Visible Light Active Nitrogen and Cobalt Co-doped TiO 2 Nanoparticles: Synthesis, Characterization and Photocatalytic Activity

A facile and room temperature approach to synthesize pure TiO 2 and different variants of nirogen and cobalt co-doped TiO 2 (CoN-TiO 2 ) catalysts is reported in this study. The successful synthesis and crystalline phase analysis was carried out via X-ray diffraction (XRD) which conrmed anatase phase with tetragonal structure. The spherical morphology, uniform size distribution in the range of 20-40 nm and presence of dopants in nal product were validated by scanning electron microscopy (SEM) and Energy dispersive X-ray spectroscopy (EDS). Diffused reectance spectroscopy (DRS) is deployed for study of optical properties. A reduction in band gap from 3.2 eV for pure TiO 2 nanoparticles to 2.34 eV for the 7 wt.%. doped CoN-TiO 2 was observed. The photocatlytic activity of pure TiO 2 , and CoN-TiO 2 nanoparticles was studied against methyl orange.The photocatlytic activity of CoN-TiO 2 was almost double as compared to undoped TiO 2 which proves these catalysts to be very ecient and potential candidate for the wastewater treatment at industrial level.


Introduction
The essence of water in our lives can be understood from the fact that no living being can survive on the earth without water. However, water pollution has become a global concern because of the increased addition of different industrial contaminants to drinking water reservoirs. This polluted water is harmful to human beings, plants, animals and other aquatic life (Owa 2013). Out of different methods for wastewater treatment, photocatalysis has proven to be quite successful for converting toxic organic nanoparticles. The crystallite sizes of un-doped TiO 2 samples were higher than single, triple, and sevenlayered TiO 2 lms, which indicated reduced particle size of cobalt-doped TiO 2 than un-doped TiO 2 . Undoped TiO 2 showed maximum transmittance of ~84%, which remained uniform throughout the visible region, whereas single-layered cobalt-doped TiO 2 showed maximum transmittance of ~92%, which started decreasing afterwards(Anupama Chanda 2021).
To the best of our insight, a limited work has been stated on N, Co co-doped TiO 2 (Penghui Shao 2015) In this work, we have successfully synthesized pure TiO 2 and N, Co co-doped TiO 2 with variable dopant ratios via the sol-gel route. All the catalysts were schematically analysed by XRD, SEM, and UV-VIS DRS.
A comparative photocatalytic activity among pure and doped catalysts was performed against methyl orange, which showed an improved photocatalytic potential of nitrogen and cobalt co-doped TiO 2 .
Pulverization at high temperature such that 450 °С leads to the improvement of the crystallinity and removal of all organic residues from the sample and grinding helped in breaking the lumps and the agglomerates. As we increased the concentration of dopant the peak broadening is observed but till wt.% 7% doping no signi cant additional peak is observed. Scherrer equation (D=Kλ/ (β cos θ) was used to calculate crystallite size of each catalyst (PATTERSON 1939 CoN-TiO 2 -1 10 CoN-TiO 2 -2 10 CoN-TiO 2 -3 9.7 CoN-TiO 2 -4 8.75 CoN-TiO 2 -5 8 SEM was performed to determine the morphology and particle size of the catalysts. The average particle sizes of doped catalysts were below 40 nm. SEM images of undoped TiO 2 , and all variants of CoN-TiO 2 have been shown in the Fig. 2. It can be observed from these images that the morphology of TiO 2 remain intact even after doping with nitrogen and cobalt. In case of undoped TiO 2 , the average particle size is ~60 nm, and particle size ranges from 20-40 nm for doped catalysts. The decrease in the particle size of the catalysts after doping are in the consensus with the decrease in the crystallite size mentioned in the Table 1. The spherical morphology of these nanoparticles is important for improving the photocatalytic properties of the catalysts (R. Lakshmi Narayana 2011). As the dopant concentration is too low to be detected in XRD, EDS is used to testify the successful doping. The EDS spectrum for each sample is shown in Supplementary information (SI) Fig. S1. and relative atomic wt. % of each element in every sample is summarized in Table 2.  Fig. 3.
The photocatalytic potential of the catalysts was measured by using them for photodegradation of methyl orange. As pre-adsorption of dye is important for effective charge transfer and affects the photocatalytic degradation rate, therefore all the samples were kept in dark for two hours to attain adsorption-desorption equilibrium. After that, the solutions were kept under LED lamp with continuous stirring. All the experiments were conducted under visible light and ambient conditions. Aliquots of 5 mL were taken after every hour and after centrifugation, their absorbance was measured by using UV-VIS spectrophotometer. The decrease in absorbance showed that concentration of methyl orange was also decreasing as shown in Fig. S2. Activity and e ciency of all catalysts against degradation of methyl orange have been shown below in Fig. 4 and 5 respectively.
It can be seen from above gures that undoped TiO 2 showed lowest degradation e ciency than other catalysts. Furthermore, nitrogen and cobalt co-doping improved the photocatalytic performance of TiO 2 .
The photo degradation e ciency of the catalysts has been shown below in the Table 3. CoN-TiO 2 -1 59 CoN-TiO 2 -2 65 CoN-TiO 2 -3 69 CoN-TiO 2 -4 75 CoN-TiO 2 -5 80 The degradation rate calculation parameters for methyl orange have been shown in Table S1. This table shows that absorbance and concentration of methyl orange was decreasing with time. Among all catalysts the highest photocatalytic activity was observed for CoN-TiO 2 -5. This improvement in photocatalytic potential of all the co-doped catalysts can be the result of decreased particle size due to the introduction of dopants into TiO 2 . Reduction in particle size results in an increase in surface area, which eventually improves adsorption of dye on the surface of catalyst, and hence increase the photocatalytic activity. In addition, anatase phase was dominant in all samples which is found to be the most active phase in the photocatalytic degradation process. Along with that, intense absorption of light in the visible range and a red shift in band gap energy resulted in generating more charge carriers thereby increasing the e ciency of photocatalytic process. More generation of hydroxyl free radicals means more degradation of methyl orange. Low degradation e ciency of undoped TiO 2 can be the result of high band gap (3.17 eV), which produces a smaller number of OH free radicals and therefore less degradation of methyl orange occurs.

Mechanism of Methyl orange degradation
The mechanism of photodegradation of methyl orange was explained by Akpan and Hameed in 2009 (U.G.Akpan 2009). According to this mechanism, when a photon of light (hv ≥ E g ) falls on catalyst, valance electrons are excited to conduction band leaving behind the holes in the valence band. These photogenerated electrons react with the oxidant to produce a reduced product, and photogenerated holes react with a reductant to produce an oxidized product. In case of methyl orange degradation, the photogenerated electrons can either reduce the dye or can produce superoxide radical anion O 2− by reacting with water present on the surface of TiO 2 . The photogenerated holes can either directly oxidize the methyl orange or can produce hydroxyl free radical by reacting with water or OH − . The OH ⋅ is such a strong oxidizing agent that it can produce mineral end products by complete oxidation of methyl orange.
According to this mechanism, most of the reactions occurring during the photodegradation of methyl The order of reaction was determined for CoN-TiO 2 -5 by plotting a graph between -ln C t /C o versus time and has been shown below in Fig. 6.
The relationship between concentration and time can be explained by following equation (Ananpattarachai 2009 Where C t is the concentration of methyl orange at a particular time, C o is the initial concentration of methyl orange, and k app refers to apparent reaction rate constant. The slope of the graph indicated the apparent reaction rate constant (K app ) and the linearity of graph represents that the reaction is pseudo rst order.

Samples preparation
Typical sol gel route was deployed for the synthesis of samples (Dongdong Liang 2019). A solution comprising of 5 mL of Titanium (IV) tetraisopropoxide (TTIP) and 6 mL of 2-propanol was labelled as A.
An aqueous solution of HNO 3 (pH=2) was designated solution B. Solution A was added dropwise into solution B along with vigorous stirring. The mixture was stirred overnight at room temperature. Resulting product was dried in rotary evaporator and annealed at 450°C for 4 hours in the furnace followed by the 60 min. grinding.
For nitrogen, cobalt co-doping of TiO 2 , urea was added to solution B and different amounts of cobalt nitrate were added to solution B. Just like before, the solution A was added to solution B followed by overnight stirring. The solvent from resulting product was evaporated prior to calcination and grinding. The obtained powder was of green colour and darkening of colour was observed with the increase in dopant concentration. As prepared samples were subjected to characterization by XRD, SEM, EDX and UV-VIS spectroscopy.

Characterization
The crystal structure of pure and doped TiO 2 nanoparticles was characterized by using JEOL-JDX-II, Xray diffractometer with Cu Kα radiation (λ = 0.1542 nm). Samples were scanned with an incident beam in scan range between 10-80° operated at 40 kV and 30 mA. The morphology, particle size and elemental composition were estimated via JEOL JSM-6460 equipped with energy dispersive X-ray spectroscopy (EDS) operated at 10 kV. Optical properties of the catalysts were studied by UV-VIS-NIR spectrophotometer and calibrated powdered barium sulphate (BaSO 4 ) was used as reference for the baseline correction.

Photocatalytic Degradation Experiment
As prepared catalysts were used for the photocatalytic degradation of pure methyl orange. An aqueous stock solution of 0.1 mM was prepared and further diluted to 0.01 mM. For the photocatalytic experiment, 50 ml solution was taken in a conical ask to which 50 mg catalyst was added. These solutions were allowed to stir for two hours to ensure adsorption-desorption equilibrium. Then solutions were kept under LED lamp with continuous stirring. All the experiments were performed at ambient conditions. After an interval of 60 min., 5 mL aliquots were taken and after centrifugation, their absorbance was measured by using UV-VIS spectrophotometer. The photocatalytic degradation rate of methyl orange was determined by using the following equation: Degradation rate= (1-A/A 0 ) × 100% Where A o represents the initial concentration and A represents the concentration of methyl orange at a particular time.

Conclusions
In summary, visible light active nitrogen and cobalt co-doped TiO 2 catalysts were successfully fabricated through sol-gel method by using TTIP, 2-propanol, urea and cobalt nitrate. The ratio of nitrogen was xed for all catalysts whereas cobalt doping was carried out in different ratios. Different characterization methods e.g., XRD, SEM, EDX and DRS con rmed the successful synthesis of all the catalysts. XRD results con rmed the presence of anatase phase in all the synthesized catalysts. SEM results showed that all the catalysts have their particle size <40 nm. UV-DRS results showed reduction in band gap from 3.21eV for undoped TiO 2 to 2.34 eV for CoN-TiO 2 -5. It was found from the photocatalytic degradation experiment that all the nitrogen, cobalt co-doped catalysts showed superior performance compared to undoped TiO 2 . However, CoN-TiO 2 -5 showed highest degradation e ciency of 80% among all the catalysts, which can be the result of lowest band gap, decreased particle size and high surface area. These results show that nitrogen, cobalt co-doped TiO 2 has the potential to be used for photocatalytic degradation of harmful organic contaminants in polluted water.

Figure 4
Photocatalytic activity of all catalysts against methyl orange degradation in visible light Plot between Ct/CoVS time for methyl orange degradation by usingCoN-TiO2-5

Supplementary Files
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