In the modern world, consumption of non-renewable fossil fuels by the growing human population is resulting in a rapid depletion of fuels and ecological pollution at a frightening rate [1
]. Currently, water pollution is highlighted as a vital concern for human beings, as it is frequently being influenced by numerous toxic pollutants such as textiles, cosmetics, food, and paint industries [3
]. In this regard, there are several techniques available for handling polluted water, visible-light-based photocatalysis is considered to be a green approach and has fascinated globally owing to its inexhaustible solar energy [5
]. Typically, photocatalytic features are generally ruled by its inherent physicochemical natures of materials, including its band-gap position, surface area, pore size, and morphological structures [8
] Until now, the existing photocatalytic semiconductors such as the metal oxides and sulfides, oxynitrides, polymers, and organic metal complexes have been demonstrated to possess effective photocatalytic features. Among these, semiconductor metal oxides, such as ZnO, TiO2
O, CuO, NiO, BiVO4
, and ZnO, have the efficiency to maximize the absorption of incident photons [12
]. More importantly, wider band-gap semiconductor materials such as TiO2
, ZnO, and SrTiO3
are already recognized as effective photocatalytic materials due to its great redox potential of photoinduced charge carriers [15
]. In recent years, n-type ZnO semiconductors are preferred, due to its low cost, eco-friendly, simple synthetic procedure, and wide bandgap with comparison to TiO2
. Particularly, the photocatalytic performance of ZnO was comparatively better than TiO2
in few reports. For instance, Sakthivel et al., also reported visible-light assisted photodegradation of azo dye: competition of photocatalytic efficiency [17
]. However, ZnO or other single metal-based materials mostly suffered from photocorrosion and have average performance. On this regard, few scientific efforts are now dedicated to finding suitable approaches for decreasing the recombination rate of charge carriers working under VLI [18
In recent years, various reports are carried out to promote the PCA employing conventional several supporting candidates which can haste an efficient separation of photoinduced charge carriers, namely semiconductors, graphene, CNT, and other carbonaceous materials. In particular, Vadivel et al. reported the BiPO4
/multi-walled carbon nanotubes (MWCNTs) composites for the photocatalyst and supercapacitor applications [20
]. Similarly, the fabrication of graphene and MWCNTs supported materials are economically expensive and produce toxicity during functionalization [21
]. To solve the problems, the researcher introduced polymeric g-CN materials incorporates with semiconductor materials. More importantly, g-CN is used to improve the electron-hole pair recombination rate, stable, inexpensive, proper band position, low-cost characteristic, easily separable, unique properties, and also examined as the visible-light active materials [22
]. The CN photocatalyst materials possessing conjugated electronic alignment with the band-gap of nearly 2.7 eV, holds greater thermal, chemical, electro-optical properties [26
]. During the past decades, Wang et al. demonstrated the polymeric g-CN photocatalytic materials for a visible-light-assisted water-splitting reaction [28
]. The metal-free g-CN is restricted by lower photocatalytic activity due to fast charge recombination. Afterward, Pragati et al., studied the degradation of methylene blue (MB) using ZnO/g-CN as a photocatalyst, urea carbon source [29
The metal-free g-CN can be fabricated by the direct pyrolysis of different organic precursors. The commercially accessible urea and thiourea were recognized as green organic precursors for the fabrication of g-CN. It was evidenced that the usage of urea and thiourea might evolve CO2
, and H2
O vapor during the pyrolysis process [30
], which can be employed as a bubble soft template. Also, g-CN with the enriched surface area was fabricated by annealing both precursors of urea and thiourea, which were recrystallized in ethanol. More importantly, Zhang et al. revealed that the fabricated g-CN with the mixture of thiourea and urea exhibits the greater specific surface area and enhanced PCA compared to g-CN derivatives from the mixture of urea and thiourea [31
]. Also, numerous research efforts have been found for homogeneous carbon sources like urea, melamine, formaldehyde resin and thioureas derived graphitic carbon and are incorporated with semiconductor nanoparticle [32
]. Numerous g-CN-based heterojunctions photocatalysts have been developed by coupling g-CN with different inorganic photocatalysts [35
]. For instance, effective combinations of catalysts comprise heterojunctions of graphene/g-CN, Au/g-CN, TiO2
/g-CN, TaON/g-CN, ZnO/g-CN, Bi2
/g-CN, CdS/g-CN, WO3
/g-CN, and BiOBr/g-CN [35
The PCA of ZnO is restricted owing to its wider bandgap and photoinduced charge carriers are easy to recombine. Also, the incorporation of various metal ions onto ZnO photocatalysts can efficiently enhance PCA [44
]. Recently, doped ZnO-based photocatalysts have been demonstrated improved photocatalytic features for MB degradation [49
]. Similarly, it is also revealed that the stability and PCA of ZnO can be promoted by loading with carbonaceous materials. Recently, Raghavan et al. fabricated a reduced graphene oxide (rGO)/TiO2
/ZnO system via a two-step solvothermal process [50
]. Tien et al. demonstrated a hybrid photocatalyst composed of rGO and ZnO spheres by an easy and rapid microwave-assisted solvothermal reaction [51
]. The formation of heterojunctions between ZnO and g-CN has the potential to develop efficient materials for photocatalytic applications. In this regard, it is imperious to synthesize a different variety of carbon sources to fabricate polymeric g-CN catalysts by a simple and cost-effective method. Hence, in this work, we have fabricated g-CN-based materials derived from the mixture of urea/thiourea.
Herein, ZnO/g-CN nanocomposites were fabricated by a simple and cost-efficient deposition-precipitation method, and its catalytic activity was investigated through photodegradation of MB. The graphitic CN was prepared by urea and thiourea mixture using the pyrolysis method. More importantly, ZnO/CN nanocomposite shows excellent activity in as than its corresponding pristine ZnO and g-CN. Because of the type-II band position of ZnO and g-CN, the interface between g-CN and ZnO benefits the rapid transport of photoinduced charge carriers, thus influenced ZnO/CN to be efficient photocatalysts. More importantly, photostability and reusability of fabricated composites investigation were also completed to identify the reactive species and explore the stability and reusability of composite materials for long time use. Lastly, a feasible photodegradation mechanism of MB above synthesized ZnO/g-CN nanocomposites has also been proposed.
3. Results and Discussion
XRD analysis was used to assess the phase purity and crystalline nature of the fabricated samples. Figure 1
a shows the powder XRD patterns of the prepared bare-ZnO, g-CN, and a series of g-CN/ZnO nanocomposites. As seen from Figure 1
a, observed peaks at 31.8°, 34.43°, 36.27°, 47.57°, 56.67°, 62.93°, 67.99°, and 69.20° and the corresponding diffraction plans from (100), (002), (101), (102), (110), (103), (112), and (201) planes clearly indicated that hexagonal wurtzite crystal phase of ZnO (JCPDS card # 89-0510) [12
]. Also, the powdered XRD patterns of ZnO particles display the peak of Zn(OH)2
a). Particularly, the pure g-CN sample has two distinct peaks at 2θ = 13.34° and 27.38° which can be related to (100) and (002) diffraction planes (JCPDS No. 87-1526) [22
]. Also, the peak positioned at 13.34° related to the in-plane packing motif (100) peak of tristriazine units. In particular, the distance is estimated to be 0.675 nm, which is agreed to the hole-to-hole distance in the nitride pores. Simultaneously, another intense peak of 27.38° is related to C–N aromatic stacking units with a distance of 0.324 nm, attributing to the (002) plane of the interlayer stacking of the conjugated aromatic system. In this regard, the sharp and intense diffraction peaks of both g-CN and ZnO evidenced their crystallinity nature. In fabricated composites, all the series of g-CN/ZnO nanocomposites shows the identical distinctive diffraction peaks with the bare-ZnO. With the ZnO incorporation over the g-CN, the ZnO crystalline peaks appeared at g-CN/ZnO composites. There is no impurity found in the g-CN/ZnO composites, thus confirmed the successfully prepared high purity of g-CN and g-CN/ZnO nanocomposites. Further, the average particle size of the ZnO particles was used to calculate Scherrer’s equation, the obtained particle size was 40 ± 2 nm. These results revealed that the ZnO fabricated onto g-CN with the chemical bonding of Zn–N was successfully achieved via condensation reactions.
b shows the FT-IR spectra of ZnO, g-CN, and a series of g-CN/ZnO photocatalysts with different ZnO contents. FT-IR spectra of g-CN showed the following characteristic bands; C–N stretching vibration mode at the wavenumber of 1628 cm−1
and the aromatic C–N stretching modes are observed at 1230, 1398, and 1543 cm−1
]. Also, the band witnessed at 802 cm−1
is credited to out-of-plane bending modes of C–N heterocycles [53
]. More importantly, the FT-IR spectrum of independent-ZnO, the peak located at 591 cm−1
was assigned to the stretching vibration of Zn–O [53
]. Further, a broad absorption peak positioned at 3500 cm−1
was owing to the existence of water molecules [54
]. More importantly, Figure 1
b also displays the FT-IR spectra of a series of ZnO/g-CN composite materials, the observed peaks are identical to those of peaks of g-CN are observed in the composite. However, with the increase in the percentage of ZnO (60% to 75%), the peak intensity also increased. Also, it is evidenced that the obtained peaks correspond to the g-CN and ZnO are witnessed in the g-CN/ZnO composite materials. All results have shown that the fabricated materials were nanocomposite rather than a physical combination of two distinct phases ZnO and g-CN.
The surface morphology and microstructure of the g-CN, ZnO and g-CN/ZnO (75%) composites were inspected through FE-SEM as presented in Figure 2
. Figure 2
a shows the FE-SEM images of g-CN which is composed of nanosheets sheets like structures and fluffier. Also, hydrothermally fabricated ZnO particles are obtained with an irregularly aggregated spherical particle (Figure 2
b). Interestingly, the surface morphology of fabricated g-CN/ZnO (75%) composites (Figure 2
c,d) contains both spherical ZnO particles and nanosheet morphologies, which were homogeneously dispersed. Thus the fabricated binary nanocomposites materials are expected to play a vital role in determining its photocatalytic performances.
To further assess the morphological features of fabricated composites, the HR-TEM measurements were carried out and are depicted in Figure 3
. HR-TEM images of g-CN/ZnO (75%) composite as seen in Figure 3
a,b, clearly observed the darker particles of ZnO over the thin layer sheets of g-CN, which is consistent with FE-SEM analysis. Furthermore, HR-TEM of a composite at higher magnifications to observe more number of fringe patterns of ZnO particles over g-CN sheets (Figure 3
c,d). The observed interplanar distance (d) of 0.252 nm is observed to have matched with the lattice fringe spacing of the (101) ZnO hexagonal wurtzite phase, which also reveals the interface between the hexagonal ZnO and g-CN sheets [12
]. Inset of Figure 3
d reveals the respective selected area electron diffraction (SAED) pattern of g-CN. These morphological investigations evidenced the interface between g-CN nanosheets and ZnO particles in the composites and are appropriate candidate materials to enhance the charge-separation and thereby enhancement of the PCA.
3.1. Photocatalytic Activity
The photocatalytic MB dye degradation performances of ZnO and g-CN/ZnO photocatalyst were studied under VLI. In a typical process, the maximum absorption concentration MB dye peak at 664 nm was taken to investigate the catalytic degradation of MB dye. Particularly, the time-dependent absorption spectra of MB with differently prepared photocatalysts were displayed in Figure 4
. The maximum absorption intensity MB dye was slowly decreased in the presence of pristine ZnO with the illumination of VLI. The observed result clearly shows the low degradation efficiency of pristine ZnO nanoparticles (Figure 4
a). Moreover, after the addition of g-CN into the ZnO nanoparticles, the catalytic activity was gradually increased which means the g-CN nanosheets are influenced toward the enhanced PCA. Interestingly, the photodegradation efficiency of the g-CN/ZnO (75%) photocatalysts was considerably greater than that of pristine ZnO and other combinations of g-CN/ZnO. Amongst all the as-prepared samples, g-CN/ZnO (75%) exhibited improved catalytic activity because of the enhanced visible-light absorption and suppressed recombination rate in comparison with bare ZnO photocatalyst or g-CN.
Similarly, g-CN provided more active surface and strong absorption capacity to MB dye molecules. Furthermore, the layered nanosheets structure of g-CN favored the photogenerated electron-hole transfer from g-CN to ZnO. It is a significant factor in photocatalytic activities of g-CN/ZnO (75%) nanocomposites as photocatalysts.
reveals the photodegradation of MB over bare-ZnO, undoped g-CN, and a series of g-CN/ZnO composites. The blank analysis was performed without a catalyst to determine the stability of MB under illumination conditions and the results indicated that the MB was stable under illumination. Figure 5
shows the kinetic plot of (C0
) vs. irradiation time (min) to examine the degradation of MB dye in the presence of as-prepared catalysts.
Further, to understand the reaction kinetics of the MB degradation catalyzed by fabricated catalysts, the experimental data were fitted by a first-order kinetic model as shown by following Equation (1):
and C are the initial and concentration of MB at a certain time interval and k
is the rate constant. Figure 6
shows the linear fitting kinetic plot for the degradation of MB for fabricated catalysts. The observed results clearly show that the prepared catalyst obeyed the pseudo-first-order kinetics. The calculated rate constants (Kapp
), agreeing with correlation coefficients (R2
), and maximal dye degradation in the presence of pristine and g-CN/ZnO nanocomposites are given Table 1
. The estimated k
values for the bare ZnO, bare g-CN, g-CN/ZnO (60%), g-CN/ZnO (70%), and g-CN/ZnO (75%) are found to be 0.0021, 0.0038, 0.0047, 0.009, and 0.0128 min−1
, correspondingly. Also, it is demonstrated that g-CN/ZnO (75%) composite has a greater rate constant which was ~6-folds superior to the bare ZnO. The existence of more reacting species, larger surface area, and enriched acting sites are improving the PCA. More importantly, the observed rate constant value also larger than that obtained for Fe-doped ZnO as reported by Isai et al [55
] and larger with that observed for boron-doped g-CN-based composite [56
shows the photodegradation efficiency of as-prepared nanocomposite samples in comparison with bare samples. The bare ZnO, bare g-CN, g-CN/ZnO (60%), g-CN/ZnO (65%), g-CN/ZnO (70%), and g-CN/ZnO (75%) materials are found to degrade around 26%, 45%, 60%, 69%, 80%, and 91% the MB solution under 120 min of irradiation. These results evidenced that with an increase in the amount of ZnO into the g-CN the PCA also gradually increased. The observed degradation result confirms that the g-C3
prominently improve the PCA.
3.2. Reactive Species Studies
To know the role of active species in the MB dye degradation activity, radical trapping tests were performed, as displayed in Figure 8
. The close inter-phase contact coupling of g-CN and ZnO particles in g-CN/ZnO must play a major part in improving PCA [39
]. In general, the close coupling of ZnO/g-Cn nanoparticles results in promoting the electron transfer between interfaces and hindering the recombination of photoinduced charge carriers [58
]. Usually, during the photocatalysis process, hydroxyl, holes, and superoxide radicals are the promising reactive species for the degradation of organic pollutants [59
]. Herein, isopropyl alcohol (IPA), triethanolamine (TEOA), and p-benzoquinone (BQ) which are engaged as the scavengers for hydroxyl, holes, and superoxide radicals, correspondingly [12
]. In a scavenger free reaction, the MB dye maximum degradation efficiency was observed up to 90.64% in 180 min irradiation for g-CN/ZnO (75%). So the g-CN/ZnO (75%) samples were selected to investigate the radical trapping study. After the introduction of radical trapping agents into the photocatalytic reaction, the degradation efficiency was decreased particularly, the observed efficiency was 90.6, 42.1, 76.3, and 62.9 percentages for without scavenger, TEOA, BQ, and IPA, respectively. This investigation confirmed that hole played a major role in the photocatalytic MB degradation compared to other radicals which means that after the addition of TEOA into the reaction nearly 60% of the efficiency was hindered. Hence, these results show that MB degradation mainly is governed by photogenerated holes.
3.3. Reusability Studies
The stability of the synthesized catalyst materials was examined by recycling the photocatalysts for the photocatalytic degradation of MB dye. Each cycle, a fresh MB dye solution was used for the next photocatalytic test. The catalyst particles were collected by centrifuged and washed, with distilled water then reused. As shown in Figure 9
, no major reduction was observed in the efficiency after four consecutive cycles, as a result, the g-CN/ZnO (75%) photocatalysts were photostable.
The apparent charge-transfer mechanism for MB degradation over g-CN/ZnO system was presented in Figure 10
. As it is known that, the pure ZnO cannot be excited by visible light irradiation, the photodegradation of MB can mainly be attributed to the photogenerated hole oxidation and photoreduction process. On comparing pure g-CN, the g-CN/ZnO photocatalysts exhibited remarkable enhancement for degrading MB under visible light. The CB and VB edge potentials of as-synthesized pristine ZnO and g-CN were acquired from Butler–Ginley method [60
]. The CB and VB edge potentials of the g-CN were calculated to be −1.12 and +1.57 eV, while the CB and VB edge position of ZnO are −0.26 and +2.83 eV, respectively. The appropriate band positions of g-CN and ZnO endorse the creation of the heterojunction and thereby improvement in PCA. According to the proposed mechanism, both ZnO and g-CN are expected to be photoinduced to create carriers. Under VLI conditions, since the CB potential of g-CN is more negative than that of ZnO, the photogenerated electrons on g-CN particle surfaces transfer easily to ZnO through the well-developed interface [62
]. So the excited electron on g-CN could directly inject into the CB of ZnO. The electron-hole separations are also driven by the internal reassembly rebuilt electric fields in the two semiconductors. This decreases the strength of electron-hole recombination and leads to large numbers of electrons on the ZnO surface and holes on the g-CN surface, respectively, thus promoting the photocatalytic reactions to degradation MB.