Facile preparation of ZnO:g-C3N4 heterostructures and their application in amiloride photodegradation and CO2 photoreduction

Article abstract of DOI:10.1016/j.jallcom.2020.156798

Semiconductor heterojunctions are interesting strategies for the development of efficient photocatalysts. In this paper we report the synthesis of ZnO:g-C₃N₄ heterostructures with different percentages of g-C₃N₄

Full text of DOI:10.1016/j.jallcom.2020.156798

Journal Pre-proof
Facile preparation of ZnO:g-C N heterostructures and their application in amiloride
3
4
photodegradation and CO photoreduction
2
Nailma de Jesus Martins, Isabel C.H. Gomes, Gelson T.S.T. da Silva, Juliana A.
Torres, Waldir Avansi, Jr., Caue Ribeiro, Andréa R. Malagutti, Henrique A.J.L.
Mourão
PII:
S0925-8388(20)33162-5
DOI:
https://doi.org/10.1016/j.jallcom.2020.156798
JALCOM 156798
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 13 April 2020
Revised Date: 22 July 2020
Accepted Date: 18 August 2020
Please cite this article as: N. de Jesus Martins, I.C.H. Gomes, G.T.S.T. da Silva, J.A. Torres, W. Avansi
Jr., C. Ribeiro, André.R. Malagutti, H.A.J.L. Mourão, Facile preparation of ZnO:g-C N heterostructures
3
4
and their application in amiloride photodegradation and CO photoreduction, Journal of Alloys and
2
Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.156798.
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CRediT author statement
Nailma de Jesus Martins: Conceptualization, Methodology, Investigation, Writing -
Original Draft.
Isabel C. H. Gomes: Methodology, Investigation.
Gelson T. S. T. da Silva: Conceptualization, Methodology, Investigation, Writing -
Original Draft.
Juliana A. Torres: Conceptualization, Methodology, Investigation, Writing - Original
Draft.
Waldir Avansi, Jr.: Conceptualization, Methodology, Investigation, Resources,
Writing - Original Draft.
Caue Ribeiro: Conceptualization, Supervision, Resources, Funding acquisition,
Writing - Original Draft.
Andréa R. Malagutti: Conceptualization, Supervision, Resources, Writing - Original
Draft.
Henrique A. J. L. Mourão: Conceptualization, Methodology, Resources, Writing -
Original Draft, Supervision, Funding acquisition, Project administration.

Facile preparation of ZnO:g-C N heterostructures and their
3
4
application in amiloride photodegradation and CO photoreduction
2
a,b*
b
c
Nailma de Jesus Martins ; Isabel C. H. Gomes ; Gelson T. S. T. da Silva ; Juliana A.
c
d
c
e
Torres ; Waldir Avansi Jr. ; Caue Ribeiro ; Andréa R. Malagutti ; Henrique A. J. L.
a,b,**
Mourão
a
Chemistry Postgraduate Program (PPGQ), Federal University of Jequitinhonha and
Mucuri Valleys (UFVJM), MGT 367 Highway – Km 583, CEP 39100-000, Diamantina,
MG, Brazil.
b
Institute of Science and Technology (ICT), Federal University of Jequitinhonha and
Mucuri Valleys (UFVJM), MGT 367 Highway – Km 583, CEP 39100-000, Diamantina,
MG, Brazil.
c
Nanotechnology National Laboratory for Agriculture (LNNA), Embrapa
Instrumentation, Zip Code 13560-970, São Carlos, SP, Brazil.
d
Department of Physics, Federal University of São Carlos (UFSCar), 13565-905, São
Carlos, SP, Brazil
e
Department of Pharmacy (DEFAR), Federal University of Jequitinhonha and Mucuri
Valleys (UFVJM), MGT 367 Highway – Km 583, CEP 39100-000, Diamantina, MG,
Brazil.
*
*
Corresponding author: nailmajanauba@gmail.com
* Corresponding author: henrique.mourao@ict.ufvjm.edu.br
1

Abstract
Semiconductor heterojunctions are interesting strategies for the development of efficient
photocatalysts. In this paper we report the synthesis of ZnO:g-C N heterostructures
3
4
with different percentages of g-C N (15%, 50% and 85%) using an easy and new
3
4
method: precipitation of ZnO by alkaline solution at room temperature on g-C N
3
4
surface. The nucleation of ZnO nanoparticles had effective interaction on the g-C N
3
4
surface, showing good crystallinity and uniformity, which improved the heterostructure
stability and the separation of photogenerated charge carriers, consequently increasing
the photocatalytic efficiency for both the degradation of the amiloride (AML) drug and
the conversion of CO into products such as CO, CH and C H .
2
4
2
4
Keywords: Sol-gel synthesis; zinc oxide; graphitic carbon nitride; heterostructure;
photodegradation; CO photoreduction.
2
2

1
. INTRODUCTION
Investigations on photocatalytic materials have been carried out to show their
importance in different topics, including environmental remediation (i.e. oxidation of
organic compounds) and new energy production (i.e. H evolution from water and CO
2
2
photoreduction into hydrocarbons) [1,2]. Semiconductors are considered promising
materials for such applications since their electronic and structural properties allow the
charge transfer and the generation of reactive species responsible for oxidation-
reduction reactions in the heterogeneous medium [3]. The conversion efficiency of these
processes depends essentially on the light absorption capacity, the generation and
separation of photogenerated charge carriers, the adsorption and diffusion of molecules,
among others [4,5]. However, a clear understanding of these properties–the oxidative
and reductive capacity of semiconductors–and how they influence each process is still a
matter of study, essential for the suitable design of future catalysts according to the
intended application.
Among several semiconductors, zinc oxide (ZnO) has been widely studied for
photocatalysis application due to its several excellent properties [6]. Nevertheless, this
oxide is traditionally synthesized by thermal processes, either at high temperatures
(above the ambient temperature) or high pressure, in order to obtain the crystalline
phase [7–9]. These conditions not only make the synthetic process more complex and
costly, but also can decrease the specific surface area that is essential for catalysis.
However, to the best of our knowledge, synthetic methods at ambient temperature for
the crystalline ZnO is still little explored in the literature.
3

The use of ZnO as photocatalyst under prolonged light irradiation can promote
its photocorrosion. In addition, for being an amphoteric oxide, ZnO can suffer acid-base
chemical reactions in drastic values of pH, either under alkaline or acid conditions
[
10,11]. These two effects can decrease the effectiveness of the reaction due to ZnO
dissolution in the reaction medium by chemical or photocorrosion mechanisms.
Therefore, these barriers must be overcome to increase its photocatalytic efficiency. In
this light, the coupling of ZnO with other semiconductors appears as a promising
alternative [12,13].
The graphitic carbon nitride, g-C N , is an organic semiconductor widely
3
4
studied in photocatalytic processes due to its good stability and band gap (2.7 eV) [14–
7]. However, in most cases it is coupled to another semiconductor because of its high
1
electron/hole recombination rate, forming heterojunctions. This coupling is responsible
for the stabilization of the metal or oxide and the increase in the solar radiation
absorption, thus favoring a reaction direction. Although heterojunctions between ZnO
and g-C N are not new [18–20], there is an increasing necessity for more studies
3
4
related to methods of impregnation under milder reaction conditions, which can provide
good migration of photogenerated charges as a result of the efficient nucleation of ZnO
nanoparticles without any loss of surface properties, for instance. Futhermore, the
development of simple, efficient and low-cost methods for the synthesis of
heterostructures with high stability and photocatalytic properties, especially under
visible-light or solar irradiation, can contribute to the consolidation of the use of
heterogeneous photocatalysis on a large scale.
Therefore, we describe herein a facile method to synthesize ZnO:g-C N
3
4
heterostructures with different proportions in mass of g-C N (15%, 50% and 85%). The
3
4
4

synthesized heterostructures were characterized by several techniques and applied for
the photodegradation of amiloride and the photoreduction of CO2.
2
2
. EXPERIMENTAL SECTION
.1. Materials preparation
The reagents used to prepare the materials were zinc acetate (ISOFAR),
melamine (Sigma-Aldrich) and sodium hydroxide (Synth). All reagents were used
without further purification.
Firstly, pristine materials were synthesized. The ZnO was prepared by a facile
method based on sol-gel hydrolysis as follows: in a typical procedure, a sodium
-1
hydroxide solution (1.0 mol L ) was added to the zinc acetate aqueous solution (0.10
-1
mol L ) under vigorous stirring until pH=10, resulting in a white precipitate. After that,
the as-formed precipitates were filtrated, washed with deionized water and isopropyl
alcohol several times and dried in air at 75 °C. The pristine g-C N , denoted as CN, was
3
4
obtained by thermal polycondensation of melamine at 550 °C for 2 h in a conventional
furnace in the air atmosphere [21].
The ZnO:g-C N heterostructures were synthesized by a simple procedure, as
3
4
subsequently described. The as-synthesized g-C N was dispersed in a zinc acetate
3
4
-1
aqueous solution at about 0.10 mol L , and then a hydroxide sodium solution (1.0 mol
-1
L ) was added under vigorous stirring until pH=10. Then, the suspensions were also
washed and dried at the same conditions of the pure ZnO to produce the powder
samples. To obtain heterostructures with different mass contents of ZnO and g-C N ,
3
4
the synthetic procedure was performed with 15%, 50% and 85% of g-C N . These
3
4
samples were referred to as ZnO:CN15%, ZnO:CN50%, and ZnO:CN85%, respectively.
5

2
.2. Materials characterization
X-ray diffraction (XRD) patterns were obtained with an XRD 6000
diffractometer (CuKα radiation) at a scanning speed of 2°/min in the 2θ range from 10°
to 90°. The crystallite sizes were calculated using Scherrer’s equation [22]. TG-DTA
measurements were carried out on a TGA Q500 thermal analyzer (TA Instruments, New
-1
Castle, DE) under airflow at a heating rate of 10 °C min at temperatures varying from
room temperature to 700 °C. Fourier transform infrared (FTIR) spectra were obtained
-1
from 400 to 4000 cm using a VARIAN 640-IR FT_IR Spectrometer in ATR mode.
The images of field emission gun scanning electron microscopy (FEG/SEM) were
obtained using a JEOL JSM-6701F microscope. Energy dispersive X-ray spectroscopy
(EDS) measurements were performed using an EDS Thermo-Noran equipped with a Si
detector attached to the JEOL JEM 2010 microscope. Transmission electron microscopy
was performed on a FEI Tecnai G2 F20 microscope. The values of specific surface area
(SSA) were estimated according to the Brunauer–Emmett–Teller (BET) method, by
using N adsorption data obtained in Micromeritics ASAP-2020 equipment. Diffuse
2
reflectance spectrometry (DRS) analyses were performed with a UV-vis Shimadzu UV-
2
600 spectrophotometer in the range of 300-700 nm. The band gap energy was
estimated by using the Tauc equation from the x-axis intercept of the tangent lines of
each curve, as described in a previous study [23].
The photocatalytic performances of the synthesized samples were analyzed in a
photoreactor equipped with six fluorescent lamps placed on the top of the reactor with
emission in the visible range. This photoreactor was connected to a heat exchanger that
maintained the temperature at about 20 ºC. Aqueous suspensions were obtained by the
dispersion of 10 mg of each photocatalyst in 20 mL of aqueous solutions of the drug
6

-1
amiloride (AML) at 10 mg L . Before the photocatalytic test, the suspensions were
previously kept in the dark for 12 h to achieve adsorption/desorption equilibrium. The
decrease in the AML concentration was analyzed in a Double Beam spectrophotometer
(Shimadzu UV 6300PC equipment) at 285 nm in different periods. All measurements
were performed in triplicate. The stability of the synthesized photocatalyst (ZnO:CN50%
)
was evaluated by performing three recycling experiments with cycles of 260 min each
under visible-light irradiation, a total of 780 min (13 h) of catalysis process. The main
active species in the photocatalytic degradation process was also investigated. The
+
-
hydroxyl radical (·OH), hole (h ) and electron (e ) were trapped with isopropyl alcohol
-1
-1
-1
(
20 mmol L ), potassium bromide (10 mmol L ) and sodium oxalate (10 mmol L ),
respectively.
The CO photoreduction was performed in a cylindrical steel reactor covered
2
with borosilicate glass with a total volume of 240 mL. Briefly, 0.1 g of catalyst was
suspended in 100 ml of distilled water. To remove unwanted gases (O and N ) inside
2
2
-1
the reactor, CO was bubbled in the reactor for 20 min in the flow of 20 mL min . At
2
the top of the reactor, a 5 W UVC lamp (Philips) with maximum length of 254 nm was
inserted, protected by a quartz capsule. The photocatalytic performance of all
synthesized materials was monitored for 6 h, and after this period the gaseous and liquid
products were collected with a syringe and injected into specific chromatograph
equipment. The gaseous products were determined by gas chromatography (Varian, CP-
3
800). Assays without catalyst were performed to evaluate the photolysis effect and
correct the total product formation by photocatalysis.
3
3
. RESULTS AND DISCUSSION
.1. Characterization of the synthesized materials
7

Fig. 1 shows the X-ray diffraction (XRD) patterns of all synthesized materials:
the pristine compounds (ZnO and g-C N ) and the heterostructures with different mass
3
4
contents of ZnO and g-C N . All diffraction peaks of the ZnO sample are well defined
3
4
and intense, suggesting that the treatment with sodium hydroxyl at ambient temperature
was sufficient to obtain a material with good crystallinity. Such peaks can be indexed to
the hexagonal würtzite phase of ZnO [JCPDS nº 36-1451]. The g-C N pattern shows
3
4
two peaks at 2θ equal to 13° and 27°, which can be attributed to the crystallographic
100) and (002) planes of the graphitic carbon nitride phase [JCPDS nº 87-1526]. These
(
observed peaks are related to an in-plane structural packing motif, such as the hole-to-
hole distance of the nitride pores in the crystal and the planar graphitic interlayer
distance of g-C N , respectively [24]. In the crystallographic pattern of the synthesized
3
4
compounds with different amounts of ZnO and g-C N we can observe peaks with
3
4
different intensities, indicating that those related to ZnO become more evident when the
oxide amount is increased, as expected.
The preferred position for Fig. 1
Table 1 shows the values of the crystallite size of the ZnO phase calculated
according to the diffraction peaks of the (100), (002) and (101) planes, and the values of
specific surface area (SSA) of the synthesized samples. It seems that ZnO preferably
grows in one particular direction since the crystallite sizes perpendicular to the (002)
planes were the highest for each sample containing ZnO (ZnO, ZnO:CN15%
ZnO:CN50%, and ZnO:CN85%). In addition, the CN sample and the heterostructures
containing g-C N and ZnO) showed higher SSA than the ZnO.
,
(
3
4
The preferred position for Table 1
8

The synthesized materials were also characterized by TGA, as shown in Fig. 2
a,b. Thermogravimetric analysis was used to estimate the amount of g-C N in the
3
4
heterostructures since this compound is thermally degraded above 500 °C (see Fig. 2b)
under air condition and ZnO is practically stable in this range, losing only 5% of its total
mass (see Fig. 2a). The thermogravimetric curves reveal that the pristine ZnO has a
small weight loss at low temperatures when heat-treated up to 700 °C, which can be
associated with the release of water molecules adsorbed on the material surface [25].
The same contribution was expected in the thermogravimetric results of the synthesized
heterostructures, whose curves show that the expected amounts of g-C N were very
3
4
close to those obtained, demonstrating that the method was efficient to obtain the
proportions 85%, 50% and 15%, as seen in Fig. 2a.
On the other hand, in Fig. 2b we can better observe in the derivative weight
loss curves that the heterostructures synthesized with g-C N suffered thermal
3
4
decomposition of g-C N at lower temperatures than the pristine g-C N sample. Our
3
4
3
4
previous studies already demonstrated a worse thermal stability of g-C N in the
3
4
presence of metal oxides or their precursors [21]. This effect is possibly related to the
influence of Zn sites on the thermal decomposition of g-C N . It is known that metals in
3
4
thermal reactions with g-C N , or even its precursors, can catalyze its decomposition
3
4
reactions. Even though this effect can be seen in the ZnO:CN15% and ZnO:CN85%
samples, it is more intense in the ZnO:CN50%, suggesting that its better distribution (i.e.
higher ZnO and g-C N contact) could increase the degradation to a maximum; such
3
4
distribution appears to be lower in the ZnO:CN85% sample due to its low ZnO content.
Additionally, the low amount of g-C N in the ZnO:CN sample might have
3
4
15%
influenced the reduction of the decomposition temperature.
9

The preferred position for Fig. 2
Fig. 3 shows the FTIR spectra obtained from all the synthesized samples and
precursors (melamine and zinc acetate). The spectrum of the pristine ZnO has few
peaks, which are attributed to the hydroxyl groups connected to the ZnO surface (3400
-1
-1
cm ), the Zn-O vibrations at 450 cm and the precursor (zinc acetate) residues (1565-
-1
1
438cm ) [25]. However, it is important to note that the quantity of residues is probably
the same in all samples, not compromising future performances. From the melamine
spectrum, it is possible to observe the presence of vibrational modes attributed to the
-1
-1
CN bond (1500-1430 cm ), the NH bond of the amine group (-NH ) (3500-3000 cm
2
-1
-1
and 1650-1600 cm ) and the deformation of the triazine ring (808 cm ) [26]. Many of
these vibrational modes can be visualized in the g-C N (CN sample) spectrum with the
3
4
-1
appearance of a broadband centered at approximately 3250 cm as a result of the
thermal polymerization of triazine rings and the formation of OH groups. The same
profile was exhibited for all the synthesized heterostructures, yet with a single
-1
difference: the signal at 808 cm decreased as the ZnO amount increased,
demonstrating that the presence of g-C N or its exposure on the surface of the materials
3
4
-
was reduced. Besides this, the intensity of the band in the range between 3600-2750 cm
1
refers to O-H and N-H vibrations (convoluted peak). This is better visualized in the
ZnO:CN85% sample, where the N-H vibrations are more significant and indicate a slight
shift of the sharper peak. Therefore, the higher intensity presented by the ZnO:CN50%
sample can be probably explained by the greater contributions of both shifts, i.e., the O-
H vibration is mainly caused by the presence of ZnO, while the N-H vibration mostly
appears as a result of the presence of g-C N .
3
4
The preferred position for Fig. 3
1
0

Fig. 4 shows the UV-vis spectra obtained from DRS measurements of the
synthesized materials. Both pristine g-C N and ZnO have photoabsorption under
3
4
visible and UV lights, respectively, which is consistent with previous reports [9,27]. On
the other hand, all spectra of the synthesized heterostructures showed that the ZnO:g-
C N heterostructures have absorption in the visible-light region. In addition, the band
3
4
gap values decreased to a value similar to that found in the pristine g-C N , indicating
3
4
that the presence of ZnO in the structure contributes poorly to the optical properties of
the synthesized heterostructures.
The preferred position for Fig. 4
The morphology of the synthesized samples can be seen in Fig. 5. The SEM
micrograph of the pristine g-C N (Fig. 5a) demonstrates that the synthesized powder
3
4
has the typical lamellar agglomerate morphology [21]. The SEM image of the ZnO
sample (Fig. 5b) shows irregular rod-like particles with micrometric lengths and
hexagonal base of approximately 100 nm, as seen in the inset of Fig. 5b. In contrast, the
representative image of the ZnO:CN50% sample (Fig. 5c) reveals the existence of
irregular rod-like particles and lamellar materials, indicating the presence of ZnO and g-
C N , respectively. Besides that, the inset of Fig. 5c also suggests that the synthesis
3
4
method effectively caused good nucleation of ZnO particles on the g-C N surface,
3
4
producing an intimate contact between them, consequently resulting in the formation of
heterojunctions between the semiconductors.
A representative TEM image of the ZnO:CN50% sample (Fig. 5d) reveals the
existence of two distinct morphologies: lamellar and spherical/hexagonal nanomaterials.
The HRTEM image (Fig. 5e) confirms that the spherical/hexagonal nanostructures are
related to the ZnO hexagonal wurtzite since it showed an interlayer distance of 0.25 nm,
1
1

which can be assigned to (101) crystalline plane of this phase. In addition, Fig. S1
brings a representative EDS mapping analysis (b-d) from the region shown in Fig. S1a
of the representative ZnO:CN50% sample, which confirms the presence and coexistence
of elements related to ZnO and g-C
3
N4, such as C, N, Zn and O. A good homogeneity in
the distribution of g-C and ZnO particles can also be observed.
3
N4
Therefore, according to our SEM, TEM and EDS results, it was possible to
conclude that the ZnO rods consist of agglomerates formed by a large number of
smaller ZnO nanoparticles sized between 5-20 nm. This behavior was already observed
by different materials, such as BiVO , where a larger particle is composed of smaller
4
particles [28]. We could also infer that the ZnO particles are attached to the g-C N ,
3
4
thus forming the ZnO:C N heterostructures.
3
4
The preferred position for Fig. 5
3
3
.2. Photocatalytic tests
.2.1. Photo-oxidation properties
Fig. 6 displays the photodegradation curves of the amiloride (AML) drug under
visible light. In addition, Table S1 shows the reaction rate constants (k ) that were
p
calculated using a pseudo-zero-order model [29]. In the presence of the synthesized
materials, there was an improvement in the degradation of the AML solution compared
to the photolysis process (negligible degradation), demonstrating that the such materials
are promising catalysts in the oxidation of this drug. Also, the pristine semiconductors
(
ZnO and g-C N ) showed poor activity compared to all synthesized heterostructures,
3 4
degrading about 15% and 22% of AML, respectively. The low photocatalytic activity
exhibited by these pristine materials was already expected as the tests were carried out
under visible-light irradiation, and this spectral range is not sufficient to activate ZnO. It
1
2

is worth mentioning that the g-C N is known to have a high rate of recombination of
3
4
photogenerated charge carriers [30]. Thus, the presence of g-C N was crucial for the
3
4
photoactivation under visible-light irradiation, and the coupling of ZnO with g-C N
3
4
was fundamental to improve the photocatalytic efficiency of the prepared
heterostructures compared to the pristine materials. In addition, the ZnO:CN50% sample
had the best photocatalytic performance (about 53% of degradation), according to Fig.
6
. In this way, these results suggest the formation of heterojunctions between the ZnO
and g-C N phases, which induces an increase in the separation of the photogenerated
3
4
charges through charge transfer from g-C N to ZnO, since the association of these
3
4
phases showed a positive synergistic effect for the photodegradation process. In the
intermediate proportion of each heterostructure component, i.e. 50% (ZnO:CN50%), the
number of heterojunctions is probably greater than in other samples, as the components
(
g-C N and ZnO) tend to be more dispersed and less segregated.
3 4
The preferred position for Fig. 6
For being a fundamental characteristic of a good catalyst, especially in large-
scale processes, the stability of the synthesized photocatalyst with the best performance
for AML degradation under visible light, ZnO:CN50%, was analyzed. In Fig. 7, we can
see no significant deactivation of the analyzed photocatalyst after successive re-uses,
demonstrating that the studied systems (ZnO:CN heterostructures) can be employed to
stabilize ZnO in photocatalytic processes. This may be associated with the fact that
apparently there was neither photo nor chemical corrosion with oxide, as the materials
exhibited good photostability and good performance to degrade amiloride under visible-
light irradiation even after 13 h of use.
The preferred position for Fig. 7
1
3

To investigate the active species in the AML photodegradation driven by the
ZnO:CN50% heterostructure under visible light, some scavengers were added to the
reaction, e.g., isopropyl alcohol (scavenger for the hydroxyl radical, ·OH), potassium
-
+
bromide (scavenger for the electron e ) and sodium oxalate (scavenger for the hole h )
(Fig. 8). In this experiment, we observed that the sequestering agents added to the
reactions fulfilled their role and decreased the AML degradation rate caused by the
synthesized photocatalyst. With that in mind, it was demonstrated that all the
photogenerated species participate in the AML degradation mechanism.
The preferred position for Fig. 8
Based on our results and several important studies regarding heterogeneous
photocatalysis [18,20,31,32], we proposed a hypothetical mechanism (Fig. 9a) to
illustrate the formation and migration of charges as well as the oxidation and reduction
processes in the valence (VB) and conduction (CB) bands, respectively. First, due to the
presence of g-C N in a conventional type-II heterostructure, the heterostructures are
3
4
photoactivated under visible-light irradiation–although they could also be activated by
solar radiation. In this case, the ZnO is not photoactive because of its high value of band
gap, making unfeasible the possibility of direct Z-scheme [33–35]. Second, there is the
+
-
formation of oxidizing charges in the VB (h ) and reducers in the CB (e ) in the
photoactived g-C N . The VB participates in the oxidation reactions, and the
3
4
photogenerated electrons are first migrated to the CB of g-C N , and then transferred to
3
4
the CB of ZnO. This charge transfer process is essential to decrease the charge carrier
recombination and consequently increase the photocatalytic activity; this is the principle
+
of a conventional type-II heterostructure. The charge generated (h ) in the VB can act as
a direct oxidizing agent and degrade the organic molecule, or oxidize water, generating
1
4

hydroxyl radicals, species with a high oxidizing capacity. In other words, the
degradation can occur indirectly by the oxidation of AML molecules through their
interaction with the formed species (i.e. ·OH), or directly by their interaction with the
generated charges, especially the direct oxidation of AML molecules by the holes
originating from the BV of g-C N . This direct oxidation by holes can begin with the
3
4
+
formation of AML radicals (·AML ), responsible for subsequent reactions of the AML
+
degradation. According to Fig. 8, the holes (h ) contribute to the degradation of AML
more than other species since they can participate in the formation of both ·OH and
+
·
AML . On the other hand, the electrons transferred to the CB of ZnO can reduce the
-
molecular oxygen, forming the superoxide radical (·O ), a species with a high oxidizing
2
capacity that later oxidizes the organic molecules, or oxidizes water, also generating
hydroxyl radicals.
The preferred position for Fig. 9
3
.2.2. CO photoreduction properties
2
The catalytic capacity of these materials (except the pristine ZnO) was also
monitored with respect to the reduction of CO , a molecule responsible for several
2
climatic changes. Fig. 10 shows the results of the photocatalytic reduction of CO into
2
CO, CH and C H . The pristine ZnO did not have satisfactory results in relation to CO
2
4
2
4
conversion, because under experimental conditions this oxide is completely dissolved,
-
as expected. It occurs due to acidic species (HCO and H CO ) formed when CO is
3
2
3
2
dissolved in water, leading to ZnO instability, especially under irradiation. This effect
was not observed in the experiments with other prepared samples (g-C N and
3
4
heterostructures), confirming the results observed in Fig. 7, where the use of g-C N not
3
4
1
5

only improved the activity of the synthesized materials, but also increased their
photochemical stability in photocatalytic processes.
The preferred position for Fig. 10
From Fig. 10, it is possible to observe that the pristine g-C N showed a
3
4
moderate selectivity for CO formation after 6 h of reaction despite the fact that the
observed CO production was low (0.56 µmol/g) compared to the other samples,
demonstrating that the formation of heterojunctions with ZnO makes the materials more
efficient due to the improved separation of photogenerated charge carriers [36]. It can
also be noted that the ZnO:CN85% sample (1.64 µmol/g) exhibited a greater activity than
the pristine g-C N , indicating that there is a tendency towards the improvement of such
3
4
activity as a consequence of the presence of ZnO. This influence can be better observed
in the ZnO:CN50% sample, whose production was 12.73 µmol/g. However, when more
ZnO (ZnO:CN15%) was added we could notice a drop in the production of CO (1.37
µmol/g), suggesting that although ZnO is fundamental for the catalytic activity of the
material an excessive amount can come to pile up the ZnO rods, agglomerating all of
them in the same region. As a result, the interaction with g-C N was reduced, causing a
3
4
low sharing of photogenerated charges, consequently decreasing the lifetime of the
electron/hole pairs. This decrease in the interaction with g-C N can also lead to
3
4
undesirable photo or chemical corrosion since the g-C N surface used as a stabilizing
3
4
agent will not be available for coupling.
Furthermore, the synthesized heterostructures showed good catalytic
performance for the conversion of CO into other products, such as CH and C H , as it
2
4
2
4
can be seen in Fig. 10. The results demonstrate that the synthesized heterostructures
ZnO:CN15%, ZnO:CN50%, and ZnO:CN85%) produced low amounts of C H , which were
(
2
4
1
6

close to the detection limit of the equipment, making it difficult to analyze the results.
However, it is possible to observe a trend of the ZnO:CN50% sample (0.49 µmol/g) to be
more effective than other samples (0.15 and 0.16 µmol/g for ZnO:CN15% and
ZnO:CN85%, respectively). The C H production involves high energy to promote -CH
2
4
2
dimerization, low stability of the formed intermediates and successive number of steps
that the specimens must undergo to form such product [37]. On the other hand, the CH4
production showed that the low exposure of g-C N did not affect the performance of
3
4
the ZnO:CN15% heterostructures (4.47 µmol/g) compared to 3.99 and 2.94 µmol/g for
ZnO:CN50% and ZnO:CN85%, respectively, indicating a moderate selectivity of the
sample to ZnO catalytic sites.
Therefore, the good photocatalytic performance of the analyzed materials during
the CO conversion can be mainly attributed to: (i) the stability of ZnO promoted by the
2
coupling with the g-C N surface; and (ii) the formation of type-II heterostructures that
3
4
favored the migration of the photogenerated charges and increased the reduction
potential of the photocatalyst. It is important to highlight that the direct Z-scheme would
be also possible considering that the CO photoreduction tests were carried out under
2
UV irradiation, which enables the photoativation of both semiconductors (ZnO and g-
C N ) [33–35]. This effect could positively influence the reduction potential of the
3
4
heterostructure since the CO reduction could occur in the CB of the g-C N (Fig. 9b),
2
3
4
which presents higher reduction potential than the BC of the ZnO.
4
. CONCLUSIONS
Highly crystalline ZnO nanoparticles and ZnO:g-C N heterostructures were
3
4
successfully prepared by a facile method at controlled pH. The synthesized
heterostructures showed distinct properties, such as visible light photoactivation,
1
7

chemical and photo stabilities, better separation of the photogenerated electron-hole
pairs, among others. In general, the ZnO:CN50% sample exhibited outstanding
performance for both AML photodegradation and CO reduction, especially with regard
2
to CO production, which was attributed to synergetic effects associated with the
presence of heterojunctions between the two phases: ZnO and g-C N .
3
4
Acknowledgments
The authors are grateful for the financial support of the funding agencies: CNPq (grants
number 454438/2014-1, 311463/2017-7 and 159866/2018-9), FAPESP (grants number
2
013/17639-4 and 2016/21515-7) and FAPEMIG. The authors also gratefully thank the
support from the LMMA sponsored by FAPEMIG (CEX-112-10), SECTES/MG and
RQ-MG (FAPEMIG: CEX-RED-00010-14); the Improvement of Higher Education
Personnel (CAPES) (Finance Code 001 and CAPES/Embrapa Call 15/2014 Grant
#
166); and the Agronano Network (Embrapa Research Network). The authors thank the
Laboratory of Structural Characterization (LCE/DEMa/UFSCar) for the TEM facilities.
Caue Ribeiro also acknowledges Alexander von Humboldt Foundation and CAPES for
the Experienced Research Fellowship (CAPES/Humboldt Agreement – Process
8
8881.145566/2017-1).
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(
2
2

TABLE
Table 1. Crystallite size (D ) calculated by Scherrer’s equation and specific surface
hkl
area (SSA).
Dhkl (nm)
-1
Samples
SSA (m² g )
(
100)
39
35
38
33
-
(002)
46
40
45
44
-
(101)
34
33
41
34
-
ZnO
6
ZnO:CN15%
ZnO:CN50%
ZnO:CN85%
CN
15
11
21
19

FIGURES
*
C N4
3
ZnO
ZnO:CN15%
ZnO:CN5
*
0%
*
ZnO:CN85%
CN
1
0
20
30
40
50
60
2
θ (degree)
Fig. 1. XRD patterns of the synthesized samples. The hkl and symbol (*) identify the
main diffraction peaks of the ZnO and g-C N crystalline phases, respectively.
3
4
ZnO
(b)
100
(a)
5
%
ZnO:CN
1
5%
0%
80
60
40
20
0
23%
ZnO:CN
5
5
4%
5%
ZnO:CN
CN
85%
ZnO
ZnO:CN15%
ZnO:CN50%
ZnO:CN85%
CN
8
100%
100
200
300
400
500
600
700
1
00
200
300
400
500
600
700
Temperature (°C)
Temperature (°C)
Fig. 2. (a) TG curves showing the mass percent as a function of temperature. The total
mass lost is presented at the end of each TG curve. (b) The first derivative of the mass
loss curves (%/°C).

triazine
Melamine
CN
ZnO:CN85%
ZnO:CN50%
ZnO:CN15%
ZnO
Zn-O
Acetate
O-H, N-H
4
000 3500 3000 2500 2000 1500 1000 500
-1
Wavenumber (cm )
Fig. 3. FTIR spectra obtained from the precursors and all the synthesized samples. More
conclusive peaks and bands are highlighted in the spectra.
EBG (eV)
ZnO
3.24
ZnO:CN15% 2.68
ZnO:CN50% 2.66
ZnO:CN85% 2.69
CN
2.66
2.0
2.5
3.0
3.5
4.0
hν (eV)
300
400
500
600
700
800
Wavelength (nm)
Fig. 4. UV-vis diffuse reflectance spectra and Tauc plot (in the inset) for indirectly
allowed transition obtained from UV-vis diffuse reflectance spectra data. The values of
band gap (E ) are also shown in the inset.
BG

Fig. 5. FEG/SEM images of the synthesized samples: a) CN; b) ZnO; c) ZnO:CN50%
The expanded views of ‘b’ and ‘c’ reveal rod-like ZnO particles and both materials
ZnO and g-C N ) together, respectively. TEM (d) and HRTEM (e) images of the
.
(
3
4
ZnO:CN50% sample. The inset in (e): an expanded view of the HRTEM image for a
representative ZnO nanoparticle.

1.0
0.8
0.6
0.4
0.2
AML
ZnO
ZnO:CN15%
ZnO:CN50%
ZnO:CN85%
CN
0
100
200
300
400
Time (min)
Fig. 6. Photodegradation curves of the amiloride (AML) comparing the direct
photolysis with its degradation in the presence of the synthesized photocatalysts under
visible-light irradiation.
1.0
0.8
0.6
0.4
0.2
0.0
AML
ZnO:CN
50%
1
2
3
Number of cycles
Fig. 7. Stability of the ZnO:CN50% (green bar) photocatalyst during three cycles of
AML photodegradation. AML direct photolysis was also presented for comparison.

Fig. 8. The effects of scavengers on the efficiency of AML degradation with the
ZnO:CN50% photocatalyst under visible-light irradiation.
Fig. 9. Schematic illustration of a) AML photodegradation and b) CO photoreduction
2
using the synthesized heterostructures. *Direct Z-scheme: possible recombination
-
+
between e of the ZnO with h of the g-C N .
3
4