3.1. Characterization of the as-synthesized NMs
XRD analysis was used to study the crystalline phase structures and formation of the g-C3N4, g-C3N4/ZnO and g-C3N4/Cu2O and g-C3N4/ZnO/Cu2O ternary heterostructure composites, with the outcomes of XRD pattern, were presented in Figure. 1 (a-d). The weak and strongest diffraction peak of the g-C3N4 sample (Figure. 1 (a)) corresponds to the (100) and (002) crystal plane (JCPDS No. 87-1526) at 2θ = 13.1° and 27.3° which is a typical inter-layer structured stacking peak of aromatic pieces [22]. The peaks into the g-C3N4/ZnO sample (Figure. 1 (b)) was located at 2θ = 31.76°, 34.31°, 36.1°, 47.54°, 56.57°, 62.9°, and 67.82° which related to the (100), (002), (101), (102), (110), (103), and (112) crystal planes of the hexagonal wurtzite structure of ZnO [23], besides its good contract with the standard JCPDS No. 36-1451. XRD diffraction peaks appear at 29.61°, 36.38°, 42.41° and 62.3° consistent to (110), (111), (200) and (220) crystal plane (JPCDS No. 65-3288) [24] reflections of Cu2O NMs in the g-C3N4/Cu2O heterojunction (Figure. 1 (c)). The XRD pattern of g-C3N4/ZnO/Cu2O heterostructure composite (Figure. 1 (d)) which was included all the typical peaks of g-C3N4, Cu2O and ZnO relatively. Hence, the fairly sharp and narrow diffraction peaks which indicate that the as-obtained NMs had high crystallinity and phase pureness. From the XRD analysis results, indicating the coexistence of g-C3N4/ZnO, g-C3N4/Cu2O and the chief g-C3N4/ZnO/Cu2O heterostructures and auxiliary elucidating the effective formation of heterostructures. Also, no other characteristic peaks were observed, which thus confirming the high purity phases of the as-obtained samples.
The structural foundations and interface among the components are further confirmed by the FT-IR spectra. Figure. 2 shows the FT-IR spectra of as-obtained samples. The bands at 1650 − 1250 cm− 1 are ascribed to the usual C-N-C (C-N and C = N heterocycles) stretching vibrations of the benzene ring of g-C3N4. Whereas those at 1385, 3195 and 3430 cm− 1 are due to the C-N bending, terminal -NH2 and N-H stretching vibrations (amino-groups), individually [25]. Similarly, the -OH bending and stretching vibrations were appear in the range of 3200–3500 cm− 1 recognized to the adsorbed water (H2O) on Cu2O and ZnO materials. The average peaks in 560–720 cm− 1 are consigned to the apt bridging of bending/stretching vibrational mode of Zn-O and Cu-O or Zn/Cu-OH bonds in ZnO and Cu2O networks [26]. Hence, the FT-IR results are in mark with those of XRD pattern which indicates the formation of ZnO and Cu2O on the surface of g-C3N4 nanostructure. All the g-C3N4 based NCs displays a characteristic absorption peak at 809 and 885 cm− 1, which matches to the living modes of tris-triazine ring units and the distortion of N-H structured functional group, separately [27]. It could be seen that the g-C3N4/ZnO, g-C3N4/Cu2O and g-C3N4/ZnO/Cu2O heterostructure samples have distinctive interaction absorption peaks for both between Cu2O, ZnO and g-C3N4, signifying that the effective construction of the NCs.
The microstructure and surface morphological features of g-C3N4 and g-C3N4/ZnO/Cu2O nanocomposite were analyzed FE-SEM and HR-TEM analysis. Typical FE-SEM images of (Figure. 3 (a, b) pristine g-C3N4 NMS exhibits porous-like structured rough morphology [28]. The g-C3N4 composite comprising ZnO/Cu2O shows a layered structure/wrapped exfoliated with irregular morphology on the smooth surfaces (Figure. 3 (c-e)), which are ascribed to the metal-oxide phases caused by calcination process [29]. The consistent EDX spectra (Figure. 3 (f)) results of the g-C3N4/ZnO/Cu2O has exposed that the composite material which contains the N, O, C, Cu, and Zn elements, designating that the ZnO/Cu2O has been effectively loaded onto the g-C3N4 lattice, specifying the construction of the ternary nanocomposite [30]. The corresponding atomic (at. %) and weight (wt. %) percentage of the as-synthesized nanocomposite is publicized in (inset) Figure. 5(f).
To further demonstrate the structural information of as-obtained g-C3N4/ZnO/Cu2O nanocomposite is attained by HRTEM images. As shown by HR-TEM images in Figure. 4 (a, b) the g-C3N4/ZnO/Cu2O nanocomposite has aggregated g-C3N4 layered like structure and nano-sized ZnO/Cu2O (dark colour) NPs are tightly attached and/or deposited on the surface enclosed [28]. Also, the larger amount of spherical shaped NPs was accumulated on the covered structure of g-C3N4. Similarly, the well-recognized that such heterostructures formed among g-C3N4 and the ZnO/Cu2O metal-oxide phase, which might give an intense effect also beneficial for the separation/transfer of photo-excited charge carriers, and could facilitate the enhancement of photocatalytic performance of the composite material [31, 32]. Figure. 4 (d) displays a selected area electron diffraction (SAED) pattern of the g-C3N4, ZnO and Cu2O major planes (and also the existing of the ring pattern), which were reserved at the interface of the g-C3N4/ZnO/Cu2O composite [33]. Besides, the spots which are distinct with a match to the (002), (101), and (111) planes of as-given samples.
The optical absorbance properties of the as-obtained materials were deliberated by UV-DRS spectroscopy studies, and the outcomes are revealed in Figure. 5(A). It could be seen that the absorption edge of pristine g-C3N4 displays the photo-absorption at ~ 442 nm in the visible-light region, which was owed to the photo-excitation of an electron from the valence band (VB) of N2p to the conduction band (CB) of the C2p orbital. In contrast, the g-C3N4/ZnO displays the photo-absorption band edge at ~ 448 nm in the visible-light region owing to the proper electron transition of n-π* [34]. Also, the absorption spectrum intensity of g-C3N4/ZnO heterostructures exceeds the most solid visible-light absorption region of ~ 518 nm upon the addition of Cu2O NPs. Additionally, a redshift of the absorption bands for g-C3N4/ZnO/Cu2O composite was observed as an effect of coordination bonds fashioned among Cu, Zn, O, C and N elements [35]. The consequences could clear that the g-C3N4/ZnO/Cu2O ternary nanocomposite have higher visible-light activity than as-synthesized other related g-C3N4, g-C3N4/ZnO and g-C3N4/Cu2O NMs. Instead, these results might be owing to the synergistic correlation amid g-C3N4 and ZnO/Cu2O in the composite which loading to the optimum level [12].
As realized in Figure. 5(B), the resultant optical bandgap of g-C3N4, g-C3N4/ZnO, g-C3N4/Cu2O and the chief g-C3N4/ZnO/Cu2O nanocomposite was calculated by Tauc plot via Kubelka-Munk function and the found values are 2.7, 2.91, 2.28 and 2.42 eV individually. The calculated bandgap value of the g-C3N4/ZnO/Cu2O composite was lower than those observed for g-C3N4 and g-C3N4/ZnO NMs [14, 15]. These outcomes demonstrate that the loading of ZnO/Cu2O on g-C3N4 composite reformed the electronic band structure of the graphitic provision, which became capable to absorb light at higher wavelengths in the visible-light range. It also specifies the upgrading of photo-excited electron-hole (e−/h+) charge carriers, expands the light utilization efficacy, which results in high responsive photocatalytic activity under visible-light exposure [36]. Moreover, the ability of the low bandgap intercept corresponding to Cu2O has relatively considered since it is the key factor contributing to the photoabsorption progression.
An additional feature of to recognize the photo-recombination of the photo-excited electron-hole (e−/h+) pairs, which are key contributing aspect in the photocatalytic performance [37]. Also, the probability of photo-excited charge carrier recombination, trapping efficiency, charge transfer/separation process and the migration, PL spectra of the as-obtained PCs were verified. Figure. 6 (A) displays the (excited by the light of ~ 337 nm) PL emission spectrum, g-C3N4 illustrates a high PL intensity, which is expressive of fast recombination of the photo-excited charges. Finally, the PL spectra of conforming to the composites have revealed the superficial reduction in the PL intensity of the g-C3N4/ZnO/Cu2O composite, signifying that the charge carriers are efficiently separated with inhibiting the photo-recombination of the electron-hole (e−/h+) pairs and hence increasing the lifetime of the species [38]. Further, the interfacial blend between ZnO/Cu2O and g-C3N4 in the effective composite making suitable heterojunction formation was also favourable for the charge separation, thus improving the photocatalytic performance.
Photocatalytic Properties
The photo-degradation of RhB dye over the as-prepared g-C3N4/ZnO/Cu2O composite PCs was measured in Figure. 6 (B) for the aqueous solution in the existence of visible-light exposure. The declines in the peak intensity (554 nm) which established the photo-degradation of RhB dye and it has proportionate to the exposure time [38]. The g-C3N4, g-C3N4/ZnO and g-C3N4/Cu2O PCs could 36.4 %, 53 % and 66 % of photo-degradation activity of the RhB dye in 100 min, despite the less utilization of visible-light response, owing to its high photoelectron-hole (e-/h+) pair recombination rate of g-C3N4. As exposed in Figure. 7(A), the concentration of RhB does not display in the blank test signifying that the self-degradation of RhB dye is negligible degradation under the identical condition [39]. As expected, the sharp reductions in RhB dye absorbance were detected for the g-C3N4/ZnO/Cu2O composite PCs. The photo-degradation efficiency of RhB dye on the g-C3N4/ZnO/Cu2O composite photocatalyst was greater (91.4 % for 100 min.) than that of other as-obtained catalysts (Figure. 7 (A)), owing to the most solid visible-light responsive and their absorption range also. Also, the photoelectron-hole (e-/h+) pair recombination rate of g-C3N4/ZnO/Cu2O composite PCs was the lowest as compared to other as-obtained samples [12].
Besides, the pseudo-first-order kinetic model (Figure. 7(B)) of the RhB photo-degradation reaction on the as-obtained PCs were could be defined as ln(C/C0) = -kt, where k signifies the pseudo-first-order kinetic rate constant (min-1), C0 and C are the primary concentration and at time t of the RhB dye, individually [40]. The deceptive rate constant k of g-C3N4/ZnO/Cu2O ternary nanocomposite (0.04796 min−1) which was higher results than the degradation rates of as-obtained pristine g-C3N4 (0.00744 min−1), g-C3N4/ZnO (0.01504 min−1) and g-C3N4/Cu2O (0.02117 min−1) composite PCs materials respectively. Similarly, the outcomes confirmed that the g-C3N4/ZnO/Cu2O ternary nanocomposite had the greater rate constant, which was 6.44, 3.18 and 2.26 folds more eminent than those of as-obtained for pristine g-C3N4, g-C3N4/ZnO, and g-C3N4/Cu2O NCs, individually [41]. The correlation coefficient (R2) ideals of g-C3N4, g-C3N4/ZnO, g-C3N4/Cu2O and g-C3N4/ZnO/Cu2O ternary nanocomposite materials originated to be 0.9521, 0.9939, 0.9976 and 0.9647, consistently.
Moreover, the stability of the as-obtained PCs is a critical aspect in relative to large-scale technologies practical application [42]. Also, to assess the stability of the g-C3N4/ZnO/Cu2O ternary photocatalyst, recycling tests were directed on the specified photocatalyst for the photo-degradation of RhB aqueous dye under visible-light exposure. When the recycling progression, the photocatalyst was collected by filtering and then washed by D.I. water with auxiliary dried in an oven at 60°C. The catalytic sample was reused for five succeeding degradations, and the stability outcomes plots are obtained in Figure. 8 (A). The composite photocatalyst preserved a great photocatalytic activity, and the removal rate of RhB dye on the g-C3N4/ZnO/Cu2O composite photocatalyst was 80.2 % after five consecutive recycles. It could be seen that g-C3N4/ZnO/Cu2O nanocomposite has lost ~ 7.5 to 9.5 % of its primary photocatalytic action (91.4 %) towards the dye after the third to five recycles. Besides, there was a slight reduction in the amount of PCs during the recycling progressions [43]. Thus, the g-C3N4/ZnO/Cu2O composite photocatalyst was revealed outstanding catalytic recycling stability under visible-light exposure.
Photocatalytic recycling stability
Hence, to additional evaluate the structural steadiness of the g-C3N4/ZnO/Cu2O samples were collected after five repeated cycles for XRD and FT-IR testing and whereas compared with the XRD pattern and FT-IR spectra of the samples for before cycling [44]. Also, the outcomes are accessible in Figure. 9 (A and B). No substantial changes were noticed in the structures and active functional groups of the photocatalyst before and after reused, which specifies that the g-C3N4/ZnO/Cu2O heterostructured photocatalyst was greatly stable. The as-obtained photocatalytic activity of this work was better for with the formerly reported photo-degradation [38, 45–47]. Also, the photocatalytic performance of g-C3N4/ZnO/Cu2O composite photocatalyst was greater when compared to the commercial P25 TiO2 NPs [48]. These consequences also further confirmed that the as-obtained g-C3N4/ZnO/Cu2O composite photocatalyst could be recycling process, which is active in the applied application for the effective photo-degradation in various dyes [49].
A Mechanism For Photocatalytic Dye Degradation
To further inspect the mechanism for the photo-degradation of RhB under visible light exposure, radicals and holes scavenging tests were directed to sense the chief active species in the role of photocatalytic progression. Likewise, OH˙, ˙O2− and h+ were trap reduced using IPA, BQ and EDTA, separately [50]. The photo-degradation efficiencies of RhB dye on the g-C3N4/ZnO/Cu2O PCs in the existence of relative scavengers are obtainable in Figure. 8 (B). In this existent, the removal efficiency of RhB dye was expressively reduced upon an accumulation of BQ, which signifies that the main effect of ˙O2− radicals play an energetic part in RhB degradation process. Equally, the removal efficiency of RhB was not considerably reduced in the occurrence of IPA for OH˙ radicals have a minor/essential impact on the photocatalytic action [51]. The photo-degradation efficiency has changed slightly by adding EDTA in this identical process, which implies that photo-holes have a minor influence on the RhB dye photo-degradation process [52].
Based on the photo-degradation outcomes, a possible charge transfer mechanism of g-C3N4/ZnO/Cu2O PCs under visible-light exposure has been proposed based on the energy band structure. The effective SCs VB and CB edge potential could be calculated using the resulting formula of (i) ECB = χ - Ee − 0.5 Eg; and (ii) EVB = ECB + Eg. Whereas, χ signifies the absolute electronegativity of the specified semiconductor (g-C3N4 = χ ~ 4.72 eV; ZnO = χ ~ 5.79 eV and Cu2O ~ 5.32 eV), ECB and EVB are the edge potentials of CB and VB respectively [53] [54]. Also, while the Ee and Eg are the free electron energy (~ 4.5 eV) on the hydrogen scale and the bandgap energy of the as-given semiconductor [55] (Eg = 2.7, 3.29 and 1.98 eV for g-C3N4, ZnO and Cu2O exclusively. The calculated VB and CB edge potentials were; g-C3N4 is at + 1.57 eV and − 1.13 eV; ZnO is at + 2.935 and − 0.355 eV; Cu2O is at + 1.81 and − 0.17 eV; respectively. Figure. 10 displays the probable band configuration of the g-C3N4/ZnO/Cu2O based photocatalysts.
When exposed to visible-light radiation (λ > 400 nm) of both g-C3N4 and ZnO/Cu2O could adsorb photons and generate e−/h+ pairs under visible-light exposure [6]. While the photo-excited electrons (e−) will transfer under the influence of an interface charges from g-C3N4 to Cu2O, and the photo-excited (h+) holes will migrate from the VB of ZnO to the VB of Cu2O. Since both VB of ZnO and Cu2O have very higher than g-C3N4, the photo-excited (e−) of g-C3N4 transfer directly to the CB of the ZnO and then Cu2O surface. Consecutively, the photo-excited h+ of ZnO and Cu2O tend to migrate to the VB of g-C3N4 surface [56]. In contrast, photo-holes of ZnO/Cu2O are inserted into the edge potential is fashioned in the charges, which supports the separation of the photo-excited carriers. Also, common facts that g-C3N4 and ZnO are typical n-type SCs, hence the n-n heterojunction was fashioned at the interface among the g-C3N4/ZnO NMs [57]. According to the edge potential heterojunction of g-C3N4 and ZnO/Cu2O, the photo-excited h+ on the VB of g-C3N4 cannot oxidize the adsorbed H2O molecules into OH˙/ OH % radicals, since the calculated VB potential of g-C3N4 (+ 1.57 eV vs. NHE) is less positive than the standard redox potential E(H2O/ OH˙−) (+ 2.935 eV vs. NHE). The photo-excited e− on the CB of N-ZnO cannot diminish the physisorbed O2 molecules into ˙O2−, since the CB potential of Cu2O (-0.17 eV vs. NHE) is further positive than the standard redox potential of E(O2/˙O2−) (-0.355 eV vs. NHE). Then, the photo-electrons stored in CB of g-C3N4 could respond by molecular oxygen dissolved in the solution to yield the responsive either superoxide radicals (˙O2−), and then ˙O2− radicals and h+ are capable to openly oxidize the organic impurities (RhB dye) owing to their great oxidative ability reagents for RhB [45] dye to make it degrade eventually. Thus the synergistic separation of photo-excited e−/h+ pairs could be captured on the g-C3N4 surface of the PCs to yield a variety of reactive oxygen species (ROSs) to effect the desired degradation in the aqueous dye solution [58].
Surveyed by the upstairs discussion, it was resolved that the photocatalytic activity of g-C3N4/ZnO/Cu2O composite semiconductor PCs was considerably upgraded. This was because of the resulting in some cases: (i) hetero-structured nature between g-C3N4 and ZnO/Cu2O have enriched the visible-light absorption belongings, (ii) the synergistic effect from the interfacial charge carries and the matched band structure of g-C3N4 and ZnO/Cu2O could increase the separation rate and also effectually inhibit the recombination of photo-excited charge carriers.