Highly Efficient Photocatalytic Degradation of Amido Black 10B Dye Using Polycarbazole-Decorated TiO2 Nanohybrids.

The present study reports the synthesis of polycarbazole (PCz)-decorated TiO2 nanohybrids via in situ chemical polymerization of carbazole monomers in TiO2 dispersions. The ratio of the polymer in the nanohybrid varied between 0.5 and 2 wt %. The synthesized nanohybrids were characterized using infrared and diffuse reflectance spectroscopies, whereas the morphology was analyzed using X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. XRD revealed changes in the peak corresponding to the d(001) plane of TiO2 owing to the interaction between the two components. TEM confirmed the formation of PCz-decorated nanohybrids. Amido Black 10B (AB-10B) was chosen as a model dye for the degradation studies. Sonophotocatalytic degradation of the dye was studied by varying the catalyst and dye concentrations. Results showed that PCz/TiO2 nanohybrids exhibited a complete degradation of AB-10B dye within a short span of 60-90 min, which was faster than pure TiO2 and the reported decorated TiO2 nanohybrids synthesized by other authors. The degraded dye fragments were identified using liquid chromatography-mass spectrometry (LCMS). By varying the loading of PCz in TiO2, the nanohybrids could be tuned to achieve visible light-driven degradation.

Introduction

Semiconductor-mediated photocatalysis is one of the widely adopted techniques for the degradation of dyes and organic pollutants in water.[1−5] The key function of this method is to generate species such as hydroxyl radicals (•OH) and superoxide radicals (O2•–) that can degrade the organic compounds in the dye waste water into harmless byproducts.[6−10] One of the major limitations of utilizing TiO2 as a photocatalyst is its high rate of recombination of holes and electrons owing to its wide band gap of ∼3.2 eV.[11,12] As this band gap corresponds to the wavelength of 387 nm, hardly 4% of the solar energy is utilized for the degradation of organic pollutants. Therefore, pristine TiO2 is economically unattractive as a photocatalyst for waste water remediation.[13,14] Several strategies have been employed to increase the photocatalytic activity of TiO2 such as increasing the catalyst surface to volume ratio, sensitization of the catalyst using polymers, doping of the catalyst with nonmetals such as nitrogen and carbon, and impregnation of metal ions/transition metals.[15−20] Lately, researchers have focused on combining semiconductor oxide nanoparticles with conducting polymers to achieve synergetic properties.[21−24] Conducting polymers, such as polyaniline,[6] poly(1-naphthylamine),[21] and so forth, provide moderate to high mobility of charge carriers via the extended π-conjugated electron system that can be electronically coupled to TiO2, so as to facilitate a better separation of photoinduced charges in the semiconductor oxide. Conducting polymers are also known to act as sensitizers when combined with semiconductor nanoparticles to prevent the electron–hole recombination.[25] Among the several conducting polymers, polycarbazole (PCz) has been proved to be an exceptionally efficient polymer owing to its high electrical conductivity, thermal stability, low band gap, and low toxicity.[26−30] It has been widely used in designing solar devices and organic light-emitting diodes.[31−35]

With a view to explore the photocatalytic efficiency of PCz, the present study reports the sonolytic synthesis of PCz/TiO2 nanohybrids using different weight ratios of the conducting polymer. The nanohybrids were characterized by FTIR, diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD), and transmission electron microscopy (TEM) techniques. To the best of our knowledge, the photocatalytic performance of PCz/TiO2 nanohybrids is reported for the first time. Amido Black 10B (AB-10B) was chosen as a model dye because it is widely used in staining proteins and is reported to be highly toxic.[36] The photocatalytic activity of the nanohybrids was evaluated by varying the catalyst and dye concentration upon exposure to UV irradiation for a period of 60 min. The degraded fragments were identified using the liquid chromatography–mass spectrometry (LCMS) technique, and a plausible mechanism for the efficient photocatalytic performance was proposed.

Results and Discussion

Confirmation of Nanohybrid Formation by IR Studies

Fourier transform infrared (FT-IR) spectra of pristine PCz and PCz/TiO2 nanohybrids are shown in Figure . The IR spectrum of pure PCz revealed two NH stretching vibration peaks at 3416 and 3045 cm–1. The imine stretching peak was observed at 1780 cm–1. The peaks corresponding to quinonoid and benzenoid units were noticed at 1597 and 1491 cm–1, respectively.[29,37−39] The benzenoid to quinonoid (B/Q) ratio in this case was observed to be 0.91, indicating the formation of equal number of benzenoid and quinonoid units. The CN stretching peak was observed at 1232 cm–1, whereas the peaks at 921, 850, and 752 cm–1 were correlated with the presence of unsubstituted carbazole, confirming that polymerization took place from 3 to 6 positions.[40] The IR spectrum of PCz/TiO2 (0.5:1) showed a broad NH stretching vibration peak around 3309 cm–1. The broadness of the peak was correlated with the interaction of NH of PCz with oxygen of TiO2.[37−39] The peak at 667 cm–1 was correlated with the presence of TiO2 and appeared to be broad, whereas the peaks corresponding to PCz appeared to be diminished owing to the lower loading of the polymer in this nanohybrid.

Figure 1

FTIR spectra of pure PCz and PCz/TiO2 nanohybrids.

In the case of PCz/TiO2 (1:1), NH stretching vibration peaks were noticed at 3416 and 3045 cm–1, whereas the imine stretching peak was observed around 1780 cm–1. The quinonoid and benzenoid units were observed at 1597 and 1491 cm–1, respectively. The peak associated with TiO2 appeared at 675 cm–1. The NH stretching vibration peaks for PCz/TiO2 (2:1) were noticed at 3416 and 3047 cm–1, whereas the imine stretching peak was observed at 1780 cm–1. The peaks corresponding to quinonoid and benzenoid units were noticed at 1599, 1448, and 1394 cm–1. The B/Q ratio was calculated to be 1.04. It can therefore be concluded that with the increase in the loading of PCz, slight changes were noticed in the IR spectra of the nanohybrids. Similar observations have also been reported by other authors.[29] However, the NH stretching vibration peak revealed a significant shift in the case of PCz/TiO2 (0.5:1). With the increase in the loading of PCz in TiO2, the shift was observed to decrease owing to the encapsulation of TiO2 by PCz. The % transmittance was also found to decrease with the increase in the loading of PCz. The lowest intensity was noticed for PCz/TiO2 (2:1), whereas the highest intensity was observed for PCz/TiO2 (0.5:1). The weight average molecular weight (M̅w) of pure PCz was calculated to be 3582 Da, which confirmed the polymerization of carbazole.[41] Conductivity was found to be 2.65 × 10–4 S/cm. The conductivities of PCz/TiO2 (0.5:1), PCz/TiO2 (1:1), and PCz/TiO2 (2:1) were obtained as 1.13 × 10–4, 1.75 × 10–4, and 2.12 × 10–4 S/cm, respectively.

Analysis of Electronic Transitions and Band Gap Energy PCz and PCz/TiO2 Nanohybrids via DRS

The diffuse reflectance spectrum of pure TiO2 (Figure a) revealed a sharp absorption edge around 390 nm, which could be well-correlated with the anatase form of TiO2. The diffuse reflectance spectrum of pure PCz and PCz/TiO2 nanohybrids showed absorption edge around 410 nm. The peak intensity was observed to be the lowest for PCz/TiO2 (2:1) but was found to be higher for PCz/TiO2 (1:1) and matched with that of pure PCz.

Figure 2

(a) UV–vis diffuse reflectance spectra (DRS) of TiO2, PCz, and PCz/TiO2 nanohybrids and Kubelka–Munk plot of (b) PCz and (c) TiO2.

The Kubelka–Munk[42] equation is generally applied to calculate the band gap of semiconductors which is given bywhere F(R) is the Kubelka–Munk function, “R” is reflectance of the sample, “α” is the absorption coefficient, and “s” is the scattering coefficient. The scattering coefficient, s, is ignored on the basis of wavelength dependence, thereby making F(R) proportional to α. Tauc, Davis, and Mott proposed an equation to calculate the band gap of semiconductors using the absorption coefficient given by the expression[43,44]where α is the adsorption coefficient, h is the Planck’s constant, ν is the vibrational frequency, and Eg is the band gap. The value of n is taken to be 2 for indirect band semiconductors. By substituting the value of n and α = F(R) in the above equation, the band gap was calculated, as shown in Figure b. The band gap energy of pristine TiO2 was calculated to be 3.2 eV, whereas that of pure PCz was calculated as 2.95 eV (Figure b).

Morphological Analysis of TiO2, PCz, and PCz/TiO2 Nanohybrids via XRD and TEM Studies

The XRD profiles of PCz and PCz/TiO2 nanohybrids are depicted in Figure . The XRD patterns of anatase TiO2 (inset) revealed peaks at 2θ = 25.5°, 38.2°, 48.3°, 54.2°, 55.4°, 62.5°, 68.2°, 70.7°, and 75.5° corresponding to d(101), d(004), d(200), d(105), d(211), d(204), d(116), d(220), and d(215) planes, respectively. The peaks were found to match with the tetragonal anatase form of TiO2 showing cell constants as a = b = 0.37710 nm, c = 0.9430 nm, and α = β = γ = 90°, which was found to be in agreement with the standard diffraction data (JCPDS 21-1272).[45] The XRD profile of pure PCz revealed peaks at 2θ = 18.50°, 19°, 19.5°, 22.5°, 23°, and 28° exhibiting high crystallinity as found in our previous studies.[46] The nanohybrids revealed a slight shift in the crystalline peaks as well as variation in their intensities upon nanohybrid formation. The PCz/TiO2 (0.5:1) nanohybrid revealed peaks at 2θ = 19°, 19.5°, 22.5°, 23.2°, and 28°. The intensity of the peaks corresponding to PCz appeared to be highly reduced upon the addition of TiO2. The peak observed around 2θ = 25.3° was correlated with the d(101) plane of TiO2. Upon further increasing the loading of PCz to 1 wt % (Figure ), the intensity of the peaks appeared to increase. For PCz/TiO2 (2:1), the peaks corresponding to PCz showed the highest intensity. However, the peak correlated to TiO2 revealed the highest intensity for PCz/TiO2 (1:1). The presence of the peaks corresponding to both PCz and TiO2 confirmed the formation of the nanohybrid. The variation in the intensity of the peak corresponding to d(101) plane of TiO2 confirmed its encapsulation by PCz (Table ). The area under the peak corresponding to the d(101) plane of TiO2 was found to increase with the increase in the loading of PCz while the crystallite size also showed a slight variation. Hence, it can be concluded that an intense synergistic interaction was found to exist between PCz and TiO2 because the peak corresponding to TiO2 revealed variations in the area as well as intensity upon loading of PCz.

Figure 3

XRD of PCz and PCz/TiO2 nanohybrids.

Table 1

XRD Data of PCz and PCz/TiO2 Nanohybrids

nanohybrids	peak (2θ)	area peak	height (au)	fwhm (2θ)	crystallite size (Å)
pure TiO2	25.5	947	741	0.220	6.97
PCz/TiO2 (0.5:1)	25.3	1659	1205	0.194	7.90
PCz/TiO2 (1:1)	25.2	1659	1401	0.191	8.02
PCz/TiO2 (2:1)	25.2	2118	1031	0.190	8.06

The morphology of pure PCz and PCz/TiO2 nanohybrids is shown in Figure a–d. The TEM of pure PCz revealed a mixed morphology of cubes as well as hexagonal particles (Figure a). The TEM of the PCz/TiO2 (0.5:1) nanohybrid (Figure b) revealed a fused distorted morphology in which the dense TiO2 particles were noticed to be surrounded by the PCz nanoparticles. The TEM of the PCz/TiO2 (1:1) nanohybrid (Figure c) exhibited huge clusters of distorted core–shell-like morphology, whereas the PCz/TiO2 (2:1) nanohybrid (Figure d) showed the formation of flowerlike clusters containing dense TiO2 particles surrounded by PCz petals. The TiO2 particles appeared to be decorated with PCz in the case of PCz/TiO2 (1:1) and PCz/TiO2 (2:1)[47] nanohybrids. The morphology clearly revealed the synergistic interaction of PCz with TiO2. The results were in agreement with the XRD studies that showed intact crystalline morphology of the two components. Hence, it can be confirmed that TiO2 particles are encapsulated with the PCz chain, leading to the formation of self-assembled structures.

Figure 4

TEM of (a) PCz, (b) PCz/TiO2 (0.5:1), (c) PCz/TiO2 (1:1), and (d) PCz/TiO2 (2:1).

Variation of Thermal Stability of PCz and Its Nanohybrids

The thermal stabilities of PCz and PCz/TiO2 nanohybrids were analyzed by TGA studies. The thermogram of pure PCz (Figure ) revealed 10 wt % loss at 200 °C, whereas 20 wt % loss occurred at 310 °C. The initial weight loss of around 10 wt % was attributed to the presence of unreacted monomers. Almost 60 wt % loss was noticed at 480 °C, whereas 95 wt % loss took place at 830 °C. The PCz/TiO2 (0.5:1) nanohybrid revealed 10 wt % loss around 250 °C, whereas 62 wt % loss was noticed at 830 °C. The thermal stability was found to be slightly enhanced. The thermogram of the PCz/TiO2 (1:1) nanohybrid revealed 10 wt % loss at 260 °C, whereas 50 wt % loss took place around 830 °C. Similarly, the PCz/TiO2 (2:1) nanohybrid showed 10 wt % loss at 260 °C, whereas 30 wt % loss occurred at 830 °C. With the increase in the loading of PCz in TiO2, the thermal stability was found to remarkably improve. The thermal stability of PCz/TiO2 was noticed to be far superior to that of pure PCz and was found to be in the order PCz/TiO2 (2:1) > PCz/TiO2 (1:1) > PCz/TiO2 (0.5:1) > PCz.

Figure 5

TGA thermograms of PCz and PCz/TiO2 nanohybrids.

Evaluation of Photocatalytic Properties of PCz and PCz/TiO2 Nanohybrids

The specific surface area plays a major role in photocatalysis as it leads to an increased catalytic activity. In our case, the specific surface area of pure PCz and PCz/TiO2 (2:1) nanohybrids was found to be 236.226 and 281.321 m2/g, respectively. The measured pore volume for PCz and PCz/TiO2 (2:1) nanohybrids was calculated to be 0.190 and 0.213 cc/g, respectively (given in the Supporting Information, Figure S1). The photocatalytic activity of the nanohybrids was evaluated using AB-10B dye. The UV–visible spectrum of AB-10B revealed a prominent peak at 618 nm and two small peaks at 226 and 318 nm (Figure a).

Figure 6

UV–visible spectra of (a) AB-10B dye (b) in the presence of PCz as a catalyst, (c) in the presence of PCz/TiO2 (0.5:1) as a catalyst, (d) in the presence of PCz/TiO2 (1:1) as a catalyst, and (e) in the presence PCz/TiO2 (2:1) as a catalyst.

A prominent peak at 618 nm in the visible region was observed owing to the presence of an azo group, whereas the peaks in the ultraviolet region were assigned to π–π* transition of the aromatic benzene group.[48,49] The degradation profile of pristine PCz (Figure b) revealed negligible change during 60 min exposure time. However, in the presence of the PCz/TiO2 nanohybrid, the peaks at 617 and 330 nm revealed a large decrease in the absorption intensity, indicating a complete degradation of AB-10B under similar experimental conditions (Figure c–e). Among the three nanohybrids, PCz/TiO2 (1:1) and PCz/TiO2 (2:1) revealed the complete degradation of AB-10B dye within a short span of 90 min. To study the effect of catalyst concentration, degradation of 90 ppm AB-10B dye solution was carried out using 50, 100, 200, and 300 mg of the nanohybrid as catalyst for a period of 60 min (Figure a,b). The PCz/TiO2 (1:0.5) nanohybrid revealed 100 wt % degradation of 50 ppm dye solution when the catalyst concentration was 300 mg, whereas 75 wt % degradation was achieved when the catalyst concentration was 100 mg (Figure a,b). Similarly, PCz/TiO2 (1:1) and PCz/TiO2 (2:1) nanohybrids revealed almost 96 wt % degradation in the case of 50 ppm of AB-10B dye solution, when the catalyst amount was 50 mg (Figure b). It can thus be concluded that the degradation efficiency was found to be high even at lower loading of the catalyst.

Figure 7

Effect of catalyst concentration on the percent degradation for (a) 50 ppm dye solution and (b) 90 ppm dye solution.

The C/Co plots of the nanohybrids were studied using different catalyst concentrations in 90 ppm AB-10B dye solution (Figure a–c). When 300 and 200 mg of PCz/TiO2 (0.5:1) nanohybrid was used as the catalyst, almost 75 and 70 wt % degradation was achieved in 60 min, whereas 65 wt % degradation was achieved when 100 mg catalyst was used (Figure a). Similarly, the PCz/TiO2 (1:1) nanohybrid (Figure b) showed 65–75 wt % degradation when the catalyst amount was increased from 50 to 300 mg, whereas the degradation increased from 65 to 90 wt % for the PCz/TiO2 (2:1) nanohybrid using the same amount of catalyst (Figure c). The plots confirmed that the nanohybrids containing higher PCz loading exhibited higher degradation efficiency even when used in small amounts. Upon increasing the concentration of the AB-10B dye solution from 30 to 120 ppm using a fixed catalyst amount of 150 mg, it was observed that 87 and 85 wt % degradation was achieved for 30 and 70 ppm dye solutions, respectively, using the PCz/TiO2 (0.5:1) nanohybrid as the catalyst (Figure a–c). The PCz/TiO2 (1:1) nanohybrid showed 90 and 80 wt % degradation in 60 min for 30 ppm and 70 ppm dye solutions, respectively, whereas 120 ppm dye solution showed 60 wt % degradation in 60 min (Figure b). Around 90 wt % degradation occurred when the PCz/TiO2 (2:1) nanohybrid was used as the catalyst for 120 ppm AB-10B dye solution (Figure c). It can thus be concluded that the nanohybrid containing 2 wt % PCz revealed degradation efficiency (as high as 80 wt %) for the degradation of 120 ppm AB-10B dye solution.

Figure 8

C/Co plots for (a) PCz/TiO2 (0.5:1) (inset ln C/Co), (b) PCz/TiO2 (1:11) (inset ln C/Co), and (c) PCz/TiO2 (2:11) (inset ln C/Co).

Figure 9

C/Co plots for (a) PCz/TiO2 (1:0.5) (inset ln C/Co), (b) PCz/TiO2 (1:1) (inset ln C/Co), and (c) PCz/TiO2 (2:1) (inset ln C/Co).

The plots of ln C/Co versus time (Figure a–c, insets) showed that degradation kinetics followed the pseudo first-order kinetics in all cases. When PCz/TiO2 (0.5:1) was used as the catalyst, the rate constant (k) was observed to be 0.027 min–1 for 30 ppm dye solution, whereas for 120 ppm dye solution, it was noticed to be 0.010 min–1. Similarly for the catalyst, PCz/TiO2 (1:1), the rate constant (k) decreased from 0.057 min–1 for 30 ppm dye solution to 0.015 min–1 for 120 ppm, whereas for PCz/TiO2 (2:1), the rate constant decreased from 0.090 to 0.038 min–1. Sivakumar et al.[48] carried out the degradation of AB-10B dye using metal-decorated TiO2 nanohybrids, and the rate constant values reported by them are shown in Table . The degradation time in our case was observed to be 60 min, whereas the authors have carried out the degradation for a period of 7 h. These observations clearly highlight the photocatalytic efficiency of PCz-decorated TiO2 nanohybrids.

Table 2

Comparison of the Rate Constant Values of Metal-Decorated and PCz-Decorated TiO2 Nanohybrids

catalysts	surface area (m2/g)	crystallite size (nm)	degradation time	rate constant (min–1) using catalyst 150 mg
TiO2	93	21.37	7 h	6.52 × 10–3
Ni/TiO2 (0.5%)[48]	47	22.29	7 h	8.14 × 10–3
Ru/TiO2 (0.5%)[48]	72	22.40	7 h	1.9 × 10–2
PCz/TiO2 (0.5:1)		43.43	60 min	1.1 × 10–2
Ni/TiO2 (1%)[48]	62	22.44	7 h	9.39 × 10–3
Ru/TiO2 (1%)[48]	81	22.49	7 h	2.3 × 10–2
PCz/TiO2 (1:1)		44.39	60 min	1.5 × 10–2
Ni/TiO2 (3%)[48]	41	22.46	7 h	8.05 × 10–3
Ru/TiO2 (3%)[48]	60	22.53	7 h	1.55 × 10–2
PCz/TiO2 (2:1)	281	66.34	60 min	3.8 × 10–2

Proposed Degradation Pathway and Analysis of Degraded Dye Fragments by the LCMS Technique

Although PCz failed to degrade the dye under UV–visible irradiation, the PCz/TiO2 nanohybrid was noticed to rapidly degrade the dye molecules under similar conditions. A plausible explanation for this behavior is depicted in Scheme . Low band gap of PCz facilitates high electron-hole recombination in the absence of metal oxide. When PCz/TiO2 is illuminated under UV light, it promotes the transfer of electrons from the lowest unoccupied molecular orbital of PCz molecules into the conduction band (CB) of TiO2 which reacts with oxygen and hydroxyl radicals present at the surface. The CB of PCz is lower than that of TiO2, and hence it acts like a sink for the photogenerated electrons. The holes move in the opposite direction from the electrons, and the photogenerated holes in PCz get trapped within the TiO2 particles. In this way, the charge carrier recombination is reduced, and more charge carriers are available for the production of free radicals through interfacial charge transfer (Scheme ). Radical-trapping experiments were conducted using benzoquinone as a superoxide anion radical scavenger and tert-butanol as a hydroxyl radical (given in the Supporting Information, Figure S2). The concentration of AB-10B dye solution was found to decrease drastically in the presence of 2 mM t-BuOH and pure TiO2, whereas the concentration decreased slightly using 2 mM benzoquinone.[15,16] However, when the PCz/TiO2 nanohybrid was used with the radical scavengers, the concentration of the AB-10B dye solution decreased drastically in the presence of benzoquinone, whereas the concentration changed slightly in the presence of t-BuOH. This behavior confirmed that •OH radicals were the active species that participated in the degradation of AB-10B dye solution when PCz/TiO2 nanohybrid was used as the catalyst.

Scheme 1

Mechanism of Radical Generation in (a) PCz and (b) PCz/TiO2 Nanohybrids

Degradation of Amido Black was confirmed by LCMS studies which revealed a variety of intermediate compounds formed during the course of the reaction (Scheme a). The intermediates with their increasing m/z values are shown in Scheme b. Around 100% abundance was assigned to the first intermediate that was taken as the main degradation product. Intermediates of 100% abundance with low m/z values ranging from 100 to 70 were obtained. They were labeled as G-1, G-2, G-3, G-4, and G-5. The first intermediate G-1 (m/z 540) showed 100% abundance and was taken as the main degradation product. Intermediates with m/z values 393 (80%), 291 (70%), 269 (83%), and 189 (100%) were obtained. These intermediates revealed that the degradation proceeded via elimination of azo and sulphonate groups, attacked by •OH free radicals. Interestingly, the dye degradation proceeded by the cleavage of the −N=N– group, owing to easy breakdown of π bond, bearing the unsubstituted phenyl ring. The •OH radicals preferentially attacked the electron-rich diazo functionality of the molecule to form sodium 4-amino-6-diazenyl-5-hydroxy-3-((4-nitrophenyl)diazenyl)naphthalene-2,7-disulfonate (G-1, m/z 540). The attack of •OH radicals produced sodium 3,4,6-triamino-5-hydroxynaphthalene-2,7-disulfonate (G-2, m/z 393). This fragment then degraded into sodium 3,4,6-triamino-5-hydroxynaphthalene-2-sulfonate (G-3, m/z 291). Accordingly, 3,4,6-triamino-5-hydroxynaphthalene-2-sulfonic acid (G-4, m/z 269) was obtained from the G-3 fragment. The degradation product G-5 was formed by the cleavage of the sulphonate group from the G-4 fragment (Scheme (a)).

Scheme 2

(a) Proposed Degradation Pathway of AB-10B Dye (Inset: LCMS Spectrum) and (b) LCMS Spectrum of Intermediates of AB-10B Dye Using PCz/TiO2 (2:1) as a Catalyst

Conclusions

PCz/TiO2 nanohybrids were successfully prepared via in situ chemical polymerization using the ultrasonic technique. XRD and TEM analyses confirmed the interaction between PCz chains and TiO2 nanoparticles. The thermal stability of PCz/TiO2 was noticed to be far superior than that of pure PCz and was found to be in the order PCz/TiO2 (2:1) > PCz/TiO2 (1:1) > PCz/TiO2 (0.5:1) > PCz. PCz/TiO2 nanohybrids exhibited good photocatalytic activity as compared to pristine TiO2, whereas PCz revealed no activity. Almost 100% degradation of AB-10B dye was achieved in the presence of PCz/TiO2 nanohybrid. LCMS studies confirmed the degradation of dye into fragments of low-molecular-weight compounds. Thus, by varying the loading of PCz in TiO2, the band gap of the nanohybrid could be modified and tuned to achieve visible light-driven photocatalysis. Overall, the nanohybrid was found to hold immense potential to be used as an efficient photocatalyst for waste water remediation. Studies on the degradation of other organic pollutants using this nanohybrid are under progress and will be published soon.

Materials and Methods

Amido Black-10B (AB-10B) dye was procured from S.D. Fine Chem. Pvt. Ltd., India and used without further purification. Carbazole (Sigma-Aldrich, USA), ferric chloride (Sigma-Aldrich, USA), titanium(IV) oxide (anatase ≥99%, Sigma-Aldrich, USA), and N-methyl-2-pyrrolidone (Merck, India) were also used without further purification.

Synthesis of PCz

Carbazole monomer (2.5 g, 0.014 mol) was added to a 250 mL conical flask containing methanol and water 1:1 v/v (50 mL each). Ferric chloride (2.7 g, 0.016 mol) was added as the initiator to the reaction mixture kept in an ultrasonicator maintained at 30 °C. The color of the solution changed from dusty gray to light yellow, indicating rapid polymerization of the monomer.[37] The reaction was carried out for 2 h. The synthesized PCz was then taken out and washed several times with distilled water/methanol on a Buchner funnel. PCz was then dried in a vacuum oven for 72 h at 70 °C for the complete removal of water and impurities.

Synthesis of PCz/TiO2 Nanohybrids

For the synthesis of 2:1 TiO2/PCz nanohybrid, carbazole monomer (3 g, 0.017 mol) and TiO2 (1.6 g, 0.020 mol) were added together in a 250 mL conical flask containing methanol and water (1:1 v/v; 50 mL each). Ferric chloride (3.2 g, 0.019 mol) was added to the reaction mixture keeping the monomer/initiator ratio to be 1:1. The flask was kept on an ultrasonicator maintained at 30 °C, and the reaction was carried out for 6 h. The synthesized PCz nanohybrid was then washed several times with distilled water and ethanol and dried in vacuum for 72 h at 80 °C to ensure the complete removal of water. A similar procedure was adopted for the synthesis of 1:1 and 0.5:1 nanohybrids, and they were designated as PCz/TiO2 (0.5:1), PCz/TiO2 (1:1), and PCz/TiO2 (2:1) based on the wt % loading of PCz in TiO2.

Photocatalytic Activity

A stock solution of 500 ppm AB-10B dye solution was prepared by dissolving 500 mg of AB-10B dye in 1 L of deionized water. To study the effect of dye concentration, solutions of AB-10B dye of 120 mg/L, 90 mg/L, 70 mg/L (mg), and 50 mg/L were prepared by the dilution of 500 mg/L stock solution and were designated as AB-10B-120, AB-10B-90, AB-10B-70, and AB-10B-50, respectively. Fixed catalyst amount (150 mg) was taken with 200 mL of dye solution prior to UV irradiation, and the suspension was stirred for 30 min and kept under dark conditions (for 24 h) to establish the equilibrium. Photocatalytic experiments were performed under UV irradiation in a photochemical reactor (model LELESIL), fitted with a UV lamp of LP 400 W, lamp arc: 125 mm with built in resistor, and wavelength spectrum: 200–1100 nm. The lamp was switched on to initiate the photocatalytic degradation reaction. The dye solutions were exposed to UV irradiation, and aliquots (2 mL) of the dye solution were taken out at regular intervals of 15, 30, 45, and 60 min and centrifuged for 10 min at a speed of 5000 rpm and analyzed using a UV–visible spectrophotometer model Shimadzu UV 1800 at λmax of AB-10B dye (618 nm). To study the effect of catalyst concentration, different concentrations of the nanohybrid (50, 100, 200, and 300 mg) were used for degrading 90 ppm AB-10B dye solution. A calibration plot based on Beer–Lambert’s law was obtained by plotting the absorbance against the concentration of dye in solution to determine the quantity of the dye degraded after different intervals of time. Each experiment was done in triplicate, and the deviation from the mean value of the concentration of the dye at any time was shown by error bars. For kinetics analysis, the degradation data were plotted in Origin 8.0 software. Out of the graphs plotted for different rate laws, ln C/Co, versus time gave the best fit data as the R2 value was observed to be higher than 0.995. The slope showed the rate constant value obtained using different nanohybrids as catalysts. Radical-trapping experiments were conducted to identify the radicals involved in the degradation of AB-10B dye solution using benzoquinone (O2•– radical scavenger) and tert-butanol (•OH radical scavenger). The nanohybrid (2 mg) along with the scavenger (5 mL, 2 mM) was added to 50 mL of AB-10B dye solution (50 mg L–1) and was sonicated together for 3 h in the dark to reach adsorption equilibrium. The samples were sonolytically irradiated under UV light. The suspensions were separated at fixed time intervals by centrifugation, and the decrease in the concentration of AB-10B dye solution was measured by taking the UV–vis spectra.

Instrumentation and Characterization

Molecular weight mass determination was done using the Viscotek GPCmax autosampler system consisting of a pump, a Viscotek UV detector, and a Viscotek differential refractive index detector. A ViscoGEL GPC column 151 (G2000HHR) (7.8 mm internal diameter, 300 mm length) was used. The effective molecular weight range of the column used was 456–42 800, and tetrahydrofuran was used as an eluant at a flow rate of 1.0 mL/min at 30 °C. Analysis of the data was done using Viscotek OmniSEC Omni-01 software. Conductivity was measured in a pellet form on a Keithley multimeter model DMM 2001 via four probe method. FT-IR spectra of the nanohybrid were taken in the form of KBR pellets on an FT-IR spectrophotometer model Shimadzu IRA Affinity-1. Diffuse reflectance spectra were taken on a UV–vis–NIR spectrophotometer with an integrated spherical detector (UV-2501PC, Shimadzu, Japan) in the range of 200–800 nm. XRD profiles of the nanohybrid were recorded on a Philips PW 3710 powder diffractometer (nickel-filtered Cu Kα radiations). Peak parameters were analyzed using Origin 8.0 software. Transmission electron micrographs were taken on Morgagni 268-D TEM, FEI, USA, operated at an accelerated voltage of 120 kV. The thermal stability of PCz and PCz/TiO2 nanohybrids was investigated by TGA using a thermal analyzer STA 6000, PerkinElmer. The samples were heated from 30 to 850 °C at a heating rate of 10 °C/min in N2 atmosphere at a flow rate of 20 mL/min. The specific surface area of PCz and PCz/TiO2 nanohybrids was analyzed via nitrogen adsorption isotherms (78 K) using the Brunauer–Emmett–Teller (BET) method. The pore size distributions of PCz and PCz/TiO2 (2:1) nanohybrids were derived from the absorption isotherms by using the BET surface area analyzer, Nova Station 2000e, Quantachrome Instruments Limited, USA, using the multiple-point BET method. For the detection and identification of degradation products, LCMS was conducted using a Waters Xevo G2-S TOF, USA, mass spectrometer equipped with an electrospray ionization interface source and operated in the negative polarity mode fitted with BEH C18 (1.7 × 50 mm) containing 2.1 packed particles. Acetonitrile and Milli-Q water containing 0.1% formic acid, pH 2.7, were used as eluants. The experiments were carried out in triplicate for evaluating the effect of nanohybrid catalyst dosage and AB-10B dye concentration.