3.1. XRD analysis
Figure 1 shows the XRD pattern of both the samples where the scan range was taken between 10 to 80 o. It has been seen that all the samples have a distinct peak centring around 2θ = 21.34 o. However, the peak has a considerable broadness and thus it is difficult to take the exact position of the peak. This though does not create any difficulties in assignment of the peak and it is the signature of presence of (111) plane of the crystalline SiO2 (As per ICDD No. 01-076-0931). The broadness of the peak suggests that the sample is semi crystalline which is rather expected from such sample. The Intensities of both the samples are almost same.
The crystallite sizes of the samples were calculated from the very well-known Scherrer’s equation that runs as:
$$= \frac{0.9}{\text{ß}\text{cos}}$$
1
where λ is the wavelength of the X-ray used
ß is the FWHM of peak centred at Bragg’s angle θ
The values of τ have come out to be 7.8 and 9.64 nm for the sample S1, S2 respectively suggesting an increase of crystallite size for the hybrid samples compared to pure silica.
3.3. FTIR analysis
Figure 4 shows the FTIR spectrum of both the samples where the spectra was taken in absorption mode between wavenumber 400–4500 cm− 1 with minimum resolution 2 cm− 1. It has been shown that the spectra have many distinct peaks at particular wavenumber region suggesting the existence of associated chemical bonds. The assignment of different bond has been shown in the following Table 1.
Table 1: Assignment of different FTIR peaks [32]
Sl. No.
|
Peak Position (cm− 1)
|
Assignment
|
1.
|
466
|
Si-O-Si
|
2.
|
562
|
Si-OH
|
3.
|
663
|
Si-O (symmetric Stretch)
|
4.
|
799
|
Si-O4 (Assymetric)
|
5.
|
972
|
Si-OH
|
6.
|
1093
|
Si-O-Si (asymmetric stretch)
|
7.
|
1392
|
C-H2
|
8.
|
1498
|
C-H3
|
9.
|
1668
|
C = O/C = C
|
10
|
2934
|
C-Hn band
|
11.
|
3358
|
O-H band
|
The conversion of TEOS to silica gel is identified by the IR peaks at ~ 562 and 972 cm− 1 assigned to the vibration of the Si–OH rocking and Si–OH stretching, which arise as the by-product due to incomplete condensation of silanes.
The band around 2930 cm− 1 is generally used to determine the hybridized state of the carbon present in the samples. In practice the band can be deconvoluted via the same origin software. The corresponding deconvoluted spectra of both the samples have been shown in Fig. 5 and corresponding assignment has been summarized in Table 2.
Table 2
Assignment of different C–H vibrational modes
Sl. No
|
Sample
|
Position (cm− 1)
|
Assignment
|
1
|
S1
|
2935
|
sp3 stretching
|
2960
|
sp3–CH3 asymmetric
|
3001
|
sp2–CH symmetric
|
2
|
S2
|
2935
|
sp3 stretching
|
2962
|
sp3–CH3 asymmetric
|
3005
|
sp2–CH symmetric
|
The value of sp2/sp3 ratio (in %) has been come out to be 2.4 and 33.8 for the sample S1 and S2 respectively. This has a direct consequence with its interaction with the dye molecules to be discussed in the coming section.
3.4. Raman Analysis
The Raman spectra of all the samples have been shown in Fig. 6. It has been bit surprisingly seen that for sample S1 there is a broad band between 1120–1790 cm− 1 in the high fluorescence background. In case of the sample S2 the fluorescence is even higher and thus the carbon D and G peaks get completely disappeared.
The band between 1100–1700 cm− 1 has been deconvoluted into two daughter peaks centring around as mentioned above and the corresponding spectra for sample S1 has been shown in Fig. 6. The peaks centring at 1390 and 1580 cm− 1 have been taken for the calculations. As mentioned in our previous work, the D and G band may be used to predict different features about the samples. The presence of sp3 content may be quantified using the equation:
$${\text{s}\text{p}}^{3} \text{c}\text{o}\text{n}\text{t}\text{e}\text{n}\text{t}=0.24-48.9\times ({{\gamma }}_{\text{G}}-0.1580)$$
2
here γG is the peak position of G band in inverse of micro-meter unit.
Table 3 summarizes the values of all the corresponding features
Table 3
Different parameters associated to sample S1 and S2 obtained from Raman spectra
Sl. No.
|
Parameters
|
Sample
|
S1
|
S2
|
1.
|
Position of D Band ( cm− 1)
|
1374
|
1331
|
2.
|
Position of G Band ( cm− 1)
|
1501
|
1598
|
3.
|
Area of D Band ( cm− 1)
|
6712
|
60084
|
4.
|
Area of G Band ( cm− 1)
|
4291
|
258135
|
5.
|
AG/AD
|
0.64
|
4.29
|
6.
|
FWHM of G Band ( cm− 1)
|
116
|
290
|
7.
|
Sp3 content
|
0.63
|
0.15
|
It is seen that when a-CNTs have been added the sp3 content of the system decreases considerably thus supporting the FTIR result. Sample S2 shows an abrupt increase in the AG/AD ratio.
3.5. Removal of Rh-B
Figure 7 (a-b) shows the interaction between the dye and both samples (S1, S2) for the loading amount 0.075 gm. From the Figures it is clearly seen that the absorption intensity of Rh-B decreases almost monotonically with the interaction times.
In our previous work [35] the efficiency has been defined as
$$\left(\text{%}\right)= \frac{{C}_{0}-C}{{\text{C}}_{0}} \times 100$$
3
Where C0 = Initial concentration of RhB i.e. when the UV light is turned on.
C = Concentration of RhB at time t. The variation is shown in Fig. 8
From the Fig. 8 it is clearly seen that for the sample S1 the maximum removal efficiency is around 82% whereas for the S2 it is around 93% just within 45 minutes which confirms the sample to be effective as dye remover. It is also clearly seen that during the dark stirring there is a considerable decrease in the UV absorbance intensity of the Rh-B suggesting there is an adsorption component also in the dye removal and the removal is thus a synergetic effect of adsorption and catalysis.
There are numbers of report where dyes have been removed with synergetic effects of adsorption and photocatalysis by certain materials. Sometimes it is even seen that one mechanism gets enhanced from the presence of the other. For instance, in the year of 2010, Ru et al. has shown that activated carbon/CdS/chitosan nanocomposite shows excellent removal efficiency of methyl orange under combined influence of both adsorption and photocatalysis [36]. In the year of 2014 Rioza and group did a similar experiment using activated carbon and TiO2 mixture as active species. The competitive mechanism has shown when light gets turned on removal efficiency further increases almost regularly and thus here differing from the current result [37]. Similar result has been reported by Xu et al. from their effort in removing MB by graphitic carbon nitride [38]. A detail synergetic mechanism has been shared by Liu and his group in their work describing the removal of RhB and MB by Ag3PO4/WO3 hybrid under the mutual influence of adsorption and photocatalysis [39]. There work shows that both the component materials (Ag3PO4/WO3) have selective response towards specific dyes i.e. either photocatalysis or adsorption. It is the proper combination of Ag3PO4 and WO3 that makes the composite capable of removing dyes under synergetic effects of adsorption and photocatalysis following a z scheme mechanism. Recently, Yi and co-workers also have shown that combined effect of adsorption and photocatalysis can only enhances the efficacy of the Bi4O5Br2/g-C3N4 nanosheet in removing the dyes like Rh-B [40].
There are mainly two reasons that are supposed to be playing the key role behind the present observation. The change of surface area due to a-CNT loading and the prolong separation of electron hole pair in the hybrid sample.
Due to the resource limitation we could not measure the surface area or porosity of the sample through BET surface area measurement. However, we have taken the help of image j software to calculate surface area and porosity of the sample following the work reported by Buckman [41]. It has been shown that though the micrographic image apparently shows otherwise but value of surface area is the higher for pure silica compared to all other doped sample. Thus higher adsorption is obvious for the pure sample as obtained here. The values of surface area and the porosity for all the samples have been summarized in Table 4
Table 4
The values of surface area and the porosity for all the samples
Sl. No. | Sample | Surface Area (µm2) | Porosity (%) |
---|
1 | S1 | 6.23 | 7.56 |
2 | S2 | 1.84 | 4.12 |
When the a-CNT has been added to silica and the system has been illuminated by UV light with suitable wavelength the entire situation can schematically be expressed as shown in Fig. 9. Here we have assumed that CNT to be rolled up graphene sheet and thus have almost metallic character. From the scheme it is clearly understood that when the sample is irradiated electron hole pairs get created in the SiO2 matrix and from the conduction band the electron relaxed into the Fermi level of CNT and thus there is considerable time for electron-hole pairs to remain separated taking part in the photo-catalysis
Another very inevitable component of such study is to checking the kinetics of the reaction. However, it has been shown in our previous work that the reaction kinetics best fitted with pseudo first order reaction mechanism [35, 42].
The 1st order reaction kinetics of photocatalysis can be formulated as:
$$r= - \frac{dC}{dt}= \frac{{K}_{r}{K}_{a}C}{(1+ {K}_{a}C)}$$
4
Here, kr and Ka represent rate constant and adsorption constant respectively. On simplifying, Eq. 8 takes the form
\(\text{ln}\frac{C}{{C}_{0}} = {K}_{r}{K}_{a}t={k}_{1}t\) (5)
Here k is the rate constant the value of which we basically seek. Eq. 5 suggests that the rate constant may be obtained from the slope of linear plot between ln(c/c0) and time. The plot can be seen in Fig. 10 and the corresponding values are summarized in Table 5.
The other reaction kinetics, i.e., the pseudo-second-order kinetics, has also been studied for which the kinetic equation runs as:
$$\frac{\varvec{t}}{{\varvec{C}}_{\varvec{t}}}= \frac{1}{{\varvec{K}}_{2}{\varvec{C}}_{\varvec{e}}^{2}}+ \left(\frac{1}{{\varvec{C}}_{\varvec{e}}}\right)\varvec{t}$$
6
where Ct and Ce (mg) are dye removed at time t and at equilibrium, respectively, k2 rate constant.
Figure 11 shows the corresponding plot and associated linear fitting. The associated parameters are summarized in Table 5.
Table 5
Different parameters related to photocatalysis and adsorption
Sl. No. | Sample | 1st order reaction Kinetics | Adsorption Parameter |
---|
| | K1 (Min− 1) | R2 | K2 | | R2 | |
1 | S1 | -0.039 | 0.62 | 51.71 | | 0.98 | |
2 | S3 | -0.054 | 0.75 | 41.58 | | 0.94 | |
It can be seen from the values of the regression coefficients that the reaction is more likely to follow the 2nd order kinetics.
Table 6 summarizes the performance of few materials system towards removal of different dyes and compares it with the present report. It has been see that our hybrid system performs well and comparable and sometimes even better with the further advantages of easy, cost effective and high yield synthesis process.
Table 6: Comparison of reported result regarding performance of different materials systems in removing dyes to the present work
Sl No. | Material | Dye | % of degradation/ Adsorption capacity (mg/g) | Removal time (min) | Ref |
1. | ZnO/SiO2 | Rhodamine-B | 100% | 120 min | [20] |
2. | Chitosan/SiO2/CNTs | Reactive Blue 19 | 97.08 mg/g | - | [21] |
3. | γ-Fe2O3/SiO2 | Methylene Blue | 116.09 mg/g | - | [22] |
4. | MgO-SiO2 | Congo Red | 4000 mg/g | - | [23] |
5. | SiO2NH2-GO | Rhodamine-B | 358 mg/g | - | [26] |
6. | GO/SiO2/SWCNTs | Congo Red | 390 mg/g | - | [28] |
7. | Si-CQDs/TiO2 | Acetaminophen | 100% | 240 min | [43] |
8. | TiO2/SBA-CNT | Methylene Blue | 63% | 160 min | [44] |
9. | Aminated silica carbon nanotube | Nitrite | 396 mg/g | - | [45] |
10. | Ti/Si/C-T | Methylene Blue | 100% | 120min | [46] |
11 | rice husk–derived Si-CQDs | Methylene Blue | 99% | 240min | [47] |
12. | a-CNTs-Silica hybrids | Rhodamine-B | 90% | 45mins | This work |