An Overview on Graphene-Metal Oxide Semiconductor Nanocomposite: A Promising Platform for Visible Light Photocatalytic Activity for the Treatment of Various Pollutants in Aqueous Medium

Graphene is one of the most favorite materials for materials science research owing to its distinctive chemical and physical properties, such as superior conductivity, extremely larger specific surface area, and good mechanical/chemical stability with the flexible monolayer structure. Graphene is considered as a supreme matrix and electron arbitrator of semiconductor nanoparticles for environmental pollution remediation. The present review looks at the recent progress on the graphene-based metal oxide and ternary composites for photocatalysis application, especially for the application of the environmental remediation. The challenges and perspectives of emerging graphene-based metal oxide nanocomposites for photocatalysis are also discussed.


Introduction
Graphene is a 2-D material composed of layers of carbon atoms crammed into a honeycomb network and has become an escalating star on the prospect of materials science in the past many years [1][2][3]. Graphene can be used to produce 0-D fullerene, 1-D and 3-D graphitic carbon nanotubes that had been intensively studied for the last ten years [4,5]. Graphene exhibits enthralling assets such as extraordinary conductivity, maximum surface-area-to-volume ratio, a fluorescence-quenching competence by electron or energy-allocation, a quantum Hall effect at room temperature, a bipolar electric field effect laterally with the surface conduction of charge carriers and a tunable band gap [6,7]. Narrow band gap metal oxides are of great interest, due to their efficient utilization of solar energy which signifies an auspicious technology to resolve the global energy and eco-friendly challenges [5,[8][9][10].
photocatalyst showed the similar occurrence. This study demonstrated that TiO 2 -GO cannot offer truly new visions into the assembly of TiO 2 carbon composite as high-performance photocatalysts. The TiO 2 particles were found to be in anatase phase and a narrow size distribution was dispersed on the surface of graphene sheets uniformly [25]. A comparison of photoluminescence spectra between TiO 2 and G-TiO 2 was reported with different reaction times, as shown in Figure 1. In this figure, the inset is the amplificatory image of the area in the range of 300 to 500 nm which demonstrates the quenching extent in relation with the reaction time in the Graphene-TiO 2 [26]. TiO 2 (P25)-rGO composite was found to be the most proficient photocatalyst for the degradation of Methylene Blue (MB) and the optimum mass ratio was found to be 1/0.2 [14]. Comparison has revealed that the P25-rGO composite has additional effectiveness compared to the P25-CNT (carbon nanotubes) composite ( Figure 2).
Molecules 2020, 25, x FOR PEER REVIEW 3 of 18 the TiO2-GO photocatalyst showed the similar occurrence. This study demonstrated that TiO2-GO cannot offer truly new visions into the assembly of TiO2 carbon composite as high-performance photocatalysts. The TiO2 particles were found to be in anatase phase and a narrow size distribution was dispersed on the surface of graphene sheets uniformly [25]. A comparison of photoluminescence spectra between TiO2 and G-TiO2 was reported with different reaction times, as shown in Figure 1. In this figure, the inset is the amplificatory image of the area in the range of 300 to 500 nm which demonstrates the quenching extent in relation with the reaction time in the Graphene-TiO2 [26]. TiO2 (P25)-rGO composite was found to be the most proficient photocatalyst for the degradation of Methylene Blue (MB) and the optimum mass ratio was found to be 1/0.2 [14]. Comparison has revealed that the P25-rGO composite has additional effectiveness compared to the P25-CNT (carbon nanotubes) composite ( Figure 2).
Molecules 2020, 25, x FOR PEER REVIEW 3 of 18 the TiO2-GO photocatalyst showed the similar occurrence. This study demonstrated that TiO2-GO cannot offer truly new visions into the assembly of TiO2 carbon composite as high-performance photocatalysts. The TiO2 particles were found to be in anatase phase and a narrow size distribution was dispersed on the surface of graphene sheets uniformly [25]. A comparison of photoluminescence spectra between TiO2 and G-TiO2 was reported with different reaction times, as shown in Figure 1. In this figure, the inset is the amplificatory image of the area in the range of 300 to 500 nm which demonstrates the quenching extent in relation with the reaction time in the Graphene-TiO2 [26]. TiO2 (P25)-rGO composite was found to be the most proficient photocatalyst for the degradation of Methylene Blue (MB) and the optimum mass ratio was found to be 1/0.2 [14]. Comparison has revealed that the P25-rGO composite has additional effectiveness compared to the P25-CNT (carbon nanotubes) composite ( Figure 2).
Graphene-loaded TiO 2 films were reported to be highly conductive and transparent; remarkably, graphene/TiO 2 films exhibited super hydrophilicity in a short time even under a white fluorescent light bulb. Higher photocatalytic activity owed to its efficient charge separation and electrons injection from the conduction band of TiO 2 to graphene [27].
The higher photocatalytic performance was observed in TiO 2 -graphene oxide composite due to the formation of both π-π conjugations between dye molecules and aromatic rings. The photocatalytic property was reported to be higher with the higher content of the graphene oxide. Furthermore, ionic interactions between MB and functional groups of GO on the surfaces of carbon-based nanosheets was also the reason considered for the superior property [28]. Improving graphene oxide (IGO) in strong acidic condition was reported to enhance the chemical interaction between TiO 2 and graphene sheets [29]. This study showed that IGO can react with Ti(OH) x to form graphene/TiO 2 composite in situ, with complete and near coverage of Ti-C and Ti-O-C carbonaceous bonds at the graphene/TiO 2 interface. Higher photocatalytic activity shown by graphene/TiO 2 due to effective charge transfer imparts under visible light and GO forms chemical bonds at the interface [29].
Photocatalytic experiments using sacrificial hole and radical scavenging agents demonstrated that the photogenerated holes are the main reason for the degradation of diphenhydramine (DP), both under UV and visible light. In this report, photoluminescence studies revealed discrete appeasing of the GO photoluminiscence under visible light and near infrared laser excitation. Hence, it was conferred that GO acts as either an electron acceptor or donor of TiO 2 under UV/visible light [30].
Nitrogen-doped P90 TiO 2 (N-P90), nitrogen-doped reduced graphene oxide (N-rGO), as well as their composites were studied for the photocatalytic activity. N-P90/N-rGO showed enhanced photocatalytic activity, and in the presence of this composite, around 80% MB was degraded by visible light irradiation in 160 min. Enhanced photocatalytic performance is observed in N-P90/N-rGO composites for degradation of MB due to photo-induced and electronic interaction between TiO 2 and graphene. Comparison of degradation efficiency of MB under visible light irradiation by P90 TiO 2 , N-P90, and N-P90/rGO is presented in Figure 3 [31]. It is reported that the rGO or N-rGO in the composite enables the separation of electrons and holes by performing as electron trapping/de-trapping under visible light [31]. Ultrafine TiO 2 nanofibers (~10 nm diameters) were synthesized from electrospun rice-shaped TiO 2 and potassium titanate was achieved from rice-shaped TiO 2 . The surface area was increased by 2.5 times after the nanofiber formation of TiO 2 . The results showed that the photodegradation of MO was found to be higher than bare TiO 2 (P-25) nanoparticles [32]. GO/TiO 2 of different composition ratios were tested and the formulation of catalyst with 1.2 times higher photocatalytic activity than commercial photocatalyst was reported. This catalyst was able to degrade 3 mg/L MB over 10 consecutive cycles with nominal loss in photocatalytic efficacy. Graphene plays a generous part in obstructing the accretion of TiO 2 grains upon calcination at high temperature [33]. A set of reduced graphene oxide-TiO 2 (rGO-TiO 2 ) nanocomposites was synthesized and examined for the photocatalytic activity by decolorization of Rhodamine B dye (RhB) under UV light. In this study, various parameters, such as dye concentration, rGO content, catalytic dose, and pH, were optimized for the decolorization. The catalysts were found to be more active at natural pH of the dye under the UV-illumination for the degradation of RhB dye. The presence of H 2 O 2 and K 2 S 2 O 8 increased the decolorization. Further, addition of CO 3 2− and Cl − ions decreased the dye degradation rate [34].
TiO 2 nanoparticle-attached graphene/carbon composite nanofibers (TiO 2 -CCNFs) were synthesized and reported as highly active photocatalysts for photocatalytic degradation of MB under the irradiation of visible light. Graphene was suggested to play the role of an electron acceptor and a photosensitizer, resulting in a higher photodegradation rate and reduced electron-hole pair recombination. CNFs having high surface also improved the photocatalytic activity of TiO 2 [35]. TiO2 nanoparticle-attached graphene/carbon composite nanofibers (TiO2-CCNFs) were synthesized and reported as highly active photocatalysts for photocatalytic degradation of MB under the irradiation of visible light. Graphene was suggested to play the role of an electron acceptor and a photosensitizer, resulting in a higher photodegradation rate and reduced electron-hole pair recombination. CNFs having high surface also improved the photocatalytic activity of TiO2 [35].

GO/rGO-ZnO Composite
An effective scalable method was developed to make nanocomposites of functional graphene sheets (FGS)/ZnO. In this study, poly (vinyl pyrrolidone) (PVP) component was reported to play a crucial role for loading of ZnO nanoparticles onto FGS by connecting Zn ions on the carbon materials and promoting ZnO nucleation and crystal growth in the precursor-prepared route. Further, FGS/ZnO composite was evaluated for photocatalytic activity and was found to be applicable for a number of environmental issues [36]. ZnO/rGO nanocomposite was used as a photocatalyst for the removal of MB. Observations showed that the efficiency of the photocatalyst activity of the ZnO nanoparticles was significantly increased by rGO [37].
The calcination atmosphere was found to affect the photocatalytic activity of the TiO2/graphene sheet (GS) (5%) composites for H2 evolution from water splitting. This study demonstrated that beyond the critical content of GS (5%), photocatalytic activity was decreased by initiating electronhole recombination centers. Calcination atmosphere was found to be important and better performance was observed for the samples calcined in nitrogen atmosphere [38].
The use of ZnO-graphene composites (Z-GC) was reported to remove dye from water due to the interaction between the graphene sheets and the ZnO nanoparticles [39]. rGO-ZnO (3.56%) showed higher photocurrent response and degradation of MB under illumination of UV light. Longer electron lifetime and the enhanced light absorption were verified by analytical and electrochemical technique

GO/rGO-ZnO Composite
An effective scalable method was developed to make nanocomposites of functional graphene sheets (FGS)/ZnO. In this study, poly (vinyl pyrrolidone) (PVP) component was reported to play a crucial role for loading of ZnO nanoparticles onto FGS by connecting Zn ions on the carbon materials and promoting ZnO nucleation and crystal growth in the precursor-prepared route. Further, FGS/ZnO composite was evaluated for photocatalytic activity and was found to be applicable for a number of environmental issues [36]. ZnO/rGO nanocomposite was used as a photocatalyst for the removal of MB. Observations showed that the efficiency of the photocatalyst activity of the ZnO nanoparticles was significantly increased by rGO [37].
The calcination atmosphere was found to affect the photocatalytic activity of the TiO 2 /graphene sheet (GS) (5%) composites for H 2 evolution from water splitting. This study demonstrated that beyond the critical content of GS (5%), photocatalytic activity was decreased by initiating electron-hole recombination centers. Calcination atmosphere was found to be important and better performance was observed for the samples calcined in nitrogen atmosphere [38].
The use of ZnO-graphene composites (Z-GC) was reported to remove dye from water due to the interaction between the graphene sheets and the ZnO nanoparticles [39]. rGO-ZnO (3.56%) showed higher photocurrent response and degradation of MB under illumination of UV light. Longer electron lifetime and the enhanced light absorption were verified by analytical and electrochemical technique ( Figure 4) [40]. The ZnO/GO nanocomposite consisting of flower-like ZnO nanoparticles anchored on graphene-oxide. Further, photocatalytic efficiency of ZnO/GO composite progressed by annealing the product in N2 atmosphere. The superior photocatalytic performance was due to the synergistic effect of the proficient electron inoculation and low charge carriers in the composite, where GO acted as an electron collector and transporter, leading to unceasing generation of reactive oxygen species for the degradation of MB [10]. Core-shell nanorods with ZnO core and ZnS-Bi2S3 bi-component shell anchored on the rGO sheets were synthesized and reported to show a broad and strong photoabsorption in the visible region. These nanorods also manifested better photocatalytic activity for H2 evolution from the glycerol-water mixtures. The superiority in performance is owing to the elevated light absorption and effective charge separation [41].

GO/rGO-CoFe2O4 Composite
Connexion of the graphene suggestively progressed the photocatalytic performance of the CoFe2O4 in which the graphene acts as a charge carrier to detain the delocalized electrons. Photocatalytic activity was explored with the variation of the dosage and dye concentration [42]. CoFe2O4-graphene hybrid materials (CFGHs) showed ferromagnetic behavior and enhanced photodegradation rate and amended adsorbing capacity due to the assimilation of graphene [43]. The photodegradation fallouts directed the visible light fascinating performance of the ternary photocatalysts and formation of the p-n junction between CoFe2O4 and CdS. Escalation in the concentration of MB was observed as the irradiation time increased for CoFe2O4 due to the desorption of MB during irradiation. G-CoFe2O4/CdS easily separated from aqueous solution in an external magnetic field, as seen from the digital photos of Gr-CoFe2O4/CdS after irradiation [44]. The CoFe2O4-rGO composite unveiled required photocatalytic performance with excellent recycling stability for the degradation of MB, RhB, and MO under visible-light irradiation [45].
CoFe2O4-rGO (CF-rGO) nanocomposites hold exceptional microwave absorbing properties and high photocatalytic activity for the degradation of various dyes under visible light irradiation [46]. 85 CF-15 rGO exposed admirable microwave absorption possessions with a Reflection Loss (RL) of 31.31 dB (99.94% absorption) at 9.05 GHz, with an 8.2-10.92 GHz effective bandwidth range. 75CF-25 rGO was found to be a good magnetically separable photocatalyst for the degradation of dyes, MO, MB, and RhB, under visible light irradiation emitted from a 100 W reading lamp [46]. The photocatalytic activity was found to be affected by the structural and optical properties and surface area of the samples [47]. The ZnO/GO nanocomposite consisting of flower-like ZnO nanoparticles anchored on graphene-oxide. Further, photocatalytic efficiency of ZnO/GO composite progressed by annealing the product in N 2 atmosphere. The superior photocatalytic performance was due to the synergistic effect of the proficient electron inoculation and low charge carriers in the composite, where GO acted as an electron collector and transporter, leading to unceasing generation of reactive oxygen species for the degradation of MB [10]. Core-shell nanorods with ZnO core and ZnS-Bi 2 S 3 bi-component shell anchored on the rGO sheets were synthesized and reported to show a broad and strong photo-absorption in the visible region. These nanorods also manifested better photocatalytic activity for H 2 evolution from the glycerol-water mixtures. The superiority in performance is owing to the elevated light absorption and effective charge separation [41].

GO/rGO-CoFe 2 O 4 Composite
Connexion of the graphene suggestively progressed the photocatalytic performance of the CoFe 2 O 4 in which the graphene acts as a charge carrier to detain the delocalized electrons. Photocatalytic activity was explored with the variation of the dosage and dye concentration [42]. CoFe 2 O 4 -graphene hybrid materials (CFGHs) showed ferromagnetic behavior and enhanced photodegradation rate and amended adsorbing capacity due to the assimilation of graphene [43]. The photodegradation fallouts directed the visible light fascinating performance of the ternary photocatalysts and formation of the p-n junction between CoFe 2 O 4 and CdS. Escalation in the concentration of MB was observed as the irradiation time increased for CoFe 2 O 4 due to the desorption of MB during irradiation. G-CoFe 2 O 4 /CdS easily separated from aqueous solution in an external magnetic field, as seen from the digital photos of Gr-CoFe 2 O 4 /CdS after irradiation [44]. The CoFe 2 O 4 -rGO composite unveiled required photocatalytic performance with excellent recycling stability for the degradation of MB, RhB, and MO under visible-light irradiation [45].
CoFe 2 O 4 -rGO (CF-rGO) nanocomposites hold exceptional microwave absorbing properties and high photocatalytic activity for the degradation of various dyes under visible light irradiation [46]. 85 CF-15 rGO exposed admirable microwave absorption possessions with a Reflection Loss (RL) of 31.31 dB (99.94% absorption) at 9.05 GHz, with an 8.2-10.92 GHz effective bandwidth range. 75CF-25 rGO was found to be a good magnetically separable photocatalyst for the degradation of dyes, MO, MB, and RhB, under visible light irradiation emitted from a 100 W reading lamp [46]. The photocatalytic activity was found to be affected by the structural and optical properties and surface area of the samples [47]. CoFe 2 O 4 -3D TiO 2 nanocomposite showed an enhancement in the photodegradation of MB as compared to the commercial rutile-phase TiO 2 and the pure urchin-like TiO 2 (3D TiO 2 ) microparticles. Results specified that the composite showed relatively consistent photocatalytic activity with slight degradation [48]. The photocatalytic activity of 75CF-25 rGO was found to be analogous and in some cases, superior, compared to the several reported rGO-CoFe 2 O 4 composites [46]. The photocatalytic activity of CF-RGO was increased with increasing rGO content in composites until 25 wt% of rGO, and degradation takes place around 60 min.
The photocatalytic degradation of short-chain chlorinated paraffin's over rGO/CoFe 2 O 4 /Ag under visible light (λ > 400 nm) was investigated by in-situ Fourier transform infrared spectroscopy and the correlated mechanisms were suggested. Superficial degradation ratio of 91.9% over rGO/CoFe 2 O 4 /Ag was obtained under visible light illumination of 12 h, while only about 21.7% was obtained with commercial P-25 TiO 2 [49]. Increase of rGO caused an increase in the completion time of the photocatalysis. Degradation of MO diminished with increasing catalyst dose up to 500 mgL −1 , and then, no noteworthy decrease of time was observed when more catalyst was added. Likewise, use of 2 mL of H 2 O 2 was found to be an optimum amount for the photocatalysis reaction ( Figure 5) [46]. Photocatalytic activity of the rGO-CoFe 2 O 4 nanocomposites was queried for the degradation of 4-Chlorophenol (4-CP) under visible light illumination. Activity of rGO-CoFe 2 O 4 composite was seen in the occurrence of PMS ( Figure 6) [50]. CoFe2O4-3D TiO2 nanocomposite showed an enhancement in the photodegradation of MB as compared to the commercial rutile-phase TiO2 and the pure urchin-like TiO2 (3D TiO2) microparticles. Results specified that the composite showed relatively consistent photocatalytic activity with slight degradation [48]. The photocatalytic activity of 75CF-25 rGO was found to be analogous and in some cases, superior, compared to the several reported rGO-CoFe2O4 composites [46]. The photocatalytic activity of CF-RGO was increased with increasing rGO content in composites until 25 wt% of rGO, and degradation takes place around 60 min.
The photocatalytic degradation of short-chain chlorinated paraffin's over rGO/CoFe2O4/Ag under visible light (λ > 400 nm) was investigated by in-situ Fourier transform infrared spectroscopy and the correlated mechanisms were suggested. Superficial degradation ratio of 91.9% over rGO/CoFe2O4/Ag was obtained under visible light illumination of 12 h, while only about 21.7% was obtained with commercial P-25 TiO2 [49]. Increase of rGO caused an increase in the completion time of the photocatalysis. Degradation of MO diminished with increasing catalyst dose up to 500 mgL -1 , and then, no noteworthy decrease of time was observed when more catalyst was added. Likewise, use of 2 mL of H2O2 was found to be an optimum amount for the photocatalysis reaction ( Figure 5) [46]. Photocatalytic activity of the rGO-CoFe2O4 nanocomposites was queried for the degradation of 4-Chlorophenol (4-CP) under visible light illumination. Activity of rGO-CoFe2O4 composite was seen in the occurrence of PMS ( Figure 6) [50].   CoFe2O4-3D TiO2 nanocomposite showed an enhancement in the photodegradation of MB as compared to the commercial rutile-phase TiO2 and the pure urchin-like TiO2 (3D TiO2) microparticles. Results specified that the composite showed relatively consistent photocatalytic activity with slight degradation [48]. The photocatalytic activity of 75CF-25 rGO was found to be analogous and in some cases, superior, compared to the several reported rGO-CoFe2O4 composites [46]. The photocatalytic activity of CF-RGO was increased with increasing rGO content in composites until 25 wt% of rGO, and degradation takes place around 60 min.
The photocatalytic degradation of short-chain chlorinated paraffin's over rGO/CoFe2O4/Ag under visible light (λ > 400 nm) was investigated by in-situ Fourier transform infrared spectroscopy and the correlated mechanisms were suggested. Superficial degradation ratio of 91.9% over rGO/CoFe2O4/Ag was obtained under visible light illumination of 12 h, while only about 21.7% was obtained with commercial P-25 TiO2 [49]. Increase of rGO caused an increase in the completion time of the photocatalysis. Degradation of MO diminished with increasing catalyst dose up to 500 mgL -1 , and then, no noteworthy decrease of time was observed when more catalyst was added. Likewise, use of 2 mL of H2O2 was found to be an optimum amount for the photocatalysis reaction ( Figure 5) [46]. Photocatalytic activity of the rGO-CoFe2O4 nanocomposites was queried for the degradation of 4-Chlorophenol (4-CP) under visible light illumination. Activity of rGO-CoFe2O4 composite was seen in the occurrence of PMS ( Figure 6) [50].

GO-rGO-ZnFe 2 O 4 Composite
Photocatalytic activity of ZnFe 2 O 4 -graphene catalyst demonstrated an important two-fold function as the photoelectrochemical degradation of MB and generation of hydroxyl radical for the decomposition of H 2 O 2 under visible light irradiation [51]. Graphene-ZnFe 2 O 4 photocatalyst facilitated the transport channels for photon-excited electrons from the surface of the catalyst. As a result, about 20 nm ZnFe 2 O 4 catalyst with a highly crystallized (311) plane confined in the graphene network exhibited an excellent visible-light-driven photocatalytic activity with an ultrafast degradation rate of 1.924 × 10 −7 mol g −1 s −1 for MB [52].
The boosted photocatalytic activity of ZnFe 2 O 4 -rGO nanocomposite was shown due to the active restraint of the recombination of the photo-excited electron-hole pairs by rGO sheets and the generation of ·OH-free radical [11]. The photocatalytic activity of the nanocomposite examined under visible light, for the degradation of 17 α-ethinylestradiol (EE 2 ) [50]. The pseudo rate constant of ZnFe 2 O 4 -Ag/rGO nanocomposite was higher by the factor of 14.6 and 5.6 times over its counterparts. Photosensitization effect was prevailed by good interaction ensuing in only 80% removal of EE 2 though humic acid [53]. rGO/ZnFe 2 O 4 composite exhibited the remarkable catalytic activity toward MB degradation; in the presence of H 2 O 2 , the activity enhanced, and the reaction followed a pseudo-first-order kinetics. The complete MB degradation observed at rGO/ZnFe 2 O 4 composites was attributed to the π-π interaction, hydrogen bonding, and electrostatic interaction exerted between the rGO and ZnFe 2 O 4 [54].

GO/rGO-NiFe 2 O 4 and MnFe 2 O 4 Composites
NiFe 2 O 4 -GO (0.25) hetero-architecture demonstrated a considerable lesser emission intensity. Due to their competent electron-transport property, graphene sheets can deliberately reduce the fluorescence of NiFe 2 O 4 fixed on them. Kinetic results indicated that the rate-determining step is the adsorption course of MB [55]. In this study, NiFe 2 O 4 -GO (0.25) shows the best activity compared to other NiFe 2 O 4 -G composites (Figure 7). GO-NiFe 2 O 4 showed photo-Fenton reactions for organic contaminants in the presence of both H 2 C 2 O 4 and H 2 O 2 under visible light irradiation. The photochemical reduction of Fe 3+ ions by GO was a key step in inducing the Fenton process [56]. The superior photocatalytic is due to (I) high visible absorbance for charge carrier production, (II) the electrons captured by Au nanoparticles results in the fast separation, and (III) the strong surface plasmon resonance (SPR) of Au nanoparticles permit the generation of high concentration of charge carriers [57]. MnFe 2 O 4 catalyst is photocatalytically inactive. The noteworthy higher photocatalytic activity is due to the rGO, as the excellent conductivity in the MnFe 2 O 4 and graphene composite [58].

GO-rGO-ZnFe2O4 Composite
Photocatalytic activity of ZnFe2O4-graphene catalyst demonstrated an important two-fold function as the photoelectrochemical degradation of MB and generation of hydroxyl radical for the decomposition of H2O2 under visible light irradiation [51]. Graphene-ZnFe2O4 photocatalyst facilitated the transport channels for photon-excited electrons from the surface of the catalyst. As a result, about 20 nm ZnFe2O4 catalyst with a highly crystallized (311) plane confined in the graphene network exhibited an excellent visible-light-driven photocatalytic activity with an ultrafast degradation rate of 1.924 × 10 −7 mol g −1 s −1 for MB [52].
The boosted photocatalytic activity of ZnFe2O4-rGO nanocomposite was shown due to the active restraint of the recombination of the photo-excited electron-hole pairs by rGO sheets and the generation of ·OH-free radical [11]. The photocatalytic activity of the nanocomposite examined under visible light, for the degradation of 17 α-ethinylestradiol (EE2) [50]. The pseudo rate constant of ZnFe2O4-Ag/rGO nanocomposite was higher by the factor of 14.6 and 5.6 times over its counterparts. Photosensitization effect was prevailed by good interaction ensuing in only 80% removal of EE2 though humic acid [53]. rGO/ZnFe2O4 composite exhibited the remarkable catalytic activity toward MB degradation; in the presence of H2O2, the activity enhanced, and the reaction followed a pseudofirst-order kinetics. The complete MB degradation observed at rGO/ZnFe2O4 composites was attributed to the π-π interaction, hydrogen bonding, and electrostatic interaction exerted between the rGO and ZnFe2O4 [54].

GO/rGO-NiFe2O4 and MnFe2O4 Composites
NiFe2O4-GO (0.25) hetero-architecture demonstrated a considerable lesser emission intensity. Due to their competent electron-transport property, graphene sheets can deliberately reduce the fluorescence of NiFe2O4 fixed on them. Kinetic results indicated that the rate-determining step is the adsorption course of MB [55]. In this study, NiFe2O4-GO (0.25) shows the best activity compared to other NiFe2O4-G composites (Figure 7). GO-NiFe2O4 showed photo-Fenton reactions for organic contaminants in the presence of both H2C2O4 and H2O2 under visible light irradiation. The photochemical reduction of Fe 3+ ions by GO was a key step in inducing the Fenton process [56]. The superior photocatalytic is due to (I) high visible absorbance for charge carrier production, (II) the electrons captured by Au nanoparticles results in the fast separation, and (III) the strong surface plasmon resonance (SPR) of Au nanoparticles permit the generation of high concentration of charge carriers [57]. MnFe2O4 catalyst is photocatalytically inactive. The noteworthy higher photocatalytic activity is due to the rGO, as the excellent conductivity in the MnFe2O4 and graphene composite [58].

Other Composite Systems
RGO-Bi 2 WO 6 and 3D CNT-pillared rGO nanocomposites show outstanding photocatalytic performance for the degradation of dyes under visible light [59]. BiFeO 3 -graphene nanohybrids have a six times higher rate compared to BiFeO 3 for the degradation of Congo Red (CR) under visible light due to its combined effects of modulated band gap and covalent bonding between BiFeO 3 and graphene [60].
Photoluminescence studies of Nb 3 O 7 (OH)-RGO composite supported the suggested mechanism of charge separation and transport mechanism. A higher degradation rate was obtained using the nanocomposite prepared with a graphene loading of 3 mgmL −1 , and when the rGO loading exceeded 3 mgmL −1 , degradation efficacy diminished. This arose as extra rGO sheets gathered and stuck the absorption of incident light [61].

Photocatalytic Evaluation
Pristine TiO 2 and ZnO exhibited good photocatalytic activity in UV light due to their wide band gap. These two metal oxides are stable in aqueous conditions during photocatalysis. Further, coupling of graphene with TiO 2 and ZnO increases the photocatalytic activity due to increases in the photogenerated charge carriers. Metal oxides with magnetic properties of metal ferrites (MFe 2 O 4 ) offer an added advantage as photocatalysts since they can be recovered by applying an external magnetic field after catalysis. Metal ferrites (MFe 2 O 4 , M = Co, Ni, Mn, Zn, etc.) materials are proven to be excellent candidates for visible light photocatalytic H 2 generation through water splitting. Recycling ability for metal ferrites are far better compared to nano semiconductors like TiO 2 and ZnO. MFe 2 O 4 is a class of semiconductor with narrow band gap, which exhibits characteristic visible light response, possess good photochemical stability, and exhibits excellent optical properties.
MFe 2 O 4 absorbs 42-45% of sunlight, whereas TiO 2 and ZnO absorbs 4% of sunlight. MFe 2 O 4 are efficient for the degradation of dye degradation and organic pollutant degradation compared to the other metal oxides (SnO 2 , CeO 2 , BaTiO 3 , and SrTiO 3 ), with respect to the catalyst and the light source. In MFe 2 O 4 context, recombination of photogenerated charge carriers is the major limitation in semiconductor photocatalysis as it reduces the overall quantum efficiency. In order to enhance the photocatalytic activity, graphene material is coupled with MFe 2 O 4 , where the graphene channels the electrons. Comparison of degradation rate for various photocatalytic reaction systems is incongruous since the nature of catalyst and substrate pollutant molecules are different in each reaction. Ferrite nanoparticles have a strong magnetic property, which can be easily used for magnetic separation after photo-mineralization.
The photocatalytic efficiency depends on the ratio of the photogenerated charge-carrier transfer rate to the rate of electron-hole recombination. For composite structure, M 2+ ion easily bonds with oxygen by giving an electron and super oxide radical. This super oxide radical can oxidize the organic substrate molecule. The Fe 3+ ion and Fe 2+ ions can show photo-Fenton reactions in presence of in-situ-generated H 2 O 2 . This H 2 O 2 generates hydroxyl-free radicals, which are involved in the degradation of pollutants. Predicted mechanism for the rGO-CoFe 2 O 4 composite is shown in Equations (1)- (4).
rGO-BiO 6 composite shows better photocatalyst compared to other catalysts prepared from hydrothermal method (Table 1; Table 2). The enhanced photocatalytic activity could be endorsed to the negative shift in the Fermi level of graphene-Bi2WO6 (G-BWO), decrease the conduction band potential, and elevate migration efficiency of photo-induced electrons, which may restrain the charge recombination efficiently. Superior contact between BiVO 4 and rGO scaffold subsidizes to photo-response augmentation compared to other electrochemical methods in the rGO-BiO 4 composite. I. In situ co-precipitation method for the synthesis of GO/rGO-NCs photocatalyst GO and 300 mg of P90 TiO 2 was added and stirred for 3 h. GO and P90 TiO 2 and a few drops of tetrabutyltitanate were added.
[31]   Overall, irrespective of other parameters, the solvothermal method is best and helps in crystal growing and super saturation is achieved by reducing the temperature in the crystal growth zone. Further, noble metal (Ag, Au, Cu, etc.) exhibits surface plasmon resonance (SPR), which is a characteristic feature. The SPR frequency of the metal particles can be tuned into visible light absorption by shifting the size of the deposited metal particles on the catalyst. Deposited metal is involved in multiple crucial roles, such as serving as a passive electron sink with high capacity to store electrons to suppress photogenerated charge carrier recombination, facilitates rapid dioxygen reduction to generate free radicals and direct excitation of metals, especially under visible light, and vectorial electron transfer to the conduction band (CB) of metal oxide. Thereby, showing improvement in the photocatalysis for the removal of various organic pollutants/dyes.

Perspectives and Challenges
Graphene nanosheets act as a substrate to support the metal oxides for photocatalytic activity and graphene-based semiconductor photocatalysts are used for environmental remediation. The morphologies of semiconductors, theoretical electronic-structure calculations, and experimental discovery determinations are necessary on GO to persuade the photocatalytic activity, and composition design is an operative method to enhance the photocatalytic properties. Photocatalytic properties depend on the preparative method, and various parameters like initial concentration, oxidant concentration, pH, particle size, number of GO sheets, and source of light should be explored.
The interface regulates the efficacy of the electron-hole separation. Currently, only few methods succeed in unswervingly depicting the interaction of GR and nanoparticles. Atomic force microscopy (AFM), Surface-enhanced Raman scattering (SERS), and scanning transmission electron microscope (STEM) may be the best techniques for determining the interaction of graphene and nanoparticles. Finally, studies on the preparation of a ternary composite as a photocatalyst for both UV and visible-light-driven pollutant photodegradation have been studied and reported. Especially, for the design of ternary composite, magnetic materials such as Fe, Co, Mn, etc., as a dopant, and possessing unique advantages, show a remarkable photocatalytic activity and photostability.
Further tasks exist in the application of graphene-based composite for the industrial scale. Some innovative applications of the metal oxide-graphene entail specific understanding between the metal oxides and surface of the graphene, which will have a direct impact on the properties of the composite. Designing a structure for the overall photocatalysis process may require further exploiting of GO by chemically modifying methods. A synthetic approach method of GO-based composite structure by using novel materials has not been achieved to date for photocatalysis, but the solutions to the key challenges appear within reach.
In view of this, graphene-based composites possess diverse potential applications, individually having dissimilar desires concerning material properties, and it can be projected that the research on graphene-composite materials will have an optimistic future.

Conflicts of Interest:
The authors declare no conflict of interest.