High-efficient one-pot synthesis of carbon quantum dots decorating Bi2MoO6 nanosheets heterostructure with enhanced visible-light photocatalytic properties

Article abstract of DOI:10.1016/j.jallcom.2017.06.130

In this study, we reported a one-pot hydrothermal synthesis method for the preparation of nano-heterostructured composite carbon quantum dots (CQDs)/Bi₂MoO. Carbon quantum dots were in-situ synthesized and grafted onto Bi₂MoO₆ nanosheets via hydrothermal oxidation of glucose. For comparison studies, carbon quantum dots and Bi₂MoO₆ were also synthesized by the same way. The structures and compositions of the photocatalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–vis diffuse reflectance spectroscopy and Raman spectroscopy. The photocatalytic activity of the photocatalysts was evaluated by degradation of Rhodamine B (RhB) and methylene blue (MB) under visible light irradiation. The resulting (CQDs)/Bi₂MoO₆ composites possess excellent photocatalytic activity and stability towards the degradation of Rhodamine B (RhB) and methylene blue (MB) in aqueous solution under visible light irradiation. When the mass ratio of Bi₂MoO₆ to glucose is 1:1, the composite showed the highest photocatalytic activity for RhB and MB degradation. A possible charge transfer and the degradation mechanism of CQDs/Bi₂MoO₆ materials were proposed to understand the relationship between structure and performance of the photocatalysts. The present work proposed a new method for the preparation of CQDs modified Bi₂MoO₆ using a facile method, which provided a new insight into the design and fabrication of advanced photocatalyst for energy and environmental application.

Full text of DOI:10.1016/j.jallcom.2017.06.130

Accepted Manuscript
High-efficient one-pot synthesis of carbon quantum dots decorating Bi MoO
nanosheets heterostructure with enhanced visible-light photocatalytic properties
2
6
Chaoyang Sun, Qiuhua Xu, Yu Xie, Yun Ling, Julong Jiao, Huihua Zhu, Jinsheng
Zhao, Xiaoming Liu, Bin Hu, Dan Zhou
PII:
S0925-8388(17)32129-1
10.1016/j.jallcom.2017.06.130
JALCOM 42199
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 1 April 2017
Revised Date: 2 June 2017
Accepted Date: 12 June 2017
Please cite this article as: C. Sun, Q. Xu, Y. Xie, Y. Ling, J. Jiao, H. Zhu, J. Zhao, X. Liu, B. Hu, D.
Zhou, High-efficient one-pot synthesis of carbon quantum dots decorating Bi MoO nanosheets
2
6
heterostructure with enhanced visible-light photocatalytic properties, Journal of Alloys and Compounds
(2017), doi: 10.1016/j.jallcom.2017.06.130.
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
2
3
High-efficient One-pot Synthesis of Carbon Quantum Dots
Decorating Bi2MoO6 Nanosheets Heterostructure with Enhanced
Visible-light Photocatalytic Properties
4
5
6
Chaoyang Sun1, Qiuhua Xu1*, Yu Xie1*, Yun Ling1, Julong Jiao1, Huihua
Zhu1, Jinsheng Zhao2*, Xiaoming Liu1, Bin Hu1, Dan Zhou1
7
8
1. College of Environment and Chemical Engineering, Nanchang
Hangkong University, Nanchang 330063, P. R. China
9
2. College of Chemistry and Chemical Engineering, Liaocheng University,
Liaocheng 252059, P.R..China
10
11
12 *Corresponding author: Dr. and Prof. Yu Xie, E-mail:
13 xieyu_121@163.com, Dr. and Prof. Jinsheng Zhao, E-mail:
14 j.s.zhao@163.com, Qiuhua Xu, E-mail: xuqiuwa@163.com, Tel:
15 +86(791) 83953373; Fax: +86(791) 83953373.
16
17 Abstract
18
In this study, we reported a one-pot hydrothermal synthesis
19 method for the preparation of nano-heterostructured composite carbon
20 quantum dots (CQDs)/Bi2MoO. Carbon quantum dots were in-situ

ACCEPTED MANUSCRIPT
21 synthesized and grafted onto Bi2MoO6 nanosheets via hydrothermal
22 oxidation of glucose. For comparison studies, carbon quantum dots
23 and Bi2MoO6 were also synthesized by the same way. The structures
24 and compositions of the photocatalysts were characterized by X-ray
25 diffraction (XRD), X-ray photoelectron spectroscopy (XPS),
26 scanning electron miscroscopy (SEM), transmission electron
27 microscopy(TEM), UV-vis diffuse reflectance spectroscopy and
28 Raman spectroscopy. The photocatalytic activity of the
29 photocatalysts was evaluated by degradation of Rhodamine B (RhB)
30 and methylene blue (MB) under visible light irradiation. The
31 resulting (CQDs)/Bi2MoO6 composites possess excellent
32 photocatalytic activity and stability towards the degradation of
33 Rhodamine B (RhB) and methylene blue (MB) in aqueous solution
34 under visible light irradiation. When the mass ratio of Bi2MoO6 to
35 glucose is 1:1, the composite showed the highest photocatalytic
36 activity for RhB and MB degradation. A possible charge transfer and
37 the degradation mechanism of CQDs/Bi2MoO6 materials were proposed
38 to understand the relationship between structure and performance of the
39 photocatalysts. The present work proposed a new method for the
40 preparation of CQDs modified Bi2MoO6 using a facile method,
41 which provided a new insight into the design and fabrication of
42 advanced photocatalyst for energy and environmental application.

ACCEPTED MANUSCRIPT
43 Keywords: Nano-Heterostructure, Bi2MoO6, Carbon quantum dots,
44 Photocatalysis, Visible-light

ACCEPTED MANUSCRIPT
45 1. Introduction
The rapid development of the industry in recent years has made the
46
47 issues of environment pollution and energy crisis become increasingly
48 prominent[1-3]. Realizing the seriousness of the problem, many researchers
49 are dedicated to the studies on eco-friendly technologies in the areas of
50 energy production and pollutants degradation. As a green technology,
51 semiconductor photocatalysis are considered to be a promising
52 technology in treating environment pollutants, especially the treatment of
53 environmental organic contaminants [4]. In the past few years, the
54 traditional semiconductor TiO2 has been widely studied [5-7]. However, the
55 application of TiO2 semiconductor is great limited because it responds to
56 only UV light (only around 4% of the solar spectrum) for the reason of its
57 relatively wide-band gap (3.2 eV). Therefore, the development of visible
58 light active photocatalysts became a current challenge and the focus of
59 research [8-12]
.
60
Bi2MoO6 is the simplest Aurivillius oxide and shows excellent
61 visible photocatalytic activity for degradation of organic pollutants and
62 water splitting[13,14]
.
However, the quick recombination of
63 photo-generated electrons and holes greatly reduces the quantum yield,
64 which is the major limitation to achieve a high photocatalytic efficiency
65 of the pure Bi2MoO6 [15]. In order to improve the photocatalytic activity of
66 Bi2MoO6, researchers have done much work, including coupling with

ACCEPTED MANUSCRIPT
67 various semiconductors like TiO2[16], MoS2[17], BiPO4[18], BiOI[19] and
68 Bi2O2CO3[19], loading of noblue metal Ag[20], and incorporation with
[23]
69 Carbon-based materials such as g-C3N4[21], RGO[22] and C60
.
70
It should be noted that carbon quantum dots (CQDs), as a kind of
71 novel nanostructured carbon-based materials, have recently drawn
72 increasing research attention due to their enormous potential for a wide
73 range of technical applications[24, 25]. In comparison to the traditional
74 semiconductor quantum dots, CQDs offer certain advantages on aqueous
75 solubility, photostability, nontoxicity, biocompatibility, cheaper cost and
76 good electrical conductivity[24, 26, 27]. Therefore, CQDs can be applied in
77 various fields, such as energy storage[24], nanomedicine, chemical
78 imaging, Photocatalysis, electrocatalysis[28] and fluorescent sensor[29].
79 Besides, CQDs possess excellent up-conversion luminescence properties
80 (absorb visible light and convert it to shorter wavelength light) and have
81 unique property that can trap and transfer the photo-induced electrons[27],
82 which have been applied to design a series of photocatalytic
83 nanocomposites to enhance their photocatalytic properties, such as
[30]
[31]
[33]
84 CQDs/TiO2
,
CQDs/SiO2
,
CQDs/CdS[32],
CQDs/Fe2O3
,
85 CQDs/ZnO[34], CQDs/Cu2O[35].
86
In this work, we combined the merits of CQDs and Bi2MoO6, and
87 synthesized the nano-heterostructured CQDs/Bi2MoO6 by a simple
88 one-pot hydrothermal method characterized by the decoration of

ACCEPTED MANUSCRIPT
89 two-dimensional (2D) Bi2MoO6 nanosheets with Zero-dimensional (0D)
90 CQDs. As expected, the obtained composite showed a significant
91 improvement of photocatalytic activity under visible light and excellent
92 cycling stability for removal of organic pollutants compared with the pure
93 Bi2MoO6 nanosheets.
94
95 2. Experimental
96 2.1. Materials preparation
97
The Bi2MoO6 nanosheets were synthesized by solvothermal synthesis
98 method. All chemicals were analytical grade and used without further
99 purification. The bismuth molybdate materials were also synthesized by
100 solvothermal method. In a typical synthesis, 1 mmol of
101 Bi(CH3COOH)3·5H2O (J&K Scientific) and 0.071 mmol of
102 (NH4)6Mo7O24·4H2O (Sinopharm Chemical Reagent Co., Ltd) were
103 dissolved in 20 ml of deionized water and stirred for 30 min. Then 0.3 g
104 of urea was dissolved in 20 ml of ethanol under magnetic stirring for 10
105 min, and the obtained solution was added to the former solution. The
106 mixture was dispersed by ultrasonication for 10min and then to
107 hydrothermal treatment in a 50ml Teflon-lined stainless steel autoclave at
108 160 °C for 10h in an oven. After cooling to room temperature, the solid
109 product was separated by centrifuge at 10000rpm, and then washed
110 several times with water and ethanol, successively. The resultant powder

ACCEPTED MANUSCRIPT
111 was dried at 60 °C for 6 h under ambient pressure.
112
CQDs were synthesized by the “bottom-up” method via
113 hydrothermal process taking glucose (Sinopharm Chemical Reagent Co.,
114 Ltd) as molecular precursors. 0.1 g of glucose was dissolved in 100 mL of
115 DI water to form a solution of 1g/L. After that, 40 ml of above aqueous
116 solution was transferred to a 50 ml Teflon-lined stainless steel
117 autoclave and kept at 160 °C for 6 h. The obtained solution was then
118 centrifuged to remove large particles. The resultant dark yellow solution
119 was water soluble CQDs.
120
CQDs modified Bi2MoO6 were prepared by a hydrothermal
121 method. Specifically, certain amount of glucose (0-0.15g) was dissolved
122 in 40 ml of distilled water. Then 0.1 g of Bi2MoO6 was added to the
123 above mixture. After being stirred for 30 min at room temperature,
124 the mixture was subject to ultrasonic treatment for 10 min. The
125 resulting solution was transferred to a 50 ml Teflon-lined stainless
126 steel autoclave and kept at 160 °C for 6 h. After cooled down to room
127 temperature, the solid product was collected by centrifugation at
128 10000rpm. The obtained precipitation was washed several times with
129 water and ethanol and dried at 60 °C for 10 h. After that, the resulted
130 product was milled into powder. Finally the as-prepared sample was
131 calcined at 300 °C for 4 h in air with the heating rate of 2 °C/min.
132 Depending on the mass of glucose, The as-prepared catalysts were

ACCEPTED MANUSCRIPT
133 denoted to BMOCX, where “X=1-3” represent 0.05g, 0.1g, and 0.15 g of
134 glucose, respectively, and the BMO represents the pure Bi2MoO6.
135 2.2. Characterization
136
The morphology and microstructure was examined by a scanning
137 electron microscopy (SEM, FEI, Holand) in a secondary electron
138 scattering mode at 15 kV and transmission electron microscopy (TEM;
139 JEOL,JEM-2100F) that was operated with accelerating voltage of 200 kV.
140 The crystal phase was investigated by X-ray diffraction (XRD, Rigaku,
141 RINT 2000) using Cu Kα (λ= 0.15418 Å) radiation in the 2θ range of
142 5o–80o at a scan rate of 40/min. Raman spectra were conducted on a laser
143 Raman spectrometer with a back scattering con-figuration, and an Ar+
144 laser (20 mW, 532 nm) was used as excitation source. The
145 Brunauer-Emmett-Teller (BET) surface area of samples was measured by
146 means of N2 adsorption/desorption at 77K over a NOVA 2000e
147 (Quantachrome) equipment. The chemical status and surface chemical
148 composition of Bi, Mo, O and C elements were analyzed by X-ray
149 photo-electron spectroscopy (XPS). The measurements were carried on
150 an X-ray photoelectron spectrometer (ESCALAB MK II) using MgKa
151 (1253.6 eV) X-rays as the excitation source and C1s (284.6 eV) for
152 calibration.
153
To determine the band gap of the materials, ultraviolet–visible
154 (UV-Vis) diffusion reflectance spectra were measured with a

ACCEPTED MANUSCRIPT
155 SHIMADZU UV-2550 spectrophotometer using BaSO4 as background at
156 room temperature. Valence band (VB) XPS spectra for CQDs/Bi2MoO6
157 heterostructures were also obtained by using the same XPS system.
158 Measurements were performed using an ultrahigh vacuum chamber with
159 a base pressure below 5 × 10-9 Torr. The VB spectra were recorded in the
160 binding energy range from -5 to 20 eV with analyzer pass energy of 20
161 eV and step size of 0.1 eV.
162 2.3. Measurement of Photocatalytic activity
163
Photocatalytic activitiy of the materials was evaluated in the
164 degradation of Rhodamine B (RhB) and methylene blue (MB) under
165 visible light. A 300W Xe lamp (PLS-SXE300, Perfect Light Company,
166 Beijing, China) with a cut-off filter (λ>420 nm) was used as light source,
167 and the light intensity was 100 mW/cm2. In each experiment, 50 mg of
168 photocatalyst was added in an aqueous solution of RhB (50 ml 10mg/L),
169 MB(50 ml 10mg/L), respectively. Before irradiation, the mixture was
170 kept in the dark under magnetically stirring for 1h to establish
171 adsorption–desorption equilibrium between the model pollutants and
172 photocatalyst. After that, the mixture was irradiated under visible light for
173 120 min. During the reaction process, 3 ml solution were sampled at 20
174 min intervals and centrifuged to remove photocatalyst nanoparticles. The
175 concentration was monitored by measuring the maximum absorbance at
176 554nm for RhB and 664nm for MB through a Shimadzu UV-2550

ACCEPTED MANUSCRIPT
177 UV–vis spectrophotometer.
178 2.4. Characterization
179
The photocurrent measurements were examined by a standard
180 three-electrode, single-compartment quartz cell on a CHI660D
181 electrochemical workstation. The as prepared samples electrode with an
182 active area of 1 cm2 were used as the working electrode. The working
183 electrode was prepared by dip-coating method. The as-prepared Bi2MoO6
184 and CQDs/Bi2MoO6 samples were thoroughly dispersed in deionized
185 water, and then cast onto 3cm × 1cm FTO substrates, respectively. The
186 films were sufficiently dried under room temperature to obtain the
187 Bi2MoO6 and CQDs/Bi2MoO6 thin film electrodes. A graphite electrode
188 and a standard calomel electrode were used as the counter electrode and
189 reference electrode, respectively. 0.5 M sodium sulfate was used as the
190 electrolyte solution for photocurrent measurement. A 300 W Xe lamp
191 (PLS-SXE300, Perfect Light Company, Beijing, China) was utilized as
192 light source and was positioned 10 cm away from the
193 photoelectrochemical cell, and the light intensity was turned to 100
194 mW/cm2.
195
196 3. Results and discussion
197 3.1. Crystal phase and textural properties
198
The microscopic morphologies of Bi2MoO6 and CQDs/Bi2MoO6

ACCEPTED MANUSCRIPT
199 were initially studied by SEM. As depicted in Fig. 1a-b, SEM images
200 of pure Bi2MoO6 showed a sheet-like morphology with the length in
201 the range of tens to hundreds nanometers and thickness of about 14
202 nanometers. Fig. 1c-h shows the morphology of Bi2MoO6 modified
203 by CQDs with different ratios. The CQDs are too small to see from the
204 SEM images, and we can see that there is no obvious change of
205 morphology of Bi2MoO6 after modified process.
206
TEM was used to further study the morphologies of CQDs and
207 CQDs/Bi2MoO6 composites. As shown in Fig. 2a-b, the formed
208 CQDs are mostly spherical dots with a size of approximately 9-20
209 nm and uniformly dispersed. Fig. 2c-d shows that the pure Bi2MoO6
210 is consisted of many irregular nanosheets with a smooth surface. Fig.
211 2e-j show the TEM images of CQDs modified Bi2MoO6 materials
212 with different ratio. It can be seen that there are few CQDs on the
213 surface of the Bi2MoO6 in the sample of BMOC1 (Fig. 2e-f). In
214 contrast, it can be easily seen from the pictures (Fig. 2e-f) that the a
215 number of CQDs show black and gray contrast dots with diameter
216 of approximately 7-10 nm and are evenly distributed on the surface
217 of Bi2MoO6 nanosheets in the sample of BMOC2. The TEM images
218 of the BMOC3 sample (Fig. 2i-j) show that much CQDs accumulate on
219 the surface of Bi2MoO6 nanowires
，which may lead to a decrease in
220 photocatalytic efficiency. The TEM images of all the samples

ACCEPTED MANUSCRIPT
221 confirmed that Bi2MoO6 was effectively and completely coupled
222 with CQDs and the introduction of CQDs to Bi2MoO6 doesn’t affect
223 the main morphology of Bi2MoO6.
224 3.2. Crystal phase and textural properties
225
Fig. 3a shows the XRD pattern of the as-prepared Bi2MoO6 and a
226 series of CQDs/Bi2MoO6 nanocomposites. As indicated by XRD
227 patterns in Figure. 3a, the diffraction peaks can be indexed to the
228 Bi2MoO6 without any impurity phase (PDF No.21-0102). The diffraction
229 peaks of pure Bi2MoO6 at 2θ = 10.98°, 19.56°, 23.50°, 28.36°, 32.64°,
230 33.18°, 34.40°, 36.12°, 39.58°, 39.64°, 43.02°, 44.68°, 46.86°, 47.24°,
231 55.62°, 56.32° and 58.52° can match well with the (020), (120), (111),
232 (131), (002), (060), (220), (151), (240), (042), (142), (080), (202),
233 (062)/(260), (133)/(331), (191) and (262) crystal planes of the
234 orthorhombic phase Bi2MoO6. The diffraction peaks of pure Bi2MoO6
235 have no significant change after thermal treatment at 300 °C for 4 h,
236 indicating that the thermal treatment process doesn’t change the phase
237 composition. After coupling with CQDs, similar XRD pattern are
238 obtained from CQDs/Bi2MoO6 samples. All these diffraction peaks
239 still exist, suggesting that the heterogeneous process doesn’t affect
240 the crystal structure of Bi2MoO6. Moreover, no distinctive peaks of
241 CQDs can be observed, and this can be attributed to the low content
242 of CQDs in the samples. Such phenomenon also appeared in other

ACCEPTED MANUSCRIPT
243 similar systerms[27, 36]
.
244
The Raman analysis was used to further characterize the crystal
245 structure of obtained samples. The Raman spectrum of as-prepared
246 samples proved the presence of Bi2MoO6. As shown in Fig. 3b, the A1g
247 peak at 791 cm-1 could be assigned to the symmetric stretch of a MoO6
248 octahedron, while the peak at 710 and 841 cm-1 (A2u mode) showed the
249 characteristics of orthorhombic distortions of the MoO6 octahedron in
2+
250 Bi2MoO6 with Aurivillius structure of layered slabs (Bi2O2 ) connected
251 through the corner sharing of distorted MoO6 octahedra. The presence of
252 peaks below 400cm-1 resulted from the bending modes of the MoO6
253 octahedra merging with stretching and bending modes of the Bi-O
254 polyhedra[37, 38]. Additionally, bands at 398, 350, 323, 289, 279, 192 and
255 134 were found, which correspond well to those reported for Bi2MoO6 in
256 the literature [37, 39]. Moreover, the spectrum peaks of CQDs/Bi2MoO6
257 samples showed slight shift with the modification of CQDs. This
258 change indicated that there existed some electron interactions between
259 the surface of Bi2MoO6 and CQDs, which may be beneficial to the
260 enhancement of the photocatalytic efficiency of Bi2MoO6, which
261 will be discussed subsequently.
262
N2 physical adsorption was carried out to study the specific
263 surface area of the samples. Fig. 4 showed the N2
264 adsorption-desorption isotherms and pore size distributions (PSDs)

ACCEPTED MANUSCRIPT
265 of all the samples. It could be seen from Fig. 4a, All of them
266 displayed typical type-IV isotherms with a type H3 hysteresis loop at
267 high relative pressure (0.6-1.0), suggesting the existence of the
268 mesoporous structure. The BET specific surface area of pure
269 Bi2MoO6 was 15.81 m2/g. After decorating by CQDs, the BET
270 specific surface area of the sample BMOC1, BMOC2 and BMOC3
271 were measured to be 16.27, 16.83 and 12.73 m2/g, respectively. The
272 BET specific surface area firstly increased and then decreased with
273 the increase of mass ratio of carbon quantum dots, which was
274 because the aggregation phenomenon of CQDs caused by loading
275 excessive CQDs on Bi2MoO6, and the agglomeration could cause
276 the decrease of BET specific surface area. The pore size distribution
277 curves of all the samples were shown in Fig. 4(b), we can see that
278 all of them showed a wide distribution of the pore size, including
279 small mesopores of 10 nm and some lager mesopores about 30nm,
280 which were likely originated from the space between the packed
281 nanosheets. Thereby, this hierarchical porous structure and higher
282 specific surface areas of the BMOC2 sample make it possessed
283 more active sites and fully contacted with contaminants, and it was
284 favourable to the photocatalytic performance. The above results
285 implied that the sample of BMOC2 may show the highest
286 photocatalytic activity among all the samples.

ACCEPTED MANUSCRIPT
287 3.3. Band gap and chemical state analysis
288
The optical properties of the Bi2MoO6 and a series of
289 CQDs/Bi2MoO6 nanocomposites were investigated using UV-vis
290 diffuse reflectance spectra (DRS) measurement. As shown in Figure.
291 5a, all the samples possess a similar light absorption edge. Some
292 research has shown that the absorption of some photocatalysts were
293 enhanced in visible region after coupling with CQDs[27, 40, 36]. But in
294 the present study, CQDs show a small effect on the optical trapping
295 capability of Bi2MoO6. As shown in Figure. 5a, the absorption
296 intensity of visible light of CQDs modified Bi2MoO6 materials were
297 increased slightly. The reason for this could be the fact that some
298 CQDs are partially oxidized during heat treatment, which present
299 weak absorption in the range of visible light [41]. The absorption edge
300 of Bi2MoO6 were around 479 nm, corresponding to band gaps of 2.73 eV
301 calculated via the Kubelka-Munk function, (hνF(R∞))2 vs. Because of the
302 similar band gaps of all the as-prepared samples, the light harvesting
303 capability shouldn’t be the most significant factor attributing to the
304 enhanced photocatalytic activity of Bi2MoO6.
305
The valence band (VB) of Bi2MoO6 can be calculated according
306 to the following empirical equation: EVB=X-Ee+0.5Eg[42], where Eg is
307 the band gap of the semiconductor, Ee is the energy of free electron
308 on the hydrogen scale (about 4.5 eV), and X is the electronegativity

ACCEPTED MANUSCRIPT
309 of the semiconductor (for Bi2MoO6, X is 5.55 eV)[43]. Therefore, the
310 EVB values of Bi2MoO6 are calculated to be 2.41 eV. Based on the
311 above result, the CB value of Bi2MoO6 was caculated by Mulliken
312 electronegativity theory: ECB=EVB-Eg[44]. Accordingly, the CB value
313 of Bi2MoO6 was determined to be -0.32 eV.
314
XPS was carried out to investigate the surface properties and
315 components of the Bi2MoO6 and CQDs/Bi2MoO6 samples. As shown in
316 Figure. 6, the XPS survey spectra (Figure. 6a) indicating that the Bi, Mo,
317 C and O elements were detected on the surface of Bi2MoO6 and
318 CQDs/Bi2MoO6 samples. The Bi 4f and Mo 3d spectra of all samples
319 showed that all the Bi and Mo species were present in Bi3+ and Mo6+ state,
320 respectively. Figure 6b showed the Bi 4f high-resolution XPS spectra of
321 the pure Bi2MoO6 and CQDs/Bi2MoO6 samples. In the Bi 4f spectra, The
322 existence of Bi (Ⅲ) in the samples of pure Bi2MoO6 was evidenced by a
323 shoulder observed on the main peak at about 159.1 eV, which was
324 assigned to Bi 4f7/2. The higher binding energies peak at about 164.4 eV
325 was ascribed to Bi 4f5/2. Meanwhile, the binding energies (BE) of 232.4
326 and 235.6 eV in Mo 3d high-resolution XPS spectra of pure Bi2MoO6
327 sample (Figure. 5c) were attributed to Mo 3d5/2 and Mo 3d3/2 of Mo6+,
328 respectively[45]. The O1s high-resolution XPS spectra of pure Bi2MoO6
329 (Figure. 5d) presented one peak at 530.0 eV, indicating the O species in
330 the pure Bi2MoO6 sample were present in O2- state and originated from

ACCEPTED MANUSCRIPT
331 Bi2MoO6 crystal[46]. When loading CQDs on Bi2MoO6, it was noted
332 that the peaks of O 1s, Bi 4f and Mo 3d high-resolution XPS spectra in
333 the CQDs/Bi2MoO6 samples showed apparent shift in contrast with pure
334 Bi2MoO6, The peaks corresponding to Bi 4f (4f7/2 and 4f5/2) in the
335 spectras of BMOC1, BMOC2 and BMOC3 lightly shifted (about 0.1,
336 0.2 and 0.2 eV) toward higher binding energies as compared to pure
337 Bi2MoO6. The binding energies of Mo 3d5/2, Mo 3d3/2 and O1s peaks
338 in BMOC1, BMOC2 and BMOC3 were also slightly higher than
339 those of pure Bi2MoO6. The above results indicated that there existed
340 some electron interactions between Bi2MoO6 and CQDs. Furthermore,
341 it can be seen that the main peaks of C1s high-resolution XPS spectra of
342 the CQDs/Bi2MoO6 samples (Figure. 6e) were at about 285.0 eV, 289.0
343 eV and 289.2 eV, which corresponded to the C-C bond with sp2 orbital,
344 C-O-C bond, and C=O bond, respectively[33]. The above results
345 demonstrate the coexistence of CQDs and Bi2MoO6 in the
346 CQDs/Bi2MoO6 hybrid material.
347
Moreover, the VB value of Bi2MoO6 was investigated by XPS
348 valence spectra. As shown in Fig. 8d, the Bi2MoO6 displayed the
349 maximum energy edge of VB at about 2.40 eV and the result is consistent
350 with the estimated value as before.
351 3.4. Photocatalytic performance analysis
352
The photocatalytic activities of as-prepared samples were

ACCEPTED MANUSCRIPT
353 evaluated by degradation of model pollutants in water under visible light
354 irradiation (using a 300W Xe lamp with a cut-off filter at λ>420nm). We
355 can see from Fig. 7a, there was almost no degradation of Rhodamine B
356 after irradiation for 120 min in the absence of catalyst, indicating that the
357 Rhodamine B molecules was sufficiently stable under visible light
358 irradiation for a long time and its selfphotolysis can be ignored in
359 photocatalytic process. Besides, the pure carbon quantum dots only
360 presented an excellent absorption property and have no any photocatalytic
361 activity. The CQDs/Bi2MoO6 materials show higher photocatalytic
362 efficiency than that of pure Bi2MoO6. It suggested that some
363 heterojunctions should be formed between CQDs and Bi2MoO6
364 nanosheets, which played crucial role in the transfer and separation of
365 photo-generated carriers. When irradiated by visible light, the electrons
366 Bi2MoO6 could be excited from the VB to the CB of Bi2MoO6. Due to the
367 excellent electronic conductivity of the CQDs, the photo-generated
368 electrons could be easily scavenged by CQDs, leading to efficient
369 separation and reduced possibility of the recombination of
370 photogenerated electron–hole pairs. Moreover, among all these samples,
371 BMOC2 exhibited the highest activity for the degradation of RhB under
372 visible light. After 120min visible light irradiation, about 100% of RhB
373 can be degraded in the presence of the sample BMOC2, whereas the
374 degradation rate is only about 85% over pure Bi2MoO6. This result

ACCEPTED MANUSCRIPT
375 indicates that the photocatalytic activity of composites is closely related
376 to the mass ratio of two components. In other word, suitable mass ratio
377 could form an efficient junction interface between CQDs and Bi2MoO6
378 that is favorable to the photo-generated carriers transfer, and thereby
379 effectively restrains the recombination of photo-generated carriers. In
380 contrary, excess CQDs deposited on the surface of Bi2MoO6 would
381 weaken the absorption of Bi2MoO6 to visible light. Besides, a number of
382 carbon quantum dots will reunite on the surface of Bi2MoO6, which will
383 also reduce the photocatalytic efficiency of the catalyst.
384
In order to study the wider practical application, methylene blue
385 (MB) is chosen as a different kind of degradation object to further
386 evaluate the photocatalytic properties. As shown in Fig. 7c, The CQDs
387 modified Bi2MoO6 materials still show more excellent photocatalytic
388 performance on the degradation of methylene blue (MB) than that of pure
389 Bi2MoO6. The sample BMOC2 show the highest photoactivity that
390 was about 100% removal efficiency of MB after 120min visible light
391 irradiation, only 91% of MB can be degraded at the same time for the
392 pure Bi2MoO6. Fig. 7e and f showed the temporal UV-vis absorption
393 spectral changes during the photocatalytic degradation of RhB and MB
394 solution in the presence of the sample BMOC2 under visible light
395 irradiation. It can be seen that the intensity of the characteristic absorption
396 peak of RhB and MB at about 554 nm and 664 nm decreased gradually

ACCEPTED MANUSCRIPT
397 and nearly disappeared after 120 min. All the photocatalytic degradation
398 results imply that loading CQDs on Bi2MoO6 can obviously improve
399 the photocatalytic efficiency under visible light and the strategy of
400 CQDs modification is efficient.
401
In order to further compare the photocatalytic efficiency of these
402 samples, the reaction kinetics of RhB and MB degradation were
403 investigated. The reaction rate constant were calculated by the linear
404 fitting of the degradation time against ln(CO/Ct). As shown in Figure 7b
405 and Figure 7d, the processes of RhB and MB degradation followed the
406 pseudo-first-order kinetics, and the sample BMOC2 had the maximum
407 reaction rate constants both in degradation of RhB and MB. The reaction
1
408 rate constants in degradation of RhB and MB are 0.029 min− and 0.048
1
409 min− for the sample BMOC2, which are almost 2.0 times and 2.4 times
410 as high as that of pure Bi2MoO6, respectively. Moreover, Bi2MoO6
411 nanomaterials studied for photocatalysis under visible light irradiation in
412 recent years were also shown in Table S1.
413
The stability and reusability are considerable features of the
414 photocatalyst, which are crucial to the practical application. To test the
415 stability and reusability of the prepared CQDs/Bi2MoO6 hybrid material,
416 the photocatalytic decomposition of RhB was repeated ten times. In this
417 study, BMOC2 after photocatalytic degration of RhB for 80 min under
418 UV-Vis light was collected by the centrifugal and dried for the

ACCEPTED MANUSCRIPT
419 subsequent photocatalytic reaction cycles. In contrast, the same test
420 conditions were conducted for pure Bi2MoO6 and the results are shown in
421 Fig. 8b, from which it can be seen that BMOC2 sample can keep a high
422 efficient and stable photocatalytic performance after ten consecutive
423 cycles, which demonstrates the high stability of the photocatalysts. The
424 stability of pure Bi2MoO6, in contrast, performed less well. The
425 degradation rate of RhB was down from initial 92% to 75% after ten
426 consecutive cycles. In addition, there exist some evident fluctuations
427 during the cycle experiment for pure Bi2MoO6. All the result above
428 indicated that CQDs/Bi2MoO6 hybrid material performed extremely well
429 on both the stability and reusability.
430
The transient photo-current responses measurement was carried out
431 to investigate the effect of CQDs on the recombination of
432 electron-hole pairs of Bi2MoO6. As can be seen from Fig. 8a, the
433 photocurrent intensity of CQDs/Bi2MoO6 materials were higher than
434 that of pure Bi2MoO6, confirming that loading CQDs onto Bi2MoO6
435 nanosheets remarkably enhanced the photocurrent. The sample
436 BMOC2 has the highest photocurrent of 0.57-0.96 µA/cm2,
437 obviously higher than that of other samples, and the significant
438 improvement of photocurrent reveals that it could form heterojunction
439 on the interface between CQDs and Bi2MoO6 nanosheets and
440 therefore the photogenerated electrons in Bi2MoO6 could quickly

ACCEPTED MANUSCRIPT
441 transfer to CQDs, facilitating the photoelectrons and holes
442 separation, which was closely related to the photocatalytic
443 performance.
444 3.5. Detection of active species
445
In order to reveal the photocatalytic mechanism, we carried out some
446 experiment to detect the main oxidative species in the photocatalytic
447 process. In the experiment, tert butyl alcohol (TBA) and potassium iodide
.
448 (KI) as the scavengers of OH and photo-generated holes were added in
449 the catalytic system, respectively. Besides, experiment was carried out
.
-
450 under air-equilibrated and N2-bubbled to investigate the role of O2 . The
451 result as shown in Fig. 8c, the addition of TPA (100 µL) only had little
452 impact on the photodegradation of RhB, indicating that there are
.
453 lesser OH radicals in the RhB oxidation process. On the contrary, the
454 addition of KI resulted in a significative degree of inhibition of RhB
455 degradation, which demonstrate the indispensable role of photo-generated
456 holes existed. At the same time, the degradation rate of RhB was clearly
457 decreased when bubbled N2 into the solution, which demonstrated the
-
458 crucial role of .O2 evolved produced by dissolving oxygen in the reaction
459 system.
460 3.6. The proposed photocatalytic mechanism
461
Based on the above results, a mechanism for the improvement of
462 photocatalytic action of CQDs/Bi2MoO6 hierarchical structure is proposed

ACCEPTED MANUSCRIPT
463 in Scheme 1. Carbon quantum dots exhibit a synergistic role in the
464 CQDs/Bi2MoO6 hierarchical structure. Upon visible light irradiation, the
465 electron can be excited by the high-energy photon from the valance band
466 (VB) into the conduction band (CB). The photo-generated electrons in
467 Bi2MoO6 can transfer quickly to the CQDs because of their excellent
468 electric conductivity. As a result, the separation of photo-generated
469 electrons from holes can be effectively promoted and the degradation
470 efficiency is further improved.
471
The valance band (VB) of Bi2MoO6 was calculated to be +2.41 eV
472 (νs. NHE), which was more negative than the VB of .OH/H2O (+2.68eV
473 νs. NHE), indicating that photo-generated holes couldn’t oxidize H2O to
.
474 yield OH radicals in Bi2MoO6. Nevertheless, the CB potential of
475 Bi2MoO6 was calculated to be -0.32 eV (νs. NHE) that is more negative
476 than E (O2/.O2 ) (-0.046eV νs. NHE), which can easily reduce dissolved
-
.
-
.
477 O2 to produce O2 . As is well knew, the OH radicals also could be
478 generated from photo-generated electrons besides photo-generated
479 holes[53], which is why there exists a slight decrease for the
480 photodegradation efficiency of RhB in the presence of tert butyl alcohol.
481 4. Conclusions
482
In summary, this work developed a facile and green thermochemical
483 method for the preparation of easily recycled CQDs Bi2MoO6 composite.
/

ACCEPTED MANUSCRIPT
484 The deposited CQDs enhanced the photocatalytic activity of the
485 Bi2MoO6 nanosheet towards the degradation of RhB and MB under
486 visible light irradiation. It can be mainly attributed to the forming
487 efficient junction interface between CQDs and Bi2MoO6, which inhibits
488 the recombination of charge carriers to enhance photocatalytic activity.
489 The sample BMOC2 exhibited the highest photocatalytic performance
490 among all the samples, which is due to the formation of more efficient
491 heterojunctions between CQDs and Bi2MoO6, the enhancement of light
492 absorption, and the enlarged BET surface area. The findings in this work
493 proposed a new insight into the design and fabrication of CQDs modified
494 Bi2MoO6, which provides novel inspiration in developing CQD
495 modified other semiconductor materials.
496
497 Acknowledgements
498 This work was financially supported by the National Natural Science
499 Foundation of China (No.21667019, 21501088), the Natural Science
500 Foundation of Jiangxi Province (No.20151BAB213002), the Jiangxi
501 Province
Youth
Scientists
Cultivating
Object
Program
502 (No.20112BCB23017), the Key Laboratory of Photochemical Conversion
503 and Optoelectronic Materials, TIPC, CSA (No.PCOM201401) and Key
504 Project of Science and Technology Research of Jiangxi Provincial
505 Department of Education (No.DA201602063).
506

ACCEPTED MANUSCRIPT
507 References
508 [1] M.
Namvari,
H.
Namazi,
Synthesis
of
magnetic
509 citric-acid-functionalized graphene oxide and its application in the
510 removal of methylene blue from contaminated water, Polymer
511 International, 63 (2014) 1881-1888.
512 [2] P. Xu, G.M. Zeng, D.L. Huang, C.L. Feng, S. Hu, M.H. Zhao, C. Lai,
513 Z. Wei, C. Huang, G.X. Xie, Z.F. Liu, Use of iron oxide nanomaterials in
514 wastewater treatment: a review, Sci Total Environ, 424 (2012) 1-10.
515 [3] W. Chen, T.-Y. Liu, T. Huang, X.-H. Liu, G.-R. Duan, X.-J. Yang,
516 S.-M. Chen, A novel yet simple strategy to fabricate visible light
517 responsive
C,N-TiO2/g-C3N4heterostructures
with
significantly
518 enhanced photocatalytic hydrogen generation, RSC Adv., 5 (2015)
519 101214-101220.
520 [4] X. Liu, J. Chen, P. Liu, H. Zhang, G. Li, T. An, H. Zhao, Controlled
521 growth of CuO/Cu2O hollow microsphere composites as efficient
522 visible-light-active photocatalysts, Applied Catalysis A: General, 521
523 (2016) 34-41.
524 [5] T.S. Natarajan, M. Thomas, K. Natarajan, H.C. Bajaj, R.J. Tayade,
525 Study on UV-LED/TiO2 process for degradation of Rhodamine B dye,
526 Chemical Engineering Journal, 169 (2011) 126-134.
527 [6] M. Lazar, S. Varghese, S. Nair, Photocatalytic Water Treatment by
528 Titanium Dioxide: Recent Updates, Catalysts, 2 (2012) 572-601.

ACCEPTED MANUSCRIPT
529 [7] G.H. Safari, M. Hoseini, M. Seyedsalehi, H. Kamani, J. Jaafari, A.H.
530 Mahvi, Photocatalytic degradation of tetracycline using nanosized
531 titanium dioxide in aqueous solution, International Journal of
532 Environmental Science and Technology, 12 (2014) 603-616.
533 [8] Y. Miao, G. Pan, Y. Huo, H. Li, Aerosol-spraying preparation of
534 Bi2MoO6: A visible photocatalyst in hollow microspheres with a porous
535 outer shell and enhanced activity, Dyes and Pigments, 99 (2013) 382-389.
536 [9] J. Zhao, Q. Lu, C. Wang, S. Liu, One-dimensional Bi2MoO6
537 nanotubes: controllable synthesis by electrospinning and enhanced
538 simulated sunlight photocatalytic degradation performances, Journal of
539 Nanoparticle Research, 17 (2015).
540 [10] Y. Shimodaira, H. Kato, H. Kobayashi, A. Kudo, Photophysical
541 properties and photocatalytic activities of bismuth molybdates under
542 visible light irradiation, The Journal of Physical Chemistry B, 110 (2006)
543 17790-17797.
544 [11] G. Li, Z. Lian, W. Wang, D. Zhang, H. Li, Nanotube-confinement
545 induced
size-controllable
g-C3N4
quantum
dots
modified
546 single-crystalline TiO2 nanotube arrays for stable synergetic
547 photoelectrocatalysis, Nano Energy, 19 (2016) 446-454.
548 [12] S. Ghosh, N.A. Kouame, L. Ramos, S. Remita, A. Dazzi, A.
549 Deniset-Besseau, P. Beaunier, F. Goubard, P.H. Aubert, H. Remita,

ACCEPTED MANUSCRIPT
550 Conducting polymer nanostructures for photocatalysis under visible light,
551 Nat Mater, 14 (2015) 505-511.
552 [13] T. Yan, M. Sun, H. Liu, T. Wu, X. Liu, Q. Yan, W. Xu, B. Du,
553 Fabrication of hierarchical BiOI/Bi2MoO6 heterojunction for degradation
554 of bisphenol A and dye under visible light irradiation, Journal of Alloys
555 and Compounds, 634 (2015) 223-231.
556 [14] P. Wang, Y. Ao, C. Wang, J. Hou, J. Qian, A one-pot method for the
557 preparation of graphene–Bi 2 MoO 6 hybrid photocatalysts that are
558 responsive to visible-light and have excellent photocatalytic activity in
559 the degradation of organic pollutants, Carbon, 50 (2012) 5256-5264.
560 [15] F. Chen, C. Niu, Q. Yang, X. Li, G. Zeng, Facile synthesis of
561 visible-light-active BiOI modified Bi2MoO6 photocatalysts with highly
562 enhanced photocatalytic activity, Ceramics International, 42 (2016)
563 2515-2525.
564 [16] J. Li, X. Liu, Z. Sun, L. Pan, Novel Bi2MoO6/TiO2 heterostructure
565 microspheres for degradation of benzene series compound under visible
566 light irradiation, J Colloid Interface Sci, 463 (2016) 145-153.
567 [17] Y. Chen, G. Tian, Y. Shi, Y. Xiao, H. Fu, Hierarchical
568 MoS2/Bi2MoO6 composites with synergistic effect for enhanced visible
569 photocatalytic activity, Applied Catalysis B: Environmental, 164 (2015)
570 40-47.

ACCEPTED MANUSCRIPT
571 [18] X. Lin, D. Liu, X. Guo, N. Sun, S. Zhao, L. Chang, H. Zhai, Q.
572 Wang, Fabrication and efficient visible light-induced photocatalytic
573 activity of Bi 2 MoO 6/BiPO 4 composite, Journal of Physics and
574 Chemistry of Solids, 76 (2015) 170-177.
575 [19] Y.-S. Xu, W.-D. Zhang, Anion exchange strategy for construction of
576 sesame-biscuit-like Bi 2 O 2 CO 3/Bi 2 MoO 6 nanocomposites with
577 enhanced photocatalytic activity, Applied Catalysis B: Environmental,
578 140 (2013) 306-316.
579 [20] A. Phuruangrat, S. Putdum, P. Dumrongrojthanath, N. Ekthammathat,
580 S. Thongtem, T. Thongtem, Enhanced properties for visible-light-driven
581 photocatalysis of Ag nanoparticle modified Bi2MoO6 nanoplates,
582 Materials Science in Semiconductor Processing, 34 (2015) 175-181.
583 [21] J. Lv, K. Dai, J. Zhang, L. Geng, C. Liang, Q. Liu, G. Zhu, C. Chen,
584 Facile synthesis of Z-scheme graphitic-C 3 N 4/Bi 2 MoO 6
585 nanocomposite for enhanced visible photocatalytic properties, Applied
586 Surface Science, 358 (2015) 377-384.
587 [22] Y. Zhang, Y. Zhu, J. Yu, D. Yang, T.W. Ng, P.K. Wong, C.Y. Jimmy,
588 Enhanced photocatalytic water disinfection properties of Bi 2 MoO
589 6–RGO nanocomposites under visible light irradiation, Nanoscale, 5
590 (2013) 6307-6310.

ACCEPTED MANUSCRIPT
591 [23] X. Zhao, H. Liu, Y. Shen, J. Qu, Photocatalytic reduction of bromate
592 at C 60 modified Bi 2 MoO 6 under visible light irradiation, Applied
593 Catalysis B: Environmental, 106 (2011) 63-68.
594 [24] H. Hou, C.E. Banks, M. Jing, Y. Zhang, X. Ji, Carbon Quantum Dots
595 and Their Derivative 3D Porous Carbon Frameworks for Sodium Ion
596 Batteries with Ultralong Cycle Life, Advanced Materials, 27 (2015)
597 7861-7866.
598 [25] Y. Song, S. Zhu, B. Yang, Bioimaging based on fluorescent carbon
599 dots, RSC Advances, 4 (2014) 27184-27200.
600 [26] R. Zhang, W. Chen, Nitrogen-doped carbon quantum dots: facile
601 synthesis and application as a “turn-off” fluorescent probe for detection
602 of Hg 2+ ions, Biosensors and Bioelectronics, 55 (2014) 83-90.
603 [27] K. Li, F.-Y. Su, W.-D. Zhang, Modification of gC 3 N 4 nanosheets
604 by carbon quantum dots for highly efficient photocatalytic generation of
605 hydrogen, Applied Surface Science, 375 (2016) 110-117.
606 [28] S.Y. Lim, W. Shen, Z. Gao, Carbon quantum dots and their
607 applications, Chemical Society Reviews, 44 (2015) 362-381.
608 [29] B.B. Campos, R. Contreras-Cáceres, T.J. Bandosz, J.
609 Jiménez-Jiménez, E. Rodríguez-Castellón, J.C.E. da Silva, M. Algarra,
610 Carbon dots as fluorescent sensor for detection of explosive
611 nitrocompounds, Carbon, 106 (2016) 171-178.