige (C@ZnO)
259
suggesting that visible light can be used by C@ZnO. The visible-light absorption is
260
important for C doping ZnO as a photocatalyst and in order to reveal more
261
information of the optical band gap of C@ZnO hybrids, the optical band gap energy
262
of the samples can be calculated by following calculation formula (Eq.1).
M AN U
SC
RI PT
Zn3d) [33] thus enhanced light absorption in the whole UV-visible band.
αhυ = A(hυ − Eg)/
263
(1)
α is the diffuse absorption coefficient, h is the Planck constant, and ν is the light
265
frequency. The plots of (αhν)2 and (αhν)1/2 vs. hν were plotted for calculation of
266
direct and indirect transitions band gaps, respectively. As shown in Fig. 7 (b), the
267
calculation of indirect energies of MOF-5, ZnO and C@ZnO hybrids of 350-3h,
268
400-3h, 450-1h, 450-2h, 450-3h, 450-4h and 500-3h estimated from a plot of (αhν)1/2
269
vs. photo energy (hν) according to the K-M model were 3.82 eV, 3.20 eV, 3.73 eV,
270
3.00 eV, 2.95 eV, 2.92 eV, 3.02 eV 3.06 eV and 3.07 eV respectively. Consistent with
271
the report in the reference, the band gap of ZnO is approximately 3.2 eV [3] and the C
272
doping ZnO (450-2h) in our experiment is 2.92 eV revealing the C-doping can shorten
273
the band gap of ZnO to enhance light harvesting and utilize visible light. In addition,
274
the band gap of C doping of ZnO can be observed clearly revealing the C was doped
275
into the crystal structure of ZnO than just covered on the surface of ZnO. Moreover,
276
introducing impurity carbon into the lattice of ZnO could revise the band structure of
AC C
EP
TE D
264
13
ACCEPTED MANUSCRIPT ZnO thus tune its bandgap. Besides the changed band gap of ZnO, we can see clearly
278
in Fig. 7a the visible light absorption of C@ZnO is extent to 800 nm which is a
279
typical red shift induced by impurity adsorption (C doping) or oxygen-vacancies (Ovac)
280
[25]. This result is also in line with the XPS data analysis.
RI PT
277
The recombination rate of photogenerated electrons and holes is an important
282
factor affecting the photocatalytic efficiency and PL spectra can be used to reveal it.
283
The PL spectra of MOF-5, pure ZnO and C@ZnO 450-2h are shown in Fig. 8.
284
Notably, C@ZnO 450-2h exhibit lower PL intensity than MOF-5 and pure ZnO,
285
suggesting the C@ZnO has higher efficiency of charge transmission and the C doping
286
can effectively inhibit the electrons/holes recombination thus to improve
287
photocatalytic degradation activity.
288
3.4 Photocatalytic activity
TE D
M AN U
SC
281
To investigate the advantages of C doping ZnO with 3D porous structure for
290
photocatalytic degradation, we further investigated its photodegradation performance
291
using Rhodamine B (RhB) as a target pollutant. Fig. 9 depicts the photocatalytic
292
activities for the RhB degradation on MOF-5, ZnO and C@ZnO hybrids under visible
293
light. To achieve adsorption-desorption equilibration of the photocatalyst and
294
pollutants, the system was stirred for 60 min in the dark before visible light irradiation
295
and the concentration of the RhB at beginning was used as the initial concentration C0.
296
The Y axis is set up as C/C0, where C is the actual concentration of RhB. After 210
297
min of irradiation it was found the photodegradation data was well fitted with a
AC C
EP
289
14
ACCEPTED MANUSCRIPT 298
fist-order kinetic. The rate constants (k) were gotten by fitting the data with the
299
following Eq. (2):
ln ( ) = κt
300
(2)
κ is the apparent first-order rate constant (min-1), t is the irradiation time for
302
photodegradation (min), C0 is the concentration of the RhB at beginning, C is the
303
actual concentration of RhB at the indicated reaction time t. As can be clearly seen in
304
Fig. 9b, the linearity in the kinetic plots was verified that the photodegradation of RhB
305
followed first-order kinetics.
M AN U
SC
RI PT
301
It can be seen in the Fig. 9 that in the absence of any photocatalyst there was
307
little degradation of RhB, indicating that the RhB was stable under visible light
308
irradiation. From Fig. 9a and b it can clearly see that the MOF-5 and pure ZnO
309
microspheres played only a minor role in degradation of RhB as the band gap energies
310
of them are 3.82 and 3.2 eV respectively, therefore, the visible light utilization was
311
negligible. In contrast, all the C@ZnO hybrids performed the ability to
312
photodegradation of RhB under visible light and C@ZnO 450-2h exhibited the best
313
degradation ability. The rate constant of RhB photodegradation was 0.01570 min-1 for
314
450-2h, which was at least six times higher than that for MOF-5 (0.00075 min-1), ZnO
315
(0.00076 min-1), 500-3h (0.00161 min-1) and 350-3h (0.00242 min-1). The different
316
photocatalytic effects of C@ZnO hybrids are due to the crystalline, the amount of
317
carbon doping and the porous structure. With the increase of annealing temperature,
318
the C@ZnO 450-2h exhibits enhanced crystallization. When the annealing
319
temperature increases to 500°C, the C@ZnO 500-3h exhibits the best crystal
AC C
EP
TE D
306
15
ACCEPTED MANUSCRIPT structure. Theoretically, the crystallization is an important factor influencing the
321
photocatalytic activity [33]. The highly crystalline structure means there are fewer
322
defects in the crystal lattice that facilitates the transfer/separation of photogenerated
323
electrons and holes and thus more electrons migrate to the surface of the
324
photocatalysts to generate more radicals. However, the content of carbon in the
325
C@ZnO lattice gradually decreased with the increasing annealing temperature leading
326
to a reduction of visible light response region (Fig. 7). In addition, the unique MOFs
327
structure and porous structure can provide more active sites and photocatalytic
328
reaction centres for the reactant molecules. Furthermore, the 3D porous structure also
329
can enhance the absorption of visible light and increase the utilization of visible light
330
[34]. However with the increased annealing temperature the MOFs structure gradually
331
collapse and the mesopores decreased resulting in a decrease of the specific surface
332
area. Therefore, in the balance of the above three factors, the C@ZnO 450-2h showed
333
the best photocatalytic activity. An overview of works published on C@ZnO
334
photocatalyst by other preparation methods as well as the present work were listed in
335
Table S1. As shown in Table S1, the C@ZnO 450-2h shows the biggest BET SSA
336
and excellent photodegradation ability.
SC
M AN U
TE D
EP
AC C
337
RI PT
320
The degradation of azo dye in wastewater is still a difficult problem at present,
338
but due to the sensitization, the photocatalytic effect of the azo dye may be affected.
339
Phenol is known as a stable organic pollutant with even more toxic. To avoid the
340
sensitization effect of azo dye and further verify the true photocatalytic capacity of
16
ACCEPTED MANUSCRIPT 341
our catalyst, phenol was used as pollutant, and the results are shown in Fig. S1.
342
Consistent with expectations, C@ZnO still shows excellent photodegradation activity.
343
3.5 Photocatalytic degradation mechanism To get insights into the photocatalytic mechanism and to identify the contribution
345
of the main active species, several different scavengers such as potassium iodide (KI)
346
[35], p-benzoquinone (BQ) [36], and isopropanol (IPA) [37] were used as species
347
scavengers for photoexcited hole (h+) , superoxide radicals (•O2-) and hydroxyl radical
348
(•OH) formed during the degradation of RhB in our experiment. As shown in Fig. 10,
349
the photocatalytic performance of RhB was obviously inhibited when BQ was added,
350
indicating the •O2- radical dominated the degradation of RhB and the degradation rate
351
dropped greatly. When KI scavenger was added in the system, the degradation
352
efficiency decreased a little, suggesting that h+ played a minor role in RhB
353
photodegradation. However, the photocatalytic performance of RhB was affected
354
slightly when IPA was added, indicating the •OH specie played little role in the
355
mechanism for RhB degradation.
EP
TE D
M AN U
SC
RI PT
344
To further prove the existence of •O2- and •OH, the production of •O2- and •OH
357
radicals in the reaction system were detected by the ESR technique using DMPO as a
358
trapping reagent. The result is shown in Fig. 11. It is clear that at 0 min in dark there
359
are no peaks, however when the visible light irradiated the four characteristic peaks of
360
DMPO- •O2- adducts for C@ZnO are observed. It is obvious that •O2- radicals were
361
generated on the surface of C@ZnO after irradiation. And the •OH radicals were also
362
found in the reaction system after 10 min irradiation.
AC C
356
17
ACCEPTED MANUSCRIPT In order to further verify the formation of above-mentioned active species in the
364
photocatalytic degradation process, Mott-Schottky measurement was carried out to
365
determine the band structure of C@ZnO. Mott-Schottky measurement of the C@ZnO
366
catalyst is performed in 0.5 M Na2SO4 solution under dark condition at a frequency of
367
100 Hz. As shown in Fig. 12, the position slope of the plot reveals that C@ZnO is an
368
n-type semiconductor characteristic which is the same type as ZnO [3]. The flat-band
369
potential of C@ZnO derived from Mott-Schottky plot is about -0.50 V versus SCE
370
corresponding to -0.26 V versus normal hydrogen electrode (NHE). For n-type
371
semiconductors, the flat-band potential was 0-0.1 V higher than the conduction-band
372
potential [38]. Therefore the conduction band potential (ECB) of the C@ZnO is -0.36
373
V vs. NHE. And the valence band (VB) and conduction band (CB) potentials of
374
C@ZnO at the point of zero charge can be calculated by the following Eq. (3):
SC
M AN U
TE D
375
RI PT
363
E = E − E
(3)
Combining with the band gap energy estimated from UV-vis DRS spectra, the
377
calculated optical bandgap of C@ZnO is 3.09 eV. Accordingly to the Eq. (3), the
378
valence band position (EVB) of C@ZnO is determined to be 2.73 V vs. NHE.
379
According to reports in the literature and our experiment, the band gap of ZnO is 3.2
380
eV and the valence band and conduction band position is 2.89 eV and -0.31 eV
381
respectively. Obviously, the bandgap of C@ZnO is narrow than pure ZnO due to the
382
defect produced in the crystal structure of ZnO after C doping which lifts up VB of
383
ZnO and pushes the CB of ZnO down to lower energy level [25]. Because of the wide
384
bandgap, the electrons of ZnO on VB cannot be excited by visible light therefore RhB
AC C
EP
376
18
ACCEPTED MANUSCRIPT photodegradation on ZnO is attributed to dye sensitized photocatalysis resulting in the
386
low degradation rate. In contrast, with a narrowed bandgap, C-doped ZnO can be
387
excited by visible light and the electrons on the VB of C@ZnO can be excited by
388
visible light to the CB leaving the holes on the VB. Moreover the ECB position of
389
C@ZnO (-0.36 V vs. NHE) is more negative than the oxidation potential of O2/•O2-
390
(-0.33 V vs. NHE), the molecular oxygen can be photoexcited to •O2- by
391
photoelectron on the CB of C@ZnO. At same time, the H2O and OH- in the system
392
can be taken by h+ to generate •OH radical species as the EVB position of C@ZnO
393
(2.73 V vs. NHE) is more positive than the oxidation potential of •OH/OH- (2.40 V vs.
394
NHE). In addition the holes leaving on the VB of C@ZnO also can oxidize RhB
395
directly according to the scavenger’s experiments. The proposed photocatalytic
396
mechanism of C@ZnO under visible light scheme is shown in scheme 1. In addition,
397
as the C@ZnO was derived by annealing MOF-5 leaving the unique MOF porous
398
structure
399
adsorption-diffusion-exchange of reactant and the use of visible light leading to the
400
increase of the degradation rate.
401
3.6 Stability
SC
M AN U
TE D
especially
large
surface
area
which
promotes
the
EP
and
AC C
402
RI PT
385
Regeneration and reusability are also very important for photocatalyst. The
403
results of reusability for C@ZnO hybrid is shown in Fig. 13. After 5 cycles of
404
degradation, the degradation rate of RhB retained well and the little loss might from
405
the loss of photocatalyst during recycling.
406 19
ACCEPTED MANUSCRIPT 407
4. Conclusions Carbon (C) doped octahedral porous ZnO have been synthesized annealing
409
MOF-5 at different temperature and time in air. The results show that the 450-2h
410
C@ZnO hybrid retains the octahedral porous morphology with aggregates of ZnO
411
nanoparticles and porous structure. Moreover, carbon was introduced in the ZnO
412
lattice effectively and evenly. There is no doubt that C@ZnO 450-2h exhibited best
413
photocatalytic performance on RhB degradation under visible light irradiation. The
414
enhanced photocatalytic performance was ascribed to the unique porous structure
415
from MOFs and a narrow band gap from C doping. The former can facilitate more
416
dye molecules absorbed on the active sites of photocatalysts and increase the light
417
transmittance. And the latter can improve visible light absorption. The active species
418
trapping experiments results proved that superoxide radicals (•O2-) was the main
419
active species in photodegradation of RhB, photoexcited holes (h+) played the minor
420
effect on it, however, hydroxyl radical (•OH) almost have little contribution on it.
421
This work provides a new insight into the design and synthesis of highly efficient
422
photocatalysts for organic dyes containing wastewater treatment in environmental
423
applications.
SC
M AN U
TE D
EP
AC C
424
RI PT
408
425
Acknowledgements
426
This work was supported by the National Natural Science Foundation of China (No.
427
21277108; 21476179), one hundred talents project of Guangzhou University and 2016
428
Wuhan Yellow Crane Talents (Science) Program. 20
ACCEPTED MANUSCRIPT 429 430
References
431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470
[1] D. Vidyasagar, S.G. Ghugal, A. Kulkarni, P. Mishra, A.G. Shende, Jagannath, S.S. Umare, R. Sasikala, Silver/Silver(II) oxide (Ag/AgO) loaded graphitic carbon nitride microspheres: An effective visible light active photocatalyst for degradation of acidic dyes and bacterial inactivation, Appl. Catal. B: Environ.
RI PT
221 (2018) 339-348.
[2] Y. Zhang, J. Zhou, Q. Feng, X. Chen, Z. Hu, Visible light photocatalytic degradation of MB using UiO-66/g-C3N4 heterojunction nanocatalyst, Chemosphere 212 (2018) 523-532.
[3] Y. Zhang, J. Zhou, W. Cai, J. Zhou, Z. Li, Enhanced photocatalytic performance and degradation pathway of Rhodamine B over hierarchical double-shelled zinc nickel oxide hollow sphere heterojunction, Appl. Surf. Sci. 430 (2018) 549-560.
SC
[4] Y.M. Zhou, L.J. Zhang, S.Y. Tao, Porous TiO2 with large surface area is an efficient catalyst carrier for the recovery of wastewater containing an ultrahigh concentration of dye, Rsc Adv. 8 (2018) 3433-3442. [5] R. Zhang, M. Ma, Q. Zhang, F. Dong, Y. Zhou, Multifunctional g-C3N4/graphene oxide wrapped
M AN U
sponge monoliths as highly efficient adsorbent and photocatalyst, Appl. Catal. B: Environ. 235 (2018) 17-25.
[6] Y. Zhang, J. Zhou, Z. Li, Q. Feng, Photodegradation pathway of rhodamine B with novel Au nanorods @ ZnO microspheres driven by visible light irradiation, J. Mater. Sci. 53 (2018) 3149-3162. [7] H. Wang, W. Zhang, X. Li, J. Li, W. Cen, Q. Li, F. Dong, Highly enhanced visible light photocatalysis and in situ FT-IR studies on Bi metal@defective BiOCl hierarchical microspheres, Appl. Catal. B: Environ. 225 (2018) 218-227.
TE D
[8] C.B. Ong, L.Y. Ng, A.W. Mohammad, A review of ZnO nanoparticles as solar photocatalysts: Synthesis, mechanisms and applications, Renewable & Sustainable Energy Reviews 81 (2018) 536-551. [9] D. Zhang, Y. Zhao, L. Chen, Fabrication and characterization of amino-grafted graphene oxide modified ZnO with high photocatalytic activity, Appl. Surf. Sci. 458 (2018) 638-647. [10] L.P. Zheng, X.W. Li, W.C. Du, D.W. Shi, W.S. Ning, X.Y. Lu, Z.Y. Hou, Metal-organic framework derived 146-153.
EP
Cu/ZnO catalysts for continuous hydrogenolysis of glycerol, Appl. Catal. B: Environ. 203 (2017) [11] S.W. Zhao, H.F. Zuo, Y.R. Guo, Q.J. Pan, Carbon-doped ZnO aided by carboxymethyl cellulose:
AC C
Fabrication, photoluminescence and photocatalytic applications, J. Alloys Compd. 695 (2017) 1029-1037.
[12] S. Wang, X. Zhang, S. Li, Y. Fang, L. Pan, J.-J. Zou, C-doped ZnO ball-in-ball hollow microspheres for efficient photocatalytic and photoelectrochemical applications, J. Hazard. Mater. 331 (2017) 235-245. [13] Y. Song, Y. Chen, J. Wu, Y. Fu, R. Zhou, S. Chen, L. Wang, Hollow metal organic frameworks-derived porous ZnO/C nanocages as anode materials for lithium-ion batteries, J. Alloys Compd. 694 (2017) 1246-1253. [14] H. Huang, K. Xiao, N. Tian, F. Dong, T. Zhang, X. Du, Y. Zhang, Template-free precursor-surface-etching route to porous, thin g-C3N4 nanosheets for enhancing photocatalytic reduction and oxidation activity, J. Mater. Chem. A 5 (2017) 17452-17463. [15] A. Corma, H. Garcia, F.X.L.I. Llabres i Xamena, Engineering Metal Organic Frameworks for Heterogeneous Catalysis, Chem. Rev. 110 (2010) 4606-4655. [16] W. Zhao, G. Wan, C. Peng, H. Sheng, J. Wen, H. Chen, Key Single-Atom Electrocatalysis in 21
ACCEPTED MANUSCRIPT Metal-Organic Framework (MOF)-Derived Bifunctional Catalysts, ChemSusChem 11 (2018) 3473-3479. [17] S.-J. Qin, B. Yan, Dual-emissive ratiometric fluorescent probe based on Eu3+/C-dots@MOF hybrids for the biomarker diaminotoluene sensing, Sensors and Actuators B-Chemical 272 (2018) 510-517. [18] J. Zheng, X. Cui, Q. Yang, Q. Ren, Y. Yang, H. Xing, Shaping of ultrahigh-loading MOF pellet with a strongly anti-tearing binder for gas separation and storage, Chem. Eng. J. 354 (2018) 1075-1082. [19] G. Zhu, H. Wen, M. Ma, W. Wang, L. Yang, L. Wang, X. Shi, X. Cheng, X. Sun, Y. Yao, A excellent rate performance, Chem. Commun. 54 (2018) 10499-10502.
RI PT
self-supported hierarchical Co-MOF as a supercapacitor electrode with ultrahigh areal capacitance and [20] K. Yang, Y. Yan, H. Wang, Z. Sun, W. Chen, H. Kang, Y. Han, W. Zahng, X. Sun, Z. Li, Monodisperse Cu/Cu2O@C core-shell nanocomposite supported on rGO layers as an efficient catalyst derived from a Cu-based MOF/GO structure, Nanoscale 10 (2018) 17647-17655.
[21] J. Xu, W. Zhang, Y. Chen, H. Fan, D. Su, G. Wang, MOF-derived porous N-Co3O4@N-C nanododecahedra wrapped with reduced graphene oxide as a high capacity cathode for lithium-sulfur
SC
batteries, J. Mater. Chem. A 6 (2018) 2797-2807.
[22] L. Yu, Z. Ye, J. Li, C. Ma, C. Ma, X. Liu, H. Wang, L. Tang, P. Huo, Y. Yan, Photocatalytic Degradation Mechanism of Tetracycline by Ag@ZnO/C Core-Shell Plasmonic Photocatalyst Under Visible Light,
M AN U
Nano 13 (2018).
[23] M. Sabo, A. Henschel, H. Froede, E. Klemm, S. Kaskel, Solution infiltration of palladium into MOF-5: synthesis, physisorption and catalytic properties, J. Mater. Chem. 17 (2007) 3827-3832. [24] S.S. Kaye, A. Dailly, O.M. Yaghi, J.R. Long, Impact of preparation and handling on the hydrogen storage properties of Zn4O(1,4-benzenedicarboxylate)(3) (MOF-5), JACS 129 (2007) 14176-14177. [25] A.S. Alshammari, L. Chi, X. Chen, A. Bagabas, D. Kramer, A. Alromaeh, Z. Jiang, Visible-light photocatalysis on C-doped ZnO derived from polymer-assisted pyrolysis, Rsc Adv. 5 (2015)
TE D
27690-27698.
[26] J. Xue, S. Ma, Y. Zhou, Z. Zhang, P. Jiang, Synthesis of Ag/ZnO/C plasmonic photocatalyst with enhanced adsorption capacity and photocatalytic activity to antibiotics, Rsc Adv. 5 (2015) 18832-18840.
[27] Z. Zhang, C. Shao, X. Li, C. Wang, M. Zhang, Y. Liu, Electrospun Nanofibers of p-Type NiO/n-Type 2915-2923.
EP
ZnO Heterojunctions with Enhanced Photocatalytic Activity, Acs Appl. Mater. Inter. 2 (2010) [28] S.B. Wang, X.W. Zhang, S. Li, Y. Fang, L. Pan, J.J. Zou, C-doped ZnO ball-in-ball hollow microspheres for efficient photocatalytic and photoelectrochemical applications, J. Hazard. Mater. 331 (2017)
AC C
471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514
235-245.
[29] L. Pan, T. Muhammad, L. Ma, Z.F. Huang, S.B. Wang, L. Wang, J.J. Zou, X.W. Zhang, MOF-derived C-doped ZnO prepared via a two-step calcination for efficient photocatalysis, Appl. Catal. B: Environ. 189 (2016) 181-191.
[30] S. Cho, J.-W. Jang, J.S. Lee, K.-H. Lee, Carbon-doped ZnO nanostructures synthesized using vitamin C for visible light photocatalysis, Crystengcomm 12 (2010) 3929-3935. [31] J. Zhai, L. Wang, D. Wang, Y. Lin, D. He, T. Xie, UV-illumination room-temperature gas sensing activity of carbon-doped ZnO microspheres, Sens. Actuators, B 161 (2012) 292-297. [32] M.J. Zhou, X.H. Gao, Y. Hu, J.F. Chen, X. Hu, Uniform hamburger-like mesoporous carbon-incorporated ZnO nanoarchitectures: One-pot solvothermal synthesis, high adsorption and visible-light photocatalytic decolorization of dyes, Appl. Catal. B: Environ. 138 (2013) 1-8. [33] S.W. Liu, C. Li, J.G. Yu, Q.J. Xiang, Improved visible-light photocatalytic activity of porous carbon 22
ACCEPTED MANUSCRIPT self-doped ZnO nanosheet-assembled flowers, Crystengcomm 13 (2011) 2533-2541. [34] S.J. Yang, J.H. Im, T. Kim, K. Lee, C.R. Park, MOF-derived ZnO and ZnO@C composites with high photocatalytic activity and adsorption capacity, J. Hazard. Mater. 186 (2011) 376-382. [35] L. Hu, H. Yuan, L. Zou, F. Chen, X. Hu, Adsorption and visible light-driven photocatalytic degradation of Rhodamine B in aqueous solutions by Ag@AgBr/SBA-15, Appl. Surf. Sci. 355 (2015) 706-715. [36] Y. Liu, X.Z. Yuan, H. Wang, X.H. Chen, S.S. Gu, Q. Jiang, Z.B. Wu, L.B. Jiang, G.M. Zeng, photocatalytic activity, Rsc Adv. 5 (2015) 33696-33704.
RI PT
Solvothermal synthesis of graphene/BiOCl0.75Br0.25 microspheres with excellent visible-light [37] J. Zhuang, W. Dai, Q. Tian, Z. Li, L. Xie, J. Wang, P. Liu, X. Shi, D. Wang, Photocatalytic Degradation of RhB over TiO2 Bilayer Films: Effect of Defects and Their Location, Langmuir 26 (2010) 9686-9694.
[38] Y. Gao, S. Li, Y. Li, L. Yao, H. Zhang, Accelerated photocatalytic degradation of organic pollutant over metal-organic framework MIL-53(Fe) under visible LED light mediated by persulfate, Appl. Catal. B: Environ. 202 (2017) 165-174.
SC
515 516 517 518 519 520 521 522 523 524 525 526 527 528
M AN U
529
AC C
EP
TE D
530
23
ACCEPTED MANUSCRIPT Scheme Caption
SC
RI PT
531
533
M AN U
532
Scheme 1 The proposed photocatalytic mechanism of C@ZnO under visible light.
AC C
EP
TE D
534
24
ACCEPTED MANUSCRIPT 535
Table 1 Textural parameters of MOF-5, C@ZnO 450-2h hybrid and ZnO.
536
These parameters were derived from the N2 sorption isotherms obtained at -196°Cº
537
SBET (m2/g)
VTotal (cm3/g)
MOF-5
1101
0.40
450-2h
833
0.33
ZnO
7.82
0.06
SC
RI PT
Sample
M AN U
538
AC C
EP
TE D
539
25
ACCEPTED MANUSCRIPT Figure Caption
541
Fig. 1 XRD patterns of MOF-5 and C@ZnO hybrids.
542
Fig. 2 XPS spectra of MOF-5 and C@ZnO 450-2h hybrids: (a) survey, (b) Zn 2p, (c)
543
C 1s, and (d) O 1s.
544
Fig. 3 FE-SEM images of (a) and (b) MOF-5, (c) C@ZnO 350-3h, (d) and (e)
545
C@ZnO 450-2h, (f) C@ZnO 500-3h.
546
Fig. 4 (a) FE-SEM images, (b) Electronic image, (c) mapping of C element, (d)
547
mapping of Zn element, (e) mapping of Zn element and (f) EDS analysis of C@ZnO
548
450-2h hybrid
549
Fig. 5 TG-DSC image of MOF-5.
550
Fig. 6 N2 adsorption/desorption isotherms of (a) MOF-5 and (b) C@ZnO 450-2h. The
551
inset of (a) and (b) shows the pore-size distribution of the samples respectively.
552
Fig. 7 (a) UV-vis DRS spectra, (b) (αhν)1/2 vs. photon energy (hν) of MOF-5 and
553
C@ZnO hybrids and (c) (αhν)2 vs. photon energy (hν) of MOF-5 and C@ZnO
554
hybrids .
555
Fig. 8 PL spectra of MOF-5 and C@ZnO 450-2h hybrids.
556
Fig. 9 Photocatalytic activity of MOF-5 and C@ZnO hybrids for the degradation of
557
RhB under visible light.
558
Fig. 10 Effects of different scavenger addition in the photocatalytic degradation of
559
RhB under visible light.
560
Fig. 11 ESR signals of the (a) DMPO- O2- (b) DMPO- OH- with visible light
561
irradiation for C@ZnO 450-2h.
AC C
EP
TE D
M AN U
SC
RI PT
540
26
ACCEPTED MANUSCRIPT 562
Fig. 12 Mott−Schottky plot for the C@ZnO electrodes.
563
Fig. 13 Recycling experiments of visible light photocatalytic degradation of RhB over
564
the C@ZnO 450-2h hybrids.
AC C
EP
TE D
M AN U
SC
RI PT
565
27
ACCEPTED MANUSCRIPT 566
ZnO
Intensity (a.u.)
500-3h
RI PT
450-4h 450-3h
450-2h
SC
450-1h 400-3h
M AN U
350-3h MOF-5
ZnO PDF 36-1451
10
20
567
571
EP
570
50
60
70
Fig. 1 XRD patterns of MOF-5 and C@ZnO hybrids.
AC C
569
40
2θ θ (degree)
TE D
568
30
28
ACCEPTED MANUSCRIPT
Zn 2p1/2
MOF-5
200
400
600
800
1000 1200
1020
1030
1040
Bind Energy (eV)
Bind Energy (eV)
579
c
d
O1s
M AN U
Counts/s
C 1s
450-2h
583
450-2h
Counts/s
580
MOF-5
280
585
285
TE D
MOF-5
584
290
1050
SC
0
578
582
Zn 2p1/2
450-2h
577
581
b
Zn 2p3/2
RI PT
576
a
Counts/s
Zn 2p3/2
O 1S O 1S
C 1S
MOF-5 C 1S
575
Counts/s
574
450-2h
Zn 2p1/2
573
Zn 2p3/2
572
295 525
Bind Energy (eV)
530
535
Bind Energy (eV)
Fig. 2 XPS spectra of MOF-5 and C@ZnO 450-2h hybrids: (a) survey, (b) Zn 2p, (c)
587
C 1s, and (d) O 1s.
AC C
588
EP
586
29
540
ACCEPTED MANUSCRIPT 589 590 591
RI PT
592 593 594
SC
595
M AN U
596 597 598 599
603 604 605
EP
602
AC C
601
TE D
600
606
Fig. 3 FE-SEM images of (a) and (b) MOF-5, (c) C@ZnO 350-3h, (d) and (e)
607
C@ZnO 450-2h, (f) C@ZnO 500-3h.
608
30
ACCEPTED MANUSCRIPT 609
RI PT
610
SC
f
C
0
O Zn Zn
M AN U
Zn
1
2
3
4
5
6
7
8
KeV
Fig. 4 (a) FE-SEM images, (b) Electronic image, (c) mapping of C element, (d)
612
mapping of Zn element, (e) mapping of Zn element and (f) EDS analysis of C@ZnO
613
450-2h hybrid
EP AC C
614
TE D
611
31
9
10
ACCEPTED MANUSCRIPT 615
516.3
DSC
10
60
SC
40 20
5
510.7
0
200
616
400
600
Temperature (°°C)
800
Fig. 5 TG-DSC image of MOF-5.
AC C
EP
TE D
617
32
0 1000
DSC (mW/mg)
RI PT
80
0
618
15
TG
M AN U
Weight loss (%)
100
300 a
0.25 0.20 0.15 0.10 0.05 0.00 0
5
SC
260
10
15
20
Pore width (nm)
240 0.0
0.4
0.6
0.8
M AN U
0.2
1.0
Relative pressure (P/P0)
260
0.4 0.3 0.2 0.1 0.0
0
5
10
15
20
Pore width (nm)
EP
220
TE D
dV/dD (cm3/g)
240
0.5
200
AC C
Adsorbed Volume (cm3/g )
619
620
RI PT
280
dV/dD (cm3/g)
Adsorbed Volume (cm3/g )
ACCEPTED MANUSCRIPT
180 0.0
b 0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0)
621
Fig. 6 N2 adsorption/desorption isotherms of (a) MOF-5 and (b) C@ZnO 450-2h. The
622
inset of (a) and (b) shows the pore-size distribution of the samples respectively.
623 33
ACCEPTED MANUSCRIPT
MOF-5
450-2h
200
300
400
500
600
700
Wavelength (nm)
624
c
SC
b
800
M AN U
Ahv^(1/2) (eV )^1/2
MOF-5 ZnO 350-3h 400-3h 450-1h 450-2h 450-3h 450-4h 500-3h
3.06
2.95
3.07
2.6
2.8
3.73
3.02 3.00
2.92
3.0
3.2
3.4
3.6
3.82
3.8
4.0
hv (eV)
TE D
625
AC C
EP
Ahv^2 (eV)^2
c
2.6
626
MOF-5 ZnO 350-3h 400-3h 450-1h 450-2h 450-3h 450-4h 500-3h
RI PT
Absorbance (a.u.)
a
MOF-5 ZnO 350-3h 400-3h 450-1h 450-2h 450-3h 450-4h 500-3h
3.14 3.13 3.09
2.8
3.0
3.20 3.19 3.18
3.95 3.92
3.16
3.2
3.4
3.6
3.8
4.0
hv (eV)
627
Fig. 7 (a) UV-vis DRS spectra, (b) (αhν)1/2 vs. photon energy (hν) of MOF-5 and
628
C@ZnO hybrids and (c) (αhν)2 vs. photon energy (hν) of MOF-5 and C@ZnO
629
hybrids .
630 34
ACCEPTED MANUSCRIPT
350
400
M AN U
SC
RI PT
Intensity
MOF-5 ZnO 450-2h
450
500
550
Wavelength (nm) 631
Fig. 8 PL spectra of MOF-5 and C@ZnO 450-2h hybrids
633
(excitation wavelength: 325nm).
EP AC C
634
TE D
632
35
600
ACCEPTED MANUSCRIPT
1.0
0.5
0.0
-60 -30
0
RI PT
Self MOF-5 ZnO 350-3h 400-3h 450-1h 450-2h 450-3h 450-4h 500-3h
30 60 90 120 150 180 210
M AN U
Time (min)
635
3.0 Self MOF-5 ZnO 350-3h 400-3h 450-1h 450-2h 450-3h 450-4h 500-3h
TE D
2.0
b
1.5 1.0
EP
ln(C0/C)
2.5
SC
C/C0
a
0.5
AC C
0.0
0
636
20
40
60
80 100 120 140 160
Time (min)
637
Fig. 9 Photocatalytic activity of MOF-5 and C@ZnO hybrids for the degradation of
638
RhB under visible light.
639
36
ACCEPTED MANUSCRIPT
1.0
RI PT
C/C0
a
0.5
0.0
-60 -30
0
SC
RhB Self C@ZnO +IPA +KI +BQ
30 60 90 120 150 180 210
M AN U
Time (min)
640
2.5
b
2.0
ln(C0/C)
TE D
RhB Self C@ZnO +IPA +KI +BQ
1.5
EP
1.0 0.5
AC C
0.0
0
641
20
40
60
80 100 120 140 160
Time (min)
642
Fig. 10 Effects of different scavenger addition in the photocatalytic degradation of
643
RhB under visible light. (IPA: 100 mM, KI: 100 mM, BQ: 10 mM, C@ZnO: 100 mg,
644
RhB: 100 mL 1mg/L )
645 37
ACCEPTED MANUSCRIPT 646
a
M AN U
SC
RI PT
Intensity (a.u.)
10 min 0 min
3420 3450 3480 3510 3540 3570 3600
Magnetic Field (Gauss) 647
TE D
10 min 0 min
AC C
EP
Intensity (a.u.)
b
3420 3450 3480 3510 3540 3570 3600
Magnetic Field (Gauss)
648 649
Fig. 11 ESR signals of the (a) DMPO- O2- (b) DMPO- OH- with visible light
650
irradiation for C@ZnO 450-2h.
651 38
ACCEPTED MANUSCRIPT 652
2.0
RI PT
1.5
0.5
SC
1.0
-0.50 V
M AN U
Csc-2(1011cm4F2)
100 Hz
0.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
Potential (V vs SCE) 653
TE D EP
655
Fig. 12 Mott−Schottky plot for the C@ZnO electrodes.
AC C
654
39
ACCEPTED MANUSCRIPT 656 657
1 st
2 nd
4 th
3 rd
0.4
0.2
0.0
5 th
SC
0.6
0
100
200 0
100
M AN U
(C0-C)/C0
0.8
RI PT
1.0
200 0
100
200 0
100
200 0
100
200
Time (min)
TE D
658
Fig. 13 Recycling experiments of visible light photocatalytic degradation of RhB over
660
the C@ZnO 450-2h hybrid.
AC C
EP
659
40
ACCEPTED MANUSCRIPT
Research highlights
Carbon-doped octahedral zinc oxide with porous structure was synthesized using metal organic frameworks (MOFs) as a precursor. The band gap of ZnO was shortened from 3.20 eV to 3.09 eV by C doping.
The unique porous structure from MOFs and a narrow band gap from C doping
The superoxide radicals (•O2-) is the main oxidative species for the degradation of
EP
TE D
M AN U
RhB.
AC C
SC
allow C@ZnO octahedral to utilize visible light.
RI PT