Self-templating synthesis of nitrogen doped graphene quantum dots/3D bismuth oxyiodine hybrid hollow microspheres with improved visible-light excited photocurrent generation: Simultaneous electron transfer acceleration and bandgap narrowing

Article abstract of DOI:10.1016/j.jallcom.2017.09.120

This work presents a self-templating synthetic approach for the synthesis of nitrogen doped graphene quantum dots/3D bismuth oxyiodine (NGQDs/3DBiOI) hybrid hollow microspheres (HHMs) by a facile one-pot solvothermal reaction. In this protocol, the formation of NGQDs/3DBiOI hollow microstructure and the hybridization of NGQDs with BiOI semiconductor were achieved simultaneously in one-pot reaction using the NGQDs as template and dopant, which played a double-acting role in the reaction: (i) to modulate the morphology of BiOI microspheres with special hollow structure; (ii) to hybrid with BiOI for the formation of NGQDs/3DBiOI HHMs. Further investigations revealed that the resulted NGQDs/3DBiOI HHMs exhibit tunable visible-light driven photocurrent generation with varying the amount of NGQDs used in the reaction process, which was ascribed to the simultaneous electron transfer acceleration and bandgap narrowing of the NGQDs/3DBiOI HHMs. Moreover, when evaluated as a matrix for enzyme encapsulation, the resulting NGQDs/3DBiOI HHMs exhibited enhanced load capacity than pure BiOI microspheres by virtue of their particular nanostructure. The design of such three-dimensional NGQDs/3DBiOI HHMs with preferable performance facilitates further multifarious applications for enhanced photocatalysis, enzyme immobilization and biofuels.

Full text of DOI:10.1016/j.jallcom.2017.09.120

Journal of Alloys and Compounds 729 (2017) 27e37
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Self-templating synthesis of nitrogen doped graphene quantum
dots/3D bismuth oxyiodine hybrid hollow microspheres with
improved visible-light excited photocurrent generation: Simultaneous
electron transfer acceleration and bandgap narrowing
a
b
b
b
a
,
**
, Kun Wang b
, *
Qian Liu , Yuanyuan Yin , Nan Hao , Jing Qian , Hanping Mao
a
Key Laboratory of Modern Agriculture Equipment and Technology, School of Agricultural Equipment Engineering, Jiangsu University, Zhenjiang, 212013, PR
China
b
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 July 2017
Received in revised form
This work presents a self-templating synthetic approach for the synthesis of nitrogen doped graphene
quantum dots/3D bismuth oxyiodine (NGQDs/3DBiOI) hybrid hollow microspheres (HHMs) by a facile
one-pot solvothermal reaction. In this protocol, the formation of NGQDs/3DBiOI hollow microstructure
and the hybridization of NGQDs with BiOI semiconductor were achieved simultaneously in one-pot
reaction using the NGQDs as template and dopant, which played a double-acting role in the reaction:
11 September 2017
Accepted 12 September 2017
Available online 15 September 2017
(
i) to modulate the morphology of BiOI microspheres with special hollow structure; (ii) to hybrid with
BiOI for the formation of NGQDs/3DBiOI HHMs. Further investigations revealed that the resulted NGQDs/
DBiOI HHMs exhibit tunable visible-light driven photocurrent generation with varying the amount of
Keywords:
3
Self-templating synthesis
Nitrogen doped graphene quantum dot
Bismuth oxyiodine
Hybrid hollow microsphere
Improved photocurrent generation
NGQDs used in the reaction process, which was ascribed to the simultaneous electron transfer accel-
eration and bandgap narrowing of the NGQDs/3DBiOI HHMs. Moreover, when evaluated as a matrix for
enzyme encapsulation, the resulting NGQDs/3DBiOI HHMs exhibited enhanced load capacity than pure
BiOI microspheres by virtue of their particular nanostructure. The design of such three-dimensional
NGQDs/3DBiOI HHMs with preferable performance facilitates further multifarious applications for
enhanced photocatalysis, enzyme immobilization and biofuels.
©
2017 Elsevier B.V. All rights reserved.
1
. Introduction
induced photoactivity with the benefits from its unique layered
crystalline structure and suitable band gap [11,12]. In the past few
years, investigations on the controllable synthesis of BiOI micro-
spheres with special hollow structure have been reported [13e15].
For example, Xia et al. reported an ethylene glycol-assisted sol-
vothermal process for the synthesis of flowerlike BiOI hollow mi-
crospheres in the presence of ionic liquid 1-butyl-3-
methylimidazolium iodine [13]. Ren et al. developed a precipita-
tion route for the preparation of BiOI microspheres with hollow
structure in a water-ethanol mixed solution with the assistance of
polyvinyl pyrrolidone and citric acid [14]. It is worth mentioning
that extra additives such as ionic liquid and polyvinyl pyrrolidone
polymer were necessary in the formation of such BiOI hollow mi-
crospheres, which made the reaction system complicated and
costly. Considering the complicated and expensive methods for the
synthesis of hollow microsphere materials, it is highly desirable to
develop a facile and economic approach for the preparation of BiOI-
Owing to their unique structure-determined physical and
chemical properties, hollow micro-/nanostructures of inorganic
materials with defined shape and composition have attracted
increasing interests in a wide range of applications, such as dye-
sensitized solar cells, photocatalysis, drug delivery, and lithium-
ion batteries [1e6]. To date, numerous efforts have been devoted
to the exploration of efficient approaches for the preparation of
hollow structures inorganic materials including metallic oxides
(
2 2 2 5 3
TiO and Cu O), nonmetallic oxides (V O , WO ), and etc [3,7e10].
As an important V-VI-VII ternary semiconductor compound, bis-
muth oxyiodine (BiOI) exhibits relatively superior visible-light-
*
Corresponding author.
*
* Corresponding author.
E-mail address: wangkun@ujs.edu.cn (K. Wang).
http://dx.doi.org/10.1016/j.jallcom.2017.09.120
925-8388/© 2017 Elsevier B.V. All rights reserved.
0

28
Q. Liu et al. / Journal of Alloys and Compounds 729 (2017) 27e37
based hollow structures with improved performance.
JSM-7001F). Fourier transform infrared (FT-IR) and UVevis ab-
sorption spectra of the samples were measured by Spectrometer
(Nicolet Nexus 470 FT-IR) and UV-2450 spectrophotometer (Shi-
madzu, Japan), respectively. X-ray photoemission spectroscopy
(XPS) was recorded on a VG MultiLab 2000 system with a
Meanwhile, in order to further exploit its practical applications,
some strategies have been successfully employed to improve the
photoactivity of BiOI by hybridizing them with other nanomaterials
(
semiconductor, metal, and carbon-based materials) to facilitate
photo-induced electron-hole separation [16e22]. For example,
Chang et al. have hybridized BiOI with mesoporous graphite-like
carbon nitride to form heterojunction structure, and the photo-
catalytic performance of the BiOI-based heterojunction was
improved and much higher than pure BiOI for the degradation of
bisphenol A under visible-light irradiation [19]. Except for the po-
tential applications of BiOI-based composites in photocatalysis,
recently, Yan et al. employed the BiOI/graphene as an effective
photoactive material for the fabrication of photoelectrochemical
monochromatic Mg-Ka source operated at 20 kV. The surface
areas and pore sizes of the samples were calculated by the
Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH)
methods, respectively, and the samples were outgassed for 10 h at
ꢀ
200 C before the measurements.
The PEC and EIS measurements were conducted with a con-
ventional three-electrode system where a modified ITO electrode,
platinum wire and saturated calomel electrode electrode was
employed as working electrode, counter electrode and reference
electrode. All the PEC measurements were performed using a
CHI660 B Electrochemical Workstation (Chen Hua Instruments,
Shanghai, China) with a 250 W Xe lamp (Beijing Trusttech Co. Ltd.)
as the visible-light source. The EIS measurements were carried out
(
PEC) sensors, and the photocurrent response of photocathode was
promoted by doping BiOI with a suitable amount of graphene [20].
These investigations above showed a promising approach in the
fabrication of BiOI related functionalized composites with prefer-
able performance in the practical applications of environmental
pollutants detection and removal. Nitrogen doped graphene
quantum dots (NGQDs), a significant carbon nanomaterial doped
with nitrogen atoms, have ignited tremendous interests as a novel
enhanced material for improving the photoelectrical performance
of semiconductor materials due to their unique electrical conduc-
tivity [23,24]. In this work, the NGQDs was employed as the tem-
plate and also the dopant for the synthesis of NGQDs/3DBiOI hybrid
hollow microspheres (HHMs), and therefore, it may have the
advantage of a simple synthetic process and better chemical
properties.
Herein, we report a facile solvothermal method for the con-
struction of NGQDs supported on BiOI (NGQDs/3DBiOI) HHMs with
the assistance of NGQDs, which has great influence on the
morphology and interior hollow structure of the NGQDs/3DBiOI
microspheres. And the formation mechanism of the NGQDs/3DBiOI
HHMs in this system was systematically studied. Moreover, the
NGQDs/3DBiOI HHMs with NGQDs supported on the BiOI structure
manifest enhanced photocurrent generation than pure BiOI mi-
crospheres, and the NGQDs/3DBiOI HHMs were employed as an
ideal host for enzyme encapsulation by virtue of their particular
hollow nanostructures. Finally, with the malathion as a model, the
fabricated PEC-based platform was successfully applied in deter-
mination of organophosphate pesticide.
3
ꢁ/4ꢁ
in 0.1 M KCl solution containing 5 mM Fe(CN)
6
.
2.3. Preparation of NGQDs/3DBiOI HHMs
The NGQDs/3DBiOI HHMs were prepared via a facile one-pot
solvothermal method using NGQDs and Bi(NO $5H O as pre-
$5H O dissolved
in glycol was added drop-wisely into the same volume of
3
)
3
2
ꢁ
1
cursor. Typically, 10 mL of 0.015 mol L Bi(NO
3
)
3
2
ꢁ1
0.015 mol L
KI/glycol solution, then, a certain amount (0.17,
0.51, 0.9, 1.3, 1.9, 3, and 4.3 mL) of NGQDs was added in the above
mixed solution. The mixture was vigorously stirred at room
temperature for 60 min and transferred to a 25 mL of Teflon-
lined autoclave, which was sealed in a stainless steel tank and
ꢀ
maintained at 140 C for 12 h. Finally, the resulting precipitates
were collected and washed with ethanol and deionized for thrice,
ꢀ
followed by the drying in oven at 50 C to obtain NGQDs/3DBiOI
HHMs. The resulting samples with different amount of NGQDs
were labeled as NGQDs/3DBiOI
I
,
NGQDs/3DBiOIII
, NGQDs/
3DBiOIIII NGQDs/3DBiOIIV NGQDs/3DBiOI ,
,
,
V
NGQDs/3DBiOIVI
and NGQDs/3DBiOIVII, respectively. For comparison, pure BiOI
samples were prepared by the similar method in the absence of
NGQDs.
2
. Experimental section
2.4. Enzyme encapsulation and enzyme electrode preparation
2.1. Materials
To encapsulate the ALP in the NGQDs/3DBiOI HHMs, 16 mg of
NGQDs/3DBiOIIV samples were added to separate 4 mL aliquots of
ꢁ
1
Glycol, potassium iodide (KI), bismuth nitrate pentahydrate
Bi(NO ) $5H O), and N,N-dimethylformamide (DMF) were pur-
3 3 2
ALP solution (3 mg mL in pH 8.12 Tris-HCl), then the resulting
mixture was continuously shaken at room temperature for 12 h to
obtain ALP-entrapped NGQDs/3DBiOIIV. The amount of entrapped
ALP was measured by comparing the UVevis absorption at
~280 nm of the supernatant solution before and after uptake of ALP
by the NGQDs/3DBiOIIV samples.
(
chased from Sinopharm Chemical Reagent Co., Ltd. Alkaline phos-
phatase (ALP) and ascorbic acid 2-phosphate (AAP) were purchased
from Sigma-Aldrich. Malathion was obtained from J&K Chemical
Ltd. (Beijing). NGQDs were synthesized according to our previous
work [25]. 0.1 M phosphate buffered solution (PBS, pH 7.0) was
used as the supporting electrolyte, which was prepared by mixing
The enzyme electrode was prepared by the simple casting
method: a suspension containing 2 mg mL
2 mg mL of NGQDs/3DBiOIIV, was prepared by thoroughly
mixing appropriate volumes of ALP solution (4 mg mL , dissolved
in pH 8.12 Tris-HCl) and NGQDs/3DBiOI
in ultrapure water). Afterward, 20 L of the resulting dispersion
was drop-cast onto a piece of ITO slice with a fixed area of 0.5 cm .
Then the electrode was covered with a beaker so that the water
evaporated slowly in the air, and a modified electrode with uni-
ꢁ
1
of ALP and
ꢁ1
stock standard solutions of NaH
2
PO
4
2
and Na HPO
4
. Other reagents
ꢁ1
were of analytical grade and used as received without further pu-
rification. Double-distilled water was used throughout the study.
ꢁ1
Ⅳ
suspension (4 mg mL
m
2
2
.2. Apparatus
X-ray diffraction (XRD) patterns were conducted on a Bruker
D8 diffractometer with high-intensity Cu K
scanning electron microscopy (SEM) measurements were per-
formed on a field-emission scanning electron microscope (JEOL
a
(
l
¼ 1.54 Å). The
form film was thus obtained (denoted as ALP/NGQDs/3DBiOIIV
ITO). The dried electrode was stored at 4 C in a refrigerator when
not in use.
/
ꢀ

Q. Liu et al. / Journal of Alloys and Compounds 729 (2017) 27e37
29
3
. Results and discussion
NGQDs (curve b-c), an obvious C-N stretching vibration at
ꢁ
1
1400 cm was observed, which might correspond to the successful
3
.1. Characterization of NGQDs/3DBiOI HHMs
adulteration of N atom in the NGQDs [30]. Besides, the absorption
ꢁ1
band at 1078 cm was attributed to the stretching vibration of C-O
[31]. After being combined with NGQDs, all above absorption bands
of NGQDs could be observed in NGQDs/3DBiOI sample (curve b),
which suggested the successful coupling of BiOI material with
NGQDs. To further understand the evaluation of the purity and
surface composition of the samples, XPS spectrum was collected for
NGQDs/3DBiOI materials. The survey XPS spectrum in Fig. 1C
confirmed the existence of Bi, O, I, and C elements on the surface of
NGQDs/3DBiOI HHMs. On the side, the corresponding elemental
mapping images of the NGQDs/3DBiOI HHMs (Fig. 2) also revealed
that the samples were composed of Bi, I, O, C, and N elements with
uniform distribution.
X-ray diffraction (XRD) analysis was performed to determine the
phase and purity of the as-prepared products. Several major
diffraction peaks were observed in Fig.1A for the pure BiOI (curve a)
and NGQDs/3DBiOI HHMs (curve b), which were readily indexed
Ⅳ
to the tetragonal phase of BiOI (JCPDS card no. 10-0445) with well-
resolved (101), (102), (110), (104), (212) and (220) reflections,
respectively [26]. Besides, no other diffraction peaks was observed
for both the pure BiOI and NGQDs/3DBiOI samples, indicating the
high-purity of the obtained samples [27].
Fig. 1B displays the FT-IR spectra of NGQDs, BiOI, and NGQDs/
DBiOI HHMs. The characteristic peak at 512 cm was observed in
ꢁ1
3
the spectra of BiOI (curve a) and NGQDs/3DBiOI (curve b), which
was assigned to the Bi-O stretching vibration [28]. In addition, the
The panoramic morphologies of the as-synthesized NGQDs/
3DBiOI HHMs were examined by SEM. Fig. 3A presented a typical
Ⅳ
low-magnification SEM image of the sample, it was obvious that
the product was composed of large-scale microspheres, and most of
the microspheres were attached to each other to form large
microsphere aggregates. In addition, some of the microspheres had
ꢁ1
presence of the sharp and strong absorption at 1621 cm and
ꢁ1
3
430 cm were ascribed to the d(O-H) bending vibration and n(O-
H) stretching vibration, respectively, owing to the adsorption of free
water molecule on the material surface [29]. As for containing
Fig. 1. (A) The XRD patterns of the as-prepared pure BiOI (a) and NGQDs/3DBiOIIV samples (b); (B) FT-IR spectra of BiOI (a), NGQDs/3DBiOIIV (b) and NGQDs (c); (C) XPS survey
spectrum of the NGQDs/3DBiOIIV sample.

30
Q. Liu et al. / Journal of Alloys and Compounds 729 (2017) 27e37
Fig. 2. Elemental mapping of the as-prepared NGQDs/3DBiOIIV HHMs.
a hollow interior in the center with diameters of about 100 nm
which were easily to be seen. From the high-magnification SEM
observation in Fig. 3B, it could be easily found that each micro-
sphere was consisted of numerous nanosheets which were some-
what out-of-flatness and arranged rulessly to the center of the
sphere. To characterize the specific surface areas and pore struc-
tures of the NGQDs/3DBiOIIV HHMs, the nitrogen adsorption and
desorption isotherms was carried out. As shown in Fig. 3C, the
isotherm of the measured sample appears a distinct hysteresis loop
at high relative pressure, which was characteristic of mesoporous
materials [32]. The pore-size distribution curve of NGQDs/3DBiOIIV
HHMs analyzed through BJH method exhibited a broad peak at
that the bandgap of NGQDs/3DBiOIIV samples was calculated to be
1.93 eV, which was narrower than that of pure BiOI (2.14 eV),
implying that the visible-light response of the composite sample
was improved by hybriding BiOI matrix with NGQDs, which was
beneficial for the amplification of the visible-light driven PEC ac-
tivity of the BiOI semiconductor.
3.2. Formation mechanism
To obtain a better understanding of the growth mechanism of
NGQDs/3DBiOI HHMs, the morphology of the samples were
investigated by SEM, which provided direct evidence that the for-
mation of hollow microspheres was related to the NGQDs contents.
Fig. 5 displays the SEM images of BiOI, NGQDs/3DBiOIII, NGQDs/
17.5 nm, as displayed in Fig. 3D, further demonstrating the meso-
porous structure of the NGQDs/3DBiOIIV HHMs. The BET specific
surface areas and pore volume of the NGQDs/3DBiOIIV HHMs are
3DBiOIIV and NGQDs/3DBiOIVII microspheres, which were prepared
2
ꢁ1
3
ꢁ1
calculated to be as high as 93.672 m g and 0.55 cm g . Inter-
estingly, the pure BiOI microspheres, which were prepared in the
absence of NGQDs, presented the uniform flower-like structures, as
shown in Fig. 3E. From the high-resolution SEM image of BiOI mi-
crospheres in Fig. 3F, it was clear to be seen that such flower-like
structure were consisted of numerous nanosheets with a mean
by the addition of NGQDs solution with different volumes. As
shown in Fig. 5A, the pure BiOI microsphere (prepared without the
addition of NGQDs) presented the flower-like structures, which
were consisted of numerous nanosheets with a mean diameter of
2 mm. With the addition of 0.51 mL NGQDs, as shown Fig. 5B, no
significant differences were found between the morphology of the
resulting NGQDs/3DBiOIII and pure BiOI microspheres, except for
the nanosheets in NGQDs/3DBiOIII (Fig. 5B inset) were not as
smooth as that of pure BiOI (Fig. 5A inset). When the added NGQDs
were increased to 1.3 mL, the hollow structure in the middle of the
sphere was apparent and the diameter of the hole was about
diameter of 2
mm. These SEM results above indicated that the
addition of NGQDs in the reaction process was significant for the
formation of NGQDs/3DBiOI with special hollow morphology.
Fig. 4A shows UVevis absorption spectra of the NGQDs/3DBiOIIV
HHMs and pure BiOI microspheres. For pure BiOI, a significant
absorption at wavelength shorter than 530 nm was observed,
which can be assigned to the intrinsic bandgap absorption. After
the NGQDs were introduced, the NGQDs/3DBiOIIV HHMs further
expand the light absorption region to 570 nm. This result suggested
that the NGQDs/3DBiOIIV HHMs were able to absorb more light to
produce more electronꢁhole pairs and thus to improve their pho-
1
00 nm. Besides, the nanosheets of the hollow microspheres were
folded hardly which made the hollow microballoon more loosely
Fig. 5C). However, when the NGQDs was 4.3 mL, the hollow
(
structure of the resulting microspheres was extremely obvious,
displaying a hole with a diameter of 200 nm in the middle of the
microsphere (Fig. 5D), and the spherical structure was destroyed
seriously. This important result indicated that the addition of
NGQDs in the reaction is significant for the formation of NGQDs/
toactivity. Furthermore, the band gap energy (E
and NGQDs/3DBiOIIV HHMs was also studied. According to the
previous report, the E of semiconductor material could be calcu-
lated from the following equation:
where was absorption coefficient, Ephoton was the discrete pho-
g
) of the pure BiOI
g
3DBiOI with special hollow morphology, and the interior hollow
n/2
aE
photon ¼ K(Ephoton-E
g
)
,
structure of the microspheres can be tailored by varying the volume
of NGQDs solution added.
a
toenergy, K was a constant for the material, and n was 2 for a direct
To understand the assembling process of the hollow NGQDs/
transition or 4 for an indirect transition [13]. Fig. 4B exhibits the
3
DBiOI
Ⅳ
microspheres, the morphology observation of the inter-
1
/2
plots for the relationship of (
aE
photon
)
versus Ephoton, indicating
mediate products were performed at different precipitation

Q. Liu et al. / Journal of Alloys and Compounds 729 (2017) 27e37
31
Fig. 3. (A, B) SEM images of the NGQDs/3DBiOIIV HHMs; (C) nitrogen absorption-desorption isotherms of the NGQDs/3DBiOIIV HHMs and (D) corresponding pore-size distribution
curves of as-synthesized sample; (E, F) SEM images of the pure BiOI microspheres.
stages of 10 min, 30 min, 1 h and 12 h. The synthesis was done in
EG solution containing KI, Bi(NO and NGQDs. During the syn-
thesis process, a clear solution could be obtained when KI,
Bi(NO and NGQDs were dissolved into the EG. After 10 min
reaction, a small number of microspheres of varying sizes were
preliminarily formed (Fig. 6A); with the growing of reaction time
up to 30 min, it can be clearly seen that the flowerlike micro-
spheres which consisted of BiOI nanosheets have been generated,
in the microspheres were eventually produced (Fig. 6D). As a
comparision, a time-dependent experiment was also performed
to track the formation of pure BiOI microspheres. As displayed in
Fig. 6E, after 10 min reaction, the scattered BiOI nanosheets were
formed; and the aggregation of BiOI nanosheets was observed
when the reaction time increased to 30 min (Fig. 6F); after 1 h, the
BiOI nanosheets continued to aggregate and transformed into
flowerlike microspheres structures (Fig. 6G); after treatment for
12 h, large numbers of BiOI microspheres with uniform sizes were
finally obtained (Fig. 6H). The result above demonstrated that the
NGQDs used in the reaction process played an important role for
the formation of NGQDs/3DBiOI microspheres with hollow
structures.
3 3
)
3 3
)
the diameter of the microspheres was about 1
the reaction time lasted for 1 h, the flowerlike structures of BiOI
hollow microspheres with a diameter of 1e1.5 m were formed
mm (Fig. 6B); when
m
and no other morphology was observed (Fig. 6C); after 12 h, the
hollow microspheres with uniform size were destroyed and a hole

32
Q. Liu et al. / Journal of Alloys and Compounds 729 (2017) 27e37
Fig. 4. (A) UVevis absorption spectra and (B) (aE
photon)1/2 vs Ephoton curves of the as-prepared pure BiOI (a) and NGQDs/3DBiOIIV HHMs (b).
Based on the observations above, a plausible mechanism for the
formation of the NGQDs/3DBiOI HHMs was proposed, and the
corresponding formation process was exhibited vividly in Fig. 7.
to combine with the NGQDs spontaneously, large amounts of nuclei
of BiOI formed on the surface of NGQDs. During the hydrothermal
treatment, these BiOI nuclei grew and transformed into nano-
sheets, then the tiny nanosheets cline to form loosely attached
aggregates. Along with the reaction time increased, the aggregates
grew continuously in size and assemble into flowerlike structures.
With the reaction time further increased, the aggregates of the
flowerlike structures continuously grew to form hollow
NGQDs used in the reaction had abundant ꢁCOOH and ꢁNH
2
groups, which made the NGQDs possessing the ability for complex
formation with positive metal ions [23]. At the beginning, when the
reagents were added into the reaction system, due to the hydro-
3
þ
phobic interaction of ꢁCOOH and ꢁNH
2
groups, Bi was inclined
Fig. 5. SEM images of (A) BiOI, (B) NGQDs/3DBiOIII, (C) NGQDs/3DBiOIIV and (D) NGQDs/3DBiOIVII microspheres.

Q. Liu et al. / Journal of Alloys and Compounds 729 (2017) 27e37
33
Fig. 6. (AeD) NGQDs/3DBiOIIV and (EeH) BiOI morphologies observed by SEM images of the intermediate products performed at different precipitation stages of 10 min, 30 min, 1 h
and 12 h.
microspheres. When the reaction finished, the external tempera-
ture of the hollow microspheres was reduced more quickly than
that of the internal, which resulted in the increased pressure dif-
ference between the inner and outer of hollow structures. Finally,
NGQDs pushed out from the unsubstantial point of the micro-
spheres, and a hole on the hollow structures was eventually
formed, meanwhile, the pressure difference between the outer and
inner of the hollow microspheres disappeared. The growing pro-
cess was similar to the previous reports of hollow CuS and BiOI
structures [13,33].
3.3. Enhanced photocurrent generation of NGQDs/3DBiOI HHMs
The PEC behaviors of pure BiOI and different NGQDs/3DBiOI
HHMs were investigated upon visible-light activated photoexci-
tation. As illustrated in Fig. 8A, a small PEC response of pure BiOI
microspheres (~0.12 mA) was observed (curve a), and the photo-
current of NGQDs/3DBiOI hollow microspheres modified elec-
trode was amplified obviously (curve b-h) by virtue of the
incorporation of NGQDs. Interestingly, it should be noted that the
photocurrent of the NGQDs/3DBiOI samples were varied with the
Fig. 7. Schematic illustration for the formation of NGQDs/3DBiOI HHMs.

34
Q. Liu et al. / Journal of Alloys and Compounds 729 (2017) 27e37
Fig. 8. (A) Photocurrent and (B) EIS responses of BiOI (a), NGQDs/3DBiOI
I V
(b), NGQDs/3DBiOIII (c), NGQDs/3DBiOIIII (d), NGQDs/3DBiOIIV (e), NGQDs/3DBiOI (f), NGQDs/3DBiOIVI
(
g), NGQDs/3DBiOIVII (h) and NGQDs (i) modified ITO electrode in 0.1 M PBS (pH 7.0).
Fig. 9. (A) UVevis absorption spectra of ALP solution before (a) and after enzyme encapsulation by pure BiOI (b) and NGQDs/3DBiOIIV HHMs (c); (B) photocurrent responses of the
ALP/NGQDs/3DBiOIIV/ITO in 0.05 M Tris-HCl (pH 8.12) in the absence and presence of 70 mM AAP at the voltage of 0 V under visible-light irridiation; (C) schematic for the catalytic
reaction of encapsulated enzyme on the PEC response of NGQDs/3DBiOI HHMs under visible-light.

Q. Liu et al. / Journal of Alloys and Compounds 729 (2017) 27e37
35
increasing of the NGQDs solution volume added in the reaction
3.4. Enzyme encapsulation property of NGQDs/3DBiOIIV materials
procedure. When the NGQDs solution volume was 1.3 mL, the
resulting NGQDs/3DBiOIIV possessed larger photocurrent than
other NGQDs/3DBiOI samples.
To better understand the interface electron transfer rate of the
samples, the electrochemical impedance spectroscopy (EIS) of the
pure BiOI and NGQDs/3DBiOI microspheres were investigated.
And the equivalent circuit (inset of Fig. 8B) was used for fitting the
impedance data, where R , Ret, Z , and Q represent electrolyte
s w
solution resistance, electron-transfer resistance, Warburg
impedance, and constant phase elements, respectively. As dis-
By virtue of the particular hollow structure and enhanced
photocurrent generation of the as-prepared NGQDs/3DBiOIIV ma-
terials, the enzyme encapsulation study and PEC biosensing ap-
plications was further carried out by selecting the ALP as the model.
Fig. 9A shows the UVevis absorption spectra of ALP solution before
and after enzyme encapsulation by NGQDs/3DBiOIIV and pure BiOI
samples, respectively. The ALP solution exhibited the characteristic
absorption band at 279 nm (curve a), while after enzyme encap-
sulation by NGQDs/3DBiOIIV and pure BiOI samples, the charac-
teristic absorption band of ALP was decreased obviously due to the
fact that part of ALP was encapsulated by the nanostructure (curve
b and c). Interestingly, despite the lower surface area of the NGQDs/
3DBiOIIV HHMs, the encapsulation amount of ALP by NGQDs/
3DBiOIIV was even larger than pure BiOI samples, which might be
due to the hollow structure of NGQDs/3DBiOIIV enabled both the
shell and core of the sample to encapsulate ALP. Therefore, the
unique nanostructure of the NGQDs/3DBiOIIV hollow microspheres
make them particularly promising for applications in enzyme
immobilization.
played in Fig. 8B, the BiOI modified ITO had a Ret of 6076
a), after NGQDs doping, the Ret of NGQDs/3DBiOI HHMs with
different NGQDs doping were decreased from 4625 (curve e) to
007 (curve b-h). Among all of the NGQDs/3DBiOI samples, the
NGQDs/3DBiOI had the smallest Ret value, indicating the fast
electron transfer rate in the NGQDs/3DBiOIIV HHMs. And the
NGQDs exhibited the smallest Ret value of 950 (curve i) due to
U (curve
U
1
U
Ⅳ
U
their inherent fast electron transfer rate [23]. In addition, the EIS
results above were accordance with the PEC response of the
samples in Fig. 8A, illustrating the fact that the enhanced photo-
current of NGQDs/3DBiOI microspheres were attributed to the fast
electron transfer rate of the samples by the incorporation of
NGQDs. On the basis of the analysis above, the improved charge-
carrier-transfer behavior and good PEC stability of the NGQDs/
In the composite electrode, the NGQDs/3DBiOI HHMs were used
as an ideal photoactive material for the fabrication of PEC-based
platform, which manifested enhanced photocurrent generation
than pure BiOI microspheres. Also, as an ideal host for enzyme
encapsulation, the NGQDs/3DBiOI HHMs exhibited enhanced load
3
DBiOIIV HHMs was confirmed.
Fig. 10. (A) Photocurrent responses of the ALP/NGQDs/3DBiOIIV/ITO with different concentrations of malathion; (B) the corresponding calibration curve for malathion detection; (C)
stability test of the resulting biosensor under eight on/off cycles.

36
Q. Liu et al. / Journal of Alloys and Compounds 729 (2017) 27e37
capacity than pure BiOI microspheres by virtue of their particular
nanostructure. Subsequently, the PEC response of ALP/NGQDs/
Notes
The authors declare no competing financial interest.
3
DBiOIIV modified electrode was investigated in 0.05 M Tris-HCl
pH 8.12) in the absence and presence of AAP at the concentra-
tion of 70 mM. As shown in Fig. 9B, the PEC response of ALP/
NGQDs/3DBiOIIV modified electrode was 2.4 A (curve a), while
(
Acknowledgments
m
with 70 mM AAP added, the photocurrent was enhanced obviously.
The detailed mechanism for the enzyme catalysis process was
shown in Fig. 9C. Firstly, the ALP/NGQDs/3DBiOIIV modified elec-
trode exhibited a PEC response under visible-light irradiation,
when AAP was added, the hydrolysis of AAP was occurred by the
modified ALP on the electrode surface. As an effective electron
donor, the generated hydrolysis products ascorbic acid could
improve the separation efficiency of photo-induced electrons and
holes, and thus amplified the photocurrent of the modified elec-
trode [34]. These results above indicated that the ALP immobilized
on the ALP/NGQDs/3DBiOIIV film exhibited good bioactivity for
enzyme catalysis reaction.
Since the activity of ALP could be inhibited by same specific
molecules including pesticides (e.g.,malathion), drugs (e.g.,
caffeine), and miscellaneous medicines (e.g., isoproterenol) as well
as many other environmental contaminants [35,36], the proposed
system exhibited great potential applications of ALP/NGQDs/
The present work was financial supported by the National
Natural Science Foundation of China (Nos. 21375050, 21675066,
31671584 and 61601204), Provincial Natural Science Foundation of
Jiangsu (No. BK20160542), China Postdoctoral Science Foundation
(Nos. 2015M581745 and 2017T100330) and Jiangsu Province Post-
doctoral Science Foundation (No. 1601204C).
References
[
1] S.H. Ahn, J.K. Dong, W.S. Chi, J.H. Kim, Hierarchical double-shell nano-
structures of TiO2 nanosheets on SnO2 hollow spheres for high-efficiency,
solid-state, dye-sensitized solar cells, Adv. Funct. Mater. 24 (2014)
5037e5044.
[
2] W.M. Liao, D.J. Zheng, J.H. Tian, Z.Q. Lin, Dual-functional semiconductor-
decorated upconversion hollow spheres for high efficiency dye-sensitized
solar cells, J. Mater. Chem. A 3 (2015) 23360e23367.
[3] G.C. Xi, Y. Yan, Q. Ma, J.F. Li, H.F. Yang, X.J. Lu, C. Wang, Synthesis of multiple-
shell WO hollow spheres by a binary carbonaceous template route and their
applications in visible-light photocatalysis, Chem. Eur J. 18 (2012)
3949e13953.
3
1
3
DBiOIIV electrode in the fabrication of high-performance PEC
[
[
[
4] P. Chen, B. Cui, X.R. Cui, W.W. Zhao, Y.M. Bu, Y.Y. Wang, A microwave-trig-
gered controllable drug delivery system based on hollow-mesoporous cobalt
ferrite magnetic nanoparticles, J. Alloys Compd 699 (2017) 526e533.
biosensors in various fields from medicine analysis to environment
pollution monitoring. Herein, the as-prepared photoelectrode was
used for pesticide detection by selecting malathion as a model.
Fig. 10 depicts the photocurrent signals of ALP/NGQDs/3DBiOIIV
electrode before and after the addition of malathion with different
5] B. Wang, H.B. Wu, L. Zhang, X.W. Lou, Self-supported construction of uniform
3 4
Fe O hollow microspheres from nanoplate building blocks, Angew. Chem. Int.
Ed. 52 (2013) 4165e4168.
6] Z.M. Wan, J. Shao, J.J. Yun, H.Y. Zheng, T. Gao, M. Shen, Q.T. Qu, H.H. Zheng,
2
Coreeshell structure of hierarchical quasi-hollow MoS microspheres encap-
sulated porous carbon as stable anode for Li-ion batteries, Small 10 (2014)
4975e4981.
ꢁ
1
concentrations from 0.01 to 100
mg mL . It is clearly that the
photocurrent decreased gradually with the malathion concentra-
tion increased. In this way, a calibration curve reporting the
decrement of photocurrent signal as a function of the inhibitor
malathion concentration could be derived. As shown in Fig. 10B
inset, the inhibition effect is found to be proportional to the loga-
[
[
7] J. Hu, M. Chen, X.S. Fang, L.M. Wu, Fabrication and application of inorganic
hollow spheres, Chem. Soc. Rev. 40 (2011) 5472e5491.
8] H.S. Hou, X.Y. Cao, Y.C. Yang, L.B. Fang, C.C. Pan, X.M. Yang, W.X. Song, X.B. Ji,
NiSb alloy hollow nanospheres as anode materials for rechargeable lithium
ion batteries, Chem. Commun. 50 (2014) 8201e8203.
ꢁ1
[9] X.C. Li, W.J. Zheng, B. Chen, L. Wang, G.H. He, Rapidly constructing multiple
AuPt nanoalloy yolk@shell hollow particles in ordered mesoporous silica
microspheres for highly efficient catalysis, ACS Sustain. Chem. Eng 4 (2016)
2780e2788.
rithm of malathion concentration from 0.01 to 100 mg mL , the
ꢁ1
detection limit was estimated to be 3 ng mL . The results above
demonstrate the feasibility of using this system to determine
environmental contaminants. The photocurrent response was
recorded under eight on/off irradiation cycles in Fig. 10C and no
noticeable variation occurred, indicating the stable readout for
signal collection. Moreover, no obvious photocurrent change was
[10] T. Wu, H.Y. Pan, R.B. Chen, D. Luo, Y.H. Li, L. Wang, Preparation and properties
3 4
of magnetic Fe O hollow spheres based magnetic-fluorescent nanoparticles,
J. Alloys Compd 689 (2016) 107e113.
[11] M.L. Guan, C. Xiao, J. Zhang, S.J. Fan, R. An, Q.M. Cheng, J.F. Xie, M. Zhou, B.J. Ye,
Y. Xie, Vacancy associates promoting solar-driven photocatalytic activity of
ultrathin bismuth oxychloride nanosheets, J. Am. Chem. Soc. 135 (2013)
ꢀ
observed after storage under 4 C for 10 days, indicating an
10411e10417.
acceptable long-term stability of the electrode.
[12] Y. Mi, M. Zhou, L.Y. Wen, H.P. Zhao, Y. Lei, A highly efficient visible-light
driven photocatalyst: two dimensional square-like bismuth oxyiodine nano-
sheets, Dalton Trans. 43 (2014) 9549e9556.
[
13] J.X. Xia, S. Yin, H.M. Li, H. Xu, Y.S. Yan, Q. Zhang, Self-assembly and enhanced
photocatalytic properties of BiOI hollow microspheres via a reactable ionic
liquid, Langmuir 27 (2011) 1200e1206.
4
. Conclusions
[
14] K.X. Ren, K. Zhang, J. Liu, H.D. Luo, Y.B. Huang, X.B. Yu, Controllable synthesis
of hollow/flower-like BiOI microspheres and highly efficient adsorption and
photocatalytic activity, CrystEngComm 14 (2012) 4384e4390.
In summary, BiOI hollow microspheres incorporated with
NGQDs have been synthesized by a facile one-pot hydrothermal
method with the assistance of NGQDs. In the reaction process, the
NGQDs played a significant role for the formation of NGQDs/3DBiOI
with special hollow morphology, and the interior hollow structure
of the microspheres could be tailored by the NGQDs used. A
mechanism insight for the formation of NGQDs/3DBiOI hollow
microspheres was further explored. Moreover, the resulting
NGQDs/3DBiOI hollow microspheres exhibited improved perfor-
mance in photocurrent generation and enzyme encapsulation
properties than pure BiOI microspheres, and the detection of
pesticide was realized by monitoring the change of the PEC signal of
the electrode induced by the presence of malathion. It is anticipated
that the NGQDs/3DBiOI hollow microspheres proposed here can be
easily extended to various PEC applications and encapsulation of
enzymes for biosensors or biofuels.
[15] J. Di, J.X. Xia, Y.P. Ge, L. Xu, H. Xu, M.Q. He, Q. Zhang, H.M. Li, Reactable ionic
liquid-assisted rapid synthesis of BiOI hollow microspheres at room tem-
perature with enhanced photocatalytic activity, J. Mater. Chem. A 2 (2014)
15864e15874.
[16] H. Liu, W.R. Cao, Y. Su, Y. Wang, X.H. Wang, Synthesis, characterization and
photocatalytic performance of novel visible-light-induced Ag/BiOI, Appl.
Catal., B 111 (2012) 271e279.
[
17] M.H. Su, C. He, L.F. Zhu, Z.J. Sun, C. Shan, Enhanced adsorption and photo-
catalytic activity of BiOIeMWCNT composites towards organic pollutants in
aqueous solution, J. Hazard Mater. 229e230 (2012) 72e82.
[
18] X. Xiao, R. Hao, M. Liang, X.X. Zuo, J.M. Nan, L.S. Li, W.D. Zhang, One-pot
solvothermal synthesis of three-dimensional (3D) BiOI/BiOCl composites with
enhanced visible-light photocatalytic activities for the degradation of
bisphenol-A, J. Hazard Mater. 233e234 (2012) 122e130.
[19] C. Chang, L.Y. Zhu, S.F. Wang, X.L. Chu, L.F. Yue, Novel mesoporous graphite
carbon nitride/BiOI heterojunction for enhancing photocatalytic performance
under visible-light irradiation, ACS Appl. Mater. Interfaces
6 (2014)
5083e5093.

Q. Liu et al. / Journal of Alloys and Compounds 729 (2017) 27e37
37
[
20] K. Yan, Y. Liu, Y.H. Yang, J.D. Zhang,
A
cathodic “signal-off” photo-
[28] N.T. Hahn, S. Hoang, J.L. Self, C.B. Mullins, Spray pyrolysis deposition and
photoelectrochemical properties of n-type BiOI nanoplatelet thin films, ACS
Nano 6 (2012) 7712e7722.
electrochemical aptasensor for ultrasensitive and selective detection of
oxytetracycline, Anal. Chem. 87 (2015) 12215e12220.
[
21] K.W. Wang, N. Sun, X.P. Li, R.H. Zhang, G.Y. Zu, J. Wang, R.J. Pei, A hemin
2 3
[29] J. Cao, B. Xu, H. Lin, B. Luo, S. Chen, Novel heterostructured Bi S /BiOI pho-
2þ
binding G-quadruplex/Pb
photoelectrochemical sensor on a ZnO/BiOI heterostructure, Anal. Methods 7
2015) 9340e9346.
complex to construct a visible light activated
tocatalyst: facile preparation, characterization and visible light photocatalytic
performance, Dalton Trans. 41 (2012) 11482e11490.
(
[30] Y.J. Li, K. Ye, K. Cheng, D.X. Cao, Y. Pan, S.Y. Kong, X.M. Zhang, G.L. Wang,
Anchoring CuO nanoparticles on nitrogen-doped reduced graphene oxide
nanosheets as electrode material for supercapacitors, J. Electroanal. Chem. 727
(2014) 154e162.
[
22] J. Di, J.X. Xia, M.X. Ji, B. Wang, S. Yin, H. Xu, Z.G. Chen, H.M. Li, Carbon quantum
dots induced ultrasmall BiOI nanosheets with assembled hollow structures for
broad spectrum photocatalytic activity and mechanism insight, Langmuir 32
(
2016) 2075e2084.
[31] L. Perna, B. Sch o€ ttker, B. Holleczek, H. Brenner, Energy-level structure of
[23] Y. Li, Y. Zhao, H.H. Cheng, Y. Hu, G.Q. Shi, L.M. Dai, L.T. Qu, Nitrogen-doped
graphene quantum dots with oxygen-rich functional groups, J. Am. Chem. Soc.
nitrogen-doped graphene quantum dots, J. Mater. Chem. C 1 (2013)
4908e4915.
1
34 (2012) 15e18.
[32] Y. Liu, K. Lan, A.A. Bagabas, P.F. Zhang, W.J. Gao, J.X. Wang, Z.K. Sun, J.W. Fan,
A.A. Elzatahry, D.Y. Zhao, Ordered macro/mesoporous TiO hollow micro-
[
24] X.T. Wang, D.D. Ling, Y.M. Wang, H. Long, Y.B. Sun, Y.Q. Shi, Y.C. Chen, Y. Jing,
N-doped graphene quantum dots-functionalized titanium dioxide nanofibers
and their highly efficient photocurrent response, J. Mater. Res. 29 (2014)
2
spheres with highly crystalline thin shells for high-efficiency photo-
conversion, Small 12 (2016) 860e867.
1
408e1416.
[33] L. Ge, X.Y. Jing, J. Wang, S.B. Jamil, Q. Liu, D.L. Song, J. Wang, Y. Xie, P.P. Yang,
M.L. Zhang, Ionic liquid-assisted synthesis of CuS nestlike hollow spheres
assembled by microflakes using an oilꢁwater interface route, Cryst. Growth
Des. 10 (2010) 1688e1692.
[
[
[
25] Y.Y. Yin, Q. Liu, D. Jiang, X.J. Du, J. Qian, H.P. Mao, K. Wang, Atmospheric
pressure synthesis of nitrogen doped graphene quantum dots for fabrication
of BiOBr nanohybrids with enhanced visible-light photoactivity and photo-
stability, Carbon 96 (2016) 1157e1165.
[34] Y.C. Zhu, N. Zhang, Y.F. Ruan, W.W. Zhao, J.J. Xu, H.Y. Chen, Alkaline phos-
26] J. Di, J.X. Xia, S. Yin, H. Xu, L. Xu, Y.G. Xu, M.Q. He, H.M. Li, Preparation of
2
phatase tagged antibodies on gold nanoparticles/TiO nanotubes electrode: a
plasmonic strategy for label-free and amplified photoelectrochemical immu-
noassay, Anal. Chem. 88 (2016) 5626e5630.
sphere-like g-C
light-driven photocatalytic degradation of pollutants, J. Mater. Chem. A 2
2014) 5340e5351.
N
3 4
/BiOI photocatalysts via a reactable ionic liquid for visible-
(
[35] R.B. McComb, N. George, J. Bowers, Alkaline Phosphatase, Plenum Press, New
York, 1979.
[36] W.W. Zhao, Z.Y. Ma, J.J. Xu, H.Y. Chen, In situ modification of a semiconductor
surface by an enzymatic process: a general strategy for photoelectrochemical
bioanalysis, Anal. Chem. 85 (2013) 8503e8506.
27] T.Y. Peng, P. Zeng, D.N. Ke, X.J. Liu, X.H. Zhang, Hydrothermal preparation of
multiwalled carbon nanotubes (MWCNTs)/CdS nanocomposite and its effi-
cient photocatalytic hydrogen production under visible light irradiation, En-
ergy Fuels 25 (2011) 2203e2210.