TiO2 / Graphitic carbon nitride Z-scheme catalyst for binary dye degradation by persulphate activation

In this work, we describe the preparation of the TiO2 (P25) / Graphitic Carbon Nitride composite and the degradation of pollutants using this composite with persulphate activation. The materials are characterized by XRD, FTIR analysis, etc. XRD analysis confirms the presence of TiO2 anatase and GCN and the characteristic functional groups of both TiO2 and GCN are determined with the help of FTIR. The degradation of crystal violet and Rhodamine B in binary mixtures over this synthesized catalyst is easier than individual catalysts separately. The degradation efficiency of crystal violet over TiO2/GCN composite is 1.5 times higher than GCN and 3 times higher than TiO2 alone and the degradation efficiency of Rhodamine B over TiO2/GCN composite is 1.1 times higher than GCN and 2.4 times higher than TiO2 alone. In flat band analysis, a Z-Scheme catalyst is formed between TiO2 and GCN. The electron transfer between the bands of TiO2 and GCN occurs because of proper band alignment between these materials.


Introduction
As a consequence of economic development, the manufacture and use of organic compounds in sectors such as printing, dyeing, medicinal products, clothing, and skincare are expanding.Research on environmental and energy issues has moved clean energy generation and wastewater treatment to the forefront in recent years.Inappropriate use or storage of water resources can nevertheless lead to the development of organic pollutants in water ecosystems which is liable to have a detrimental effect on their efficiency and conservation.Due to their delayed biodegradation and deleterious consequences, hazardous pollutants such as organic dyes, heavy metals, and pesticides cause major health hazards for people as well as aquatic ecosystems.Modern methods of pollution removal from industrial waste, like photocatalysis and adsorption, have shown promise in these difficulties.Advanced oxidation based on radical reactions was demonstrated as an effective method to degrade organic refractory compounds in the field of environmental remediation. 1Transition metal ions, thermal, photolytic, oncolytic, and radiolytic methods are conventional methods of persulfate (PS) activation to generate SO4•− radicals.The fuel activation mechanism consists of the fission of OO bonds, which results in high energy consumption as they are fed from outside sources.Titanium dioxide (TiO2) is a very affordable, stable, and nontoxic semiconductor photocatalyst.TiO2's drawbacks, such as its wide band gap and the need for UV light to activate and aggregate nanoparticles, have prompted efforts to alter it to enhance performance. 2Graphitic carbon nitride g-C3N4 has a narrow band gap, is easy to manufacture, contains an extensive raw material source, and can be produced cheaply due to these it was regarded as the best carbon-based material. 3The use of carbonaceous materials is especially favored, as they have a negligible amount of toxicity, cost-effectiveness, wide areas surface, and excellent chemical stability.Carbonaceous materials are examined for the activation of persulfates when applied in contaminated groundwater remediation as well as wastewater treatment facilities, and economic concepts and perspectives on this issue are presented.Based on the composition and microstructure of these materials, they are divided into three categories, including pristine carbon materials and their derivatives, heteroatom-doped carbon materials, and carbon materials with metal particles. 4For persulphate (PS) activation, g-C3N4 can be excited by visible light-producing electrons in the conduction band.Persulphate activation processes involve the formation of the highly reactive sulfate radical (SO4 •-).Sulfate radicals (SO4 •− ) have been utilized for the degradation of organic pollutants due to the advantages of SO4 •− over •OH.SO4 •− has a longer halftime (3-4 × 10 −5 s) and has selectivity to the electron-rich compounds.SO4 •− could take effect in a wide range of pH ( 2.0-8.0).In addition, SO4 •− has a redox potential ( 2.5 V-3.1 V).Persulphates have a peroxide bonding environment that is vulnerable to one-electron reduction, allowing for SO4 •-formation through reductive transformation. 5An effective method of enhancing photogenerated electron-hole pair separation is using g-C3N4, in combination with TiO2, which forms a heterojunction.It was proven that the energy level positions of g-C3N4 and TiO2 correspond to each other quite well for TiO2 to be able to accept the electrons from g-C3N4.The efficiency of pollutant degradation in the catalytic system can be significantly enhanced by the presence of persulphate. 6A simple method for the preparation of P25/ g-C3N4 catalysts which have a uniform distribution of TiO2 on a layer of g-C3N4 has been reported in this study.More than one technology has been applied to evaluate the physical and chemical properties of a Synthetic Photocatalyst.

Materials and methods
TiO2 (P25, Degussa), Melamine (Merck), and Ammonium Chloride( Merck) were purchased and used.Graphitic Carbon Nitride was synthesized by calcining melamine at 550℃ for four hours.P25/GCN composite was synthesized by mixing a pre-determined amount of TiO2 with 100mg of prepared GCN and dispersing it in ethanol.After evaporating ethanol, the obtained slurry was calcined at 250℃ to get P25/GCN composite.

FIGURE 1a XRD diffraction pattern of TiO2, GCN, and P25/GCN composite
Figure 1b shows the FTIR data of P25 and GCN composites.
The characteristic peak of the GCN sample got was 816.7cm-1 which indicates the presence of the tri-s-triazine functional group.The second bandwidth range is the prominent one, the wavenumber observed came in the range 2981.899-3399.27cm-1 which corresponds to the bending modes vibration of NH2 and NH functional groups resulting from hydrogenation on nitrogen atoms during thermal polymerization or the presence of terminal NH and NH2 at the defect regions of the GCN.The last group of peaks observed was in the range 1202.83-1632.87cm-1 which corresponds to the stretching vibration of C=N and C-N functional groups.Figure 2a shows the degradation efficiency of 5ppm Crystal violet in a binary mixture of RhB and CV over synthesized catalysts over some time.From the graph, it is evident that the P25/GCN composite has a degradation efficiency of 88%.The remaining individual catalysts which are GCN and TiO 2 show degradation efficiency of 57% and 30% respectively.So from the plot, it is clear that the P25/GCN composite has the maximum degradation efficiency compared to the other two individual catalysts.Figure 2b shows the degradation efficiency of crystal violet in the binary mixture of CV and RhB over the synthesized catalysts.From the graph, it is evident that the P25/GCN composite has a degradation efficiency of 85%.The remaining individual catalysts which are GCN and TiO 2 show degradation efficiency of 73% and 35% respectively.So from the plot, it is clear that the P25/GCN composite has the maximum degradation efficiency compared to the other two individual catalysts. 9The valence band and conduction band potentials of P25 and GCN have been calculated and from the figure, it is evident that the transfer of electrons from the conduction band of TiO2 to the valence band of GCN is possible due to appropriate band alignment between two materials. 10Thus the effective charge separation and charge migration in the composite results in the degradation of organic pollutants, The active oxygen species involved in photocatalytic degradation include superoxide radical, hydroxyl radical, and sulfate radical.11 The active oxygen species can be successfully generated over the composite since the redox potentials are sufficiently high to produce superoxide and hydroxyl radicals from the solution. 12

Conclusion
In this study, we synthesized P25/GCN composite by successfully incorporating TiO2 into GCN and finally synthesized it into a Z-scheme catalyst.We observed XRD diffraction patterns of TiO2, GCN, and P25/GCN composite individually and the results are matched with already available literature.We also studied the degradation of the pollutants by testing the degradations of crystal violet and Rhodamine B in binary mixtures over synthesized catalysts (P25/GCN composite) and respective degradation efficiencies were calculated.The degradation efficiencies of crystal violet and Rhodamine B were calculated separately for individual species and composite and compared.For crystal violet, the P25/GCN composite has a degradation efficiency of 88% while the individual catalysts GCN and TiO 2 show degradation efficiencies of 57% and 30% respectively.Similarly for Rhodamine B, the P25/GCN composite has a degradation efficiency of 85% while the individual catalysts GCN and TiO2 have degradation efficiencies of 73% and 35% respectively.The photocatalytic degradation of pollutants using P25/GCN composite with persulphate activation has been studied.

Figure 2 :
Figure 2:Degradation efficiencies of Crystal violet and Rhodamine B in binary mixtures over the synthesized catalysts.a) crystal violet, b):Rhodamine B

Figure 3 :
Figure 3: A plausible mechanism for photocatalytic degradation over P25/GCN composite