Herein, Cu2O spheres were prepared and encapsulated with reduced graphene oxide (rGO). The Cu2O–rGO–C3N4 composite covered the whole solar spectrum with significant absorption intensity. rGO wrapped Cu2O loading caused a red shift in the absorption with respect to considering the absorption of bare C3N4. The photoluminescence study confirms that rGO exploited as an electron transport layer at the interface of Cu2O and C3N4 heterojunction. Utmost, ∼2 fold synergistic effect was achieved with Cu2O–rGO–C3N4 for the photocatalytic reduction of 4-nitrophenol to 4-aminophenol in comparison with Cu2O–rGO and C3N4. The Cu2O–rGO–C3N4 photocatalyst was reused for four times without loss in its activity.

Semiconductor photocatalysis technique is one of the best environmental friendly methods for the degradation of organic pollutants, removal of heavy metals, water purification, and disinfection.1–3 Owing to the non-toxicity, chemical stability, insolubility in water, and the favorable optical property, titania (TiO2) is regarded as the promising material to approach the universal energy and environmental crises.4,5 Unfortunately, TiO2 can only be excited by ultraviolet (UV) light, which limits its application to a great extend.6 Many approaches have been proposed to make the TiO2 as visible light active: metal ion implanted TiO2, reduced TiOx photocatalysts, non-metal doped TiO2, composites of TiO2 with semiconductor having lower band gap energy, sensitizing of TiO2 with dyes, and TiO2 with luminescence agent.7–12 On the other hand, a great research effort is being taken to replace the UV light active TiO2 photocatalyst with the visible light active stable photocatalysts. Recently, Wang et al. reported a novel metal free polymer material and graphitic carbon nitride (g-C3N4) for the production of H2 by splitting of water via photocatalysis.13 Because of its lower band gap energy, g-C3N4 (Eg = 2.7 eV) can absorb natural sunlight and thereby active under visible light. More interestingly, C3N4 possesses more favorable properties to be a suitable photocatalyst such as extremely thermally stable, chemically inert, and resistive towards photocorrosion.14 High recombination rate of electron-hole pairs makes the pristine C3N4 to be less efficient under visible light irradiation that limits its energy and environmental applications. Furthermore, C3N4 suffers with inadequate photon absorption capability after ∼460 nm.15 Many methods have been developed to overcome the above two demerits such as doping of metals or non-metals and combining with graphene.16,17 Hence, to make the C3N4 photocatalyst more efficient, copper(I) oxide (Cu2O) is deposited on the surface of C3N4. It is well known that Cu2O is a promising visible light active photocatalyst due to its low band gap energy (2.2 eV), nontoxicity, and natural abundance.18 For example, Tion et al. reported the preparation of C3N4/Cu2O heterojunction and its photocatalytic studies for the degradation of methyl orange dye under visible light irradiation.19 

To enhance the reactivity of the photocatalyst or to reduce the charged carrier recombination, rGO was used as a potential carbonaceous solid support in many of the photocatalytic systems.20 But insertion of the rGO exactly at the interfacial of the two heterojunction is another key challenge. Hence, in this particular research work, Cu2O spheres were wrapped with rGO layer and then deposited on the surface of C3N4, so that the rGO layers are precisely in between the p-n heterojunction of Cu2O and C3N4. So, rGO effectively increased the charged carrier mobility in the interfacial region of the two semiconducting materials. To prove the efficacy of the prepared composite, photocatalytic reduction of 4-nitrophenol was performed under visible light irradiation which is an important organic reaction as per industrial perspective.

The schematic representation for the preparation of Cu2O–rGO–C3N4 photocatalyst is illustrated in Figure 1. Cu2O spheres were prepared by simple wet solution method using Cu(CH3COO)2⋅H2O, glucose, and polyvinyl pyrolidone. The as-prepared Cu2O spheres were characterized by scanning electron microscope - energy dispersive spectroscopy (SEM-EDS) analysis. As depicted in Figure 2(a), the Cu2O possesses exact spherical in shape. This hierarchical spherical structure made the wrapping of rGO extremely straightforward. More importantly, no defective spots were identified in the spherical structure which emphasized the reliability of this preparation protocol [Figures 2(b)-2(d)]. Furthermore, the purity of the sample was estimated by EDS analysis which confirmed the presence of Cu and O (the elemental C peak came from double side carbon tape used as a sample holder for SEM-EDS measurements) [Figure 2(e)]. In order to find the phase purity of the prepared Cu2O spheres, XRD analysis was carried out and the pattern exactly resembled the XRD pattern of Cu2O (JCPDS card number is 77-0199).21 More importantly, no peaks were observed with respect to the CuO, which confirmed the complete reduction of Cu2+ (Cu(CH3COO)2⋅H2O) to Cu+ (Cu2O) [Figure 2(f)].22 

FIG. 1.

Schematic illustration for the preparation of Cu2O–rGO–C3N4 photocatalyst.

FIG. 1.

Schematic illustration for the preparation of Cu2O–rGO–C3N4 photocatalyst.

Close modal
FIG. 2.

(a)-(d) SEM images of hierarchically structured Cu2O spheres with different magnifications. (e) EDX graph [inset: elemental wt. % table] and (f) XRD pattern of Cu2O spheres.

FIG. 2.

(a)-(d) SEM images of hierarchically structured Cu2O spheres with different magnifications. (e) EDX graph [inset: elemental wt. % table] and (f) XRD pattern of Cu2O spheres.

Close modal

XRD patterns of Cu2O, (3-Aminopropyl)triethoxysilane (APTES) modified Cu2O, Cu2O–rGO, Cu2O–rGO–C3N4 and C3N4 are depicted in Figure 3. As discussed previously, the XRD peaks of phase pure primitive lattice Cu2O spheres were observed. The XRD pattern of surface modified Cu2O using APTES precisely resembled with the pure Cu2O spheres that revealed that surface modification only generated positive charges on the surface of Cu2O spheres but not created any changes in the phases and oxidation states. However, while wrapping the rGO layer onto the surface of Cu2O spheres, a new XRD peak centered at 2θ = 38.74° was appeared, corresponding to the (111) plane of CuO.23,24 This oxidation of Cu2O to CuO occurred during the calcination process. Nevertheless, the intensity of the peak was very less which authenticated that the over-oxidation was very minimal because the calcination performed under inert condition (in presence of N2 atmosphere). It is worth to mention here that no authenticated peaks were observed for rGO in the XRD spectra of both Cu2O–rGO and Cu2O–rGO–C3N4 photocatalysts which might be due to the lowest loading of GO (1%). The additional two peaks in the XRD pattern of Cu2O–rGO–C3N4 photocatalyst at 13.05° and 27.55° were corresponded to the (001) and (002) planes of graphitic C3N4 (g-C3N4), respectively.25 This was further confirmed by comparing the XRD spectrum of bare g-C3N4 as shown in Figure 3.

FIG. 3.

XRD patterns of Cu2O, APTES modified Cu2O, Cu2O–rGO, Cu2O–rGO–C3N4, and C3N4.

FIG. 3.

XRD patterns of Cu2O, APTES modified Cu2O, Cu2O–rGO, Cu2O–rGO–C3N4, and C3N4.

Close modal

Figure 4 represents the SEM images of APTES modified Cu2O, Cu2O–rGO and Cu2O–rGO–C3N4 photocatalysts. It is seen that APTES not only created positive charges on the surface of Cu2O spheres but also reduced the particle size which is clearly evident from the SEM images of APTES modified Cu2O [Figures 4(a) and 4(b)]. This size contraction might be due to the existence of more positive charges on the surface of Cu2O. Presence of rGO layer in Cu2O–rGO photocatalyst was verified by the SEM images as shown in Figure 4(c). Complete wrapping of rGO over Cu2O spheres were confirmed from the Figure 4(d). Nonetheless, only a meager amount of free rGO layers were identified at the background which authenticated the complete utilization of rGO for the wrapping of Cu2O spheres.26 The SEM images of Cu2O-rGO-C3N4 composite are depicted in Figures 4(e) and 4(f). In addition, even after introducing C3N4, the spherical morphology of Cu2O was maintained. Furthermore, the SEM results revealed that the rGO wrapped Cu2O spheres were dispersed homogeneously on the surface of C3N4.

FIG. 4.

SEM images of (a) and (b) APTES modified Cu2O (c) and (d), rGO wrapped Cu2O, and (e) and (f) Cu2O–rGO–C3N4 composites.

FIG. 4.

SEM images of (a) and (b) APTES modified Cu2O (c) and (d), rGO wrapped Cu2O, and (e) and (f) Cu2O–rGO–C3N4 composites.

Close modal

In order to understand the absorption behavior and band gap of Cu2O, C3N4 and Cu2O–rGO–C3N4, ultraviolet-visible (UV-vis) spectroscopy was studied in diffused reflectance spectroscopy (DRS) mode. The results are shown in Figure 5. The absorption range was restricted to 460 and 600 nm for pristine Cu2O and C3N4, respectively, which was in good agreement with the literature reports.27,28 But the Cu2O–rGO–C3N4 composite covered almost the whole solar spectrum that favors the material to be photoactive [Figure S1(a) in the supplementary material].36 The absorption intensity was also remarkably increased after loading Cu2O–rGO to C3N4 and appeared as a red shift in comparison with bare C3N4. The adsorption edge of pristine C3N4 was 450 nm which can be assigned to the band gap energy of 2.75 eV [Figure S1(b) in the supplementary material].36 Bare Cu2O possesses the band gap energy of 2.20 eV [Figure S1(c) in the supplementary material].36 Similarly, the absorption edge of Cu2O–rGO–C3N4 photocatalyst was about 477 nm and the calculated band gap was 2.60 eV [Figure S1(d) in the supplementary material].29,36

FIG. 5.

Photoluminescence spectra of C3N4, Cu2O, Cu2O–rGO, and Cu2O–rGO–C3N4 photocatalysts.

FIG. 5.

Photoluminescence spectra of C3N4, Cu2O, Cu2O–rGO, and Cu2O–rGO–C3N4 photocatalysts.

Close modal

Photoluminesence (PL) spectroscopy is one of the powerful tools to examine the charge transfer, migration, and extend of separation in photocatalysts. Figure 5 shows the PL spectra of pure C3N4, Cu2O, Cu2O–rGO, and Cu2O–rGO–C3N4 photocatalysts. In the PL spectrum of pristine C3N4, a peak centered at 450 nm, corresponding to the band gap of C3N4.29 The other three materials, namely, Cu2O, Cu2O–rGO, and Cu2O–rGO–C3N4 showed broad PL emission peak from 500 to 700 nm. Very importantly, the peak intensity at this region was in the order of Cu2O > Cu2O–rGO > Cu2O–rGO–C3N4 which confirmed that the charged carriers recombination was very high in pristine Cu2O. It is well known fact that rGO has higher carrier mobility and thereby reduced the recombination rate in Cu2O–rGO photocatalyst. As a result, the peak intensity was lesser for Cu2O–rGO than bare Cu2O. But the peak intensity was decreased further by adding C3N4 to Cu2O–rGO. It is worth to mention here that the Cu2O–rGO–C3N4 photocatalyst showed two PL emission peaks at around 500 to 700 nm and 450 nm which were emerged because of Cu2O and C3N4, respectively. Astonishingly, both the emission peaks were suppressed that inferred that the recombination of charged carriers was completely prevented.

All the characterization techniques results, especially the optical studies (UV-vis DRS and PL), indicate that Cu2O–rGO–C3N4 can be a very good material for visible light active photocatalytic reactions. Therefore, photocatalytic reduction of 4-nitrophenol to 4-aminophenol was performed in presence of Cu2O–rGO–C3N4 photocatalyst under visible light irradiation. For comparison, the photocatalytic reduction of 4-nitrophenol was performed using bare Cu2O, rGO wrapped Cu2O, pure C3N4, Cu2O–C3N4, and Cu2O–rGO–C3N4 (Figure 6). Among the photocatalysts used, bare Cu2O showed lesser activity whereas the rGO wrapping increased the photocatalytic reduction property of Cu2O. But the activity was comparatively lesser than the photocatalytic reduction of 4-nitrophenol using pristine C3N4. The photocatalytic activity of pristine C3N4 was increased slightly by loading bare Cu2O (without rGO) spheres. However, the Cu2O–rGO–C3N4 composite exhibited maximum reduction of 4-nitrophenol. This can be well explained by the fact that the recombination rate was completely prevented in Cu2O–rGO–C3N4 photocatalyst as evident by the PL studies (Figure 5) and also that it possessed utmost visible light absorption capability as supported by the UV-vis DRS absorption spectrum [Figure S1(a) in the supplementary material].36 

FIG. 6.

Photocatalytic reduction of 4-nitrophenol to 4-aminophenol in presence of different photocatalysts.

FIG. 6.

Photocatalytic reduction of 4-nitrophenol to 4-aminophenol in presence of different photocatalysts.

Close modal

Based on the results, a tentative mechanism was proposed for the photocatalytic reduction of 4-nitrophenol using Cu2O–rGO–C3N4 photocatalyst in presence of sodium sulfite under visible light irradiation (Figure 7). Both Cu2O and C3N4 are visible light active photocatalysts and hence electron-hole pairs were generated in both the catalysts under visible light irradiation. The conduction band (CB) and valance band (VB) energy levels of Cu2O are −0.7 V and 1.3 V, respectively.30 Since Cu2O is a p-type semiconductor, the Fermi level energy is close to the VB.31 On the other hand, the CB and VB potential of C3N4 are −1.13 V and 1.57 V, respectively.32 But the Fermi level is nearer to the CB because it is an n-type semiconductor.33 Apparently, the band structure of Cu2O and C3N4 is not suitable for the formation of heterojunction. Nevertheless, when the rGO wrapped Cu2O deposited on the surface of C3N4, the band structures were modified in such a way to reach the equilibrium between the Fermi levels of C3N4 and Cu2O.34 The negatively charged carriers move to the positive field (n-type C3N4) and the positive carriers migrate to the negative field (p-type Cu2O). In other words, the CB of C3N4 behaved as a sink for the photogenerated electrons and subsequently the holes accumulated in the VB of Cu2O.35 Presence of rGO facilitated this carrier transport as strongly supported by the PL studies (Figure 5) and thereby increased the photocatalytic activity. The holes were utilized for the conversion of SO 3 2 to SO 3 . On the other hand, the photoexcited electrons involved in the reduction of 4-nitrophenol to 4-aminophenol.

FIG. 7.

Plausible mechanism for the photocatalytic reduction of 4-nitrophenol to 4-aminophenol using Cu2O–rGO–C3N4 photocatalyst in presence of sodium sulfite.

FIG. 7.

Plausible mechanism for the photocatalytic reduction of 4-nitrophenol to 4-aminophenol using Cu2O–rGO–C3N4 photocatalyst in presence of sodium sulfite.

Close modal

In conclusion, rGO wrapped Cu2O spheres were synthesized and loaded over C3N4 photocatalyst. By this synthetic route, the rGO was placed exactly at the interfacial of Cu2O and C3N4 heterojunction and hence the photogenerated carrier migration became very rapid which was authenticated by the PL studies. As a result of this, the photocatalytic reduction ability of Cu2O–rGO–C3N4 was enhanced ∼2 fold synergistically as compared with that of the bare C3N4 and Cu2O–rGO photocatalysts. A tentative mechanism was proposed for the reduction of 4-nitrophenol using Cu2O–rGO–C3N4 photocatalyst under visible light irradiation. Photocatalyst recyclability was scrutinized for four times without any measurable loss in its photocatalytic activity.

We acknowledge financial support from the SERB (No. SR/FT/CS-127/2011), DST, New Delhi, India. We also acknowledge Professor B. Viswanathan, National Centre for Catalysis Research, IIT-Chennai for XPS analysis.

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See supplementary material at http://dx.doi.org/10.1063/1.4928286 for detailed experimental procedures for the preparation of materials, instrumental parameters and some of the results.

Supplementary Material