We have deployed a facile, economical, reproducible, and room temperature synthesis by using sol gel method for the successful synthesis of pure and 0.5, 1, 2, 6 and 7-wt.%. nitrogen, cobalt co-doped TiO2 nanoparticles. They were tagged as CoN-TiO2-1, CoN-TiO2-2, CoN-TiO2-3, CoN-TiO2-4 and CoN-TiO2-5. 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. Fig. 1 shows the XRD spectrums of the synthesized catalysts. The peaks located at 25.3 °, 37.8 °, 48.06 °, 55.1 ° and 62.7 ° correspond to (101), (004), (200), (105), and (204) can be well indexed with the already reported JCPDS cards 96-900-908214 to 96-900-908214. They confirm the anatase phase formation(S. Mugundan 2015, Dongdong Liang 2019).
As we increased the concentration of dopant the peak broadening is observed but till wt.% 7% doping no significant additional peak is observed. Scherrer equation (D=Kλ/ (β cos θ) was used to calculate crystallite size of each catalyst (PATTERSON 1939). The crystallite sizes of all catalysts have been summarized in the Table 1.
Table 1
Crystallite sizes of all samples
Sample Code
|
Crystallite size (nm)
|
TiO2
|
26
|
CoN-TiO2-1
|
10
|
CoN-TiO2-2
|
10
|
CoN-TiO2-3
|
9.7
|
CoN-TiO2-4
|
8.75
|
CoN-TiO2-5
|
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 TiO2, and all variants of CoN-TiO2 have been shown in the Fig. 2.
It can be observed from these images that the morphology of TiO2 remain intact even after doping with nitrogen and cobalt. In case of undoped TiO2, 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.
Table 2
Atomic percentage of constituent elements in catalysts
Sample Code
|
Atomic % of Ti
|
Atomic % of O
|
Atomic % of Co
|
TiO2
|
24.33
|
75.77
|
----
|
CoN-TiO2-1
|
27.83
|
72.03
|
0.13
|
CoN-TiO2-2
|
27.26
|
72.46
|
0.27
|
CoN-TiO2-3
|
25.75
|
73.65
|
0.60
|
CoN-TiO2-4
|
18.07
|
79.23
|
2.70
|
CoN-TiO2-5
|
24.60
|
72.64
|
2.76
|
Every element shows its own characteristic peak in EDX spectra. We can clearly see the signal for titanium, cobalt, and oxygen in the EDX spectra of samples. DRS was carried out to calculate the band gap energy co-doped catalysts. As the band gap energy of the pure TiO2 nanoparticles is 3.2 eV, so they absorb in the UV region only. After doping TiO2 with nitrogen and cobalt, the band gaps were reduced, thereby shifting the absorption of TiO2 in the visible region. The band gap energies for all catalysts along with Tauc’s plots have been presented in the 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 efficiency of all catalysts against degradation of methyl orange have been shown below in Fig. 4 and 5 respectively.
It can be seen from above figures that undoped TiO2 showed lowest degradation efficiency than other catalysts. Furthermore, nitrogen and cobalt co-doping improved the photocatalytic performance of TiO2. The photo degradation efficiency of the catalysts has been shown below in the Table 3.
Table 3
Comparison of degradation efficiency of all catalysts
Sample
|
Degradation Efficiency (%)
|
TiO2
|
32
|
CoN-TiO2-1
|
59
|
CoN-TiO2-2
|
65
|
CoN-TiO2-3
|
69
|
CoN-TiO2-4
|
75
|
CoN-TiO2-5
|
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-TiO2-5. 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 TiO2. 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 efficiency of photocatalytic process. More generation of hydroxyl free radicals means more degradation of methyl orange. Low degradation efficiency of undoped TiO2 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 ≥ Eg) 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 O2− by reacting with water present on the surface of TiO2. 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 orange can be explained by following equations (Liming Bai, et al. 2019).
$${CoN-TiO}_{2}+hv\to {h}^{+}+{e}^{-}$$
1
$${h}^{+}+{OH}^{-}\to \bullet OH$$
2
$${h}^{+}+{H}_{2}O\to \bullet OH+{H}^{+}$$
3
$${e}^{-}+{O}_{2}\to {\bullet O}_{2}^{-}$$
4
$${H}_{2}O+{\bullet O}_{2}^{-}\to {HO}_{2}\bullet +{OH}^{-}$$
5
$${HO}_{2}^{\bullet }+{e}^{-}+{H}_{2}O\to {H}_{2}{O}_{2}+{OH}^{-}$$
6
$${H}_{2}{O}_{2}+{\bullet O}_{2}^{-}\to \bullet OH+{H}^{+}$$
7
$$\bullet OH+Dye\to {CO}_{2}+{H}_{2}O$$
8
$${\bullet O}_{2}^{-}+Dye\to {CO}_{2}+{H}_{2}O$$
9
Photodegradation Kinetics
The order of reaction was determined for CoN-TiO2-5 by plotting a graph between -ln Ct/Co versus time and has been shown below in Fig. 6.
The relationship between concentration and time can be explained by following equation (Ananpattarachai 2009).
$$-\text{l}\text{n}({C}_{t}/{C}_{\text{o}})={K}_{app}t$$
Where Ct is the concentration of methyl orange at a particular time, Co is the initial concentration of methyl orange, and kapp refers to apparent reaction rate constant. The slope of the graph indicated the apparent reaction rate constant (Kapp) and the linearity of graph represents that the reaction is pseudo first order.