Synthesis and characterization of GaN/PEDOT–PPY nanocomposites and its photocatalytic activity and electrochemical detection of mebendazole

2.1 Materials

Pyrrole, 3,4-ethylenedioxythiophene, gallium, camphor sulfonic acid, and ammonium persulfate, were purchased from Sigma Aldrich. Deionized water was used as the solvent.

2.2 Synthesis of gallium nitride nanoparticles

The experimental setup consists of a long horizontal type quartz reactor of length 100 cm with suitable gas inlet and outlet orifice and the reactor is kept inside a resistively heated furnace. Required amount of gallium oxide nanoparticles was taken in a quartz reaction boat. Nitrogen gas was used as a carrier gas for ammonia molecules. Gallium oxide nanoparticles were heated at 900 °C for 9 h in the presence of ammonia. During heating, gallium nitride (GaN) nanoparticles were formed. The synthesized GaN nanoparticles were gradually cooled to room temperature by a continuous flow of ammonia and nitrogen gas.

2.3 Synthesis of poly(3,4-ethylenedioxythiophene-co-pyrrole) nanocomposite

In a typical synthesis, a mixture of pyrrole (5 mM), 3,4-ethylenedioxythiophene (5 mM), and (10 mM) camphor sulfonic acid (CSA) was taken in a 250 mL three neck round bottom flask containing 180 mL of deionized water. The above solution was ice cooled under magnetic stirring condition. Then, 20 mL of aqueous ammonium persulfate (14 mM) solution was added dropwise to the above solution under stirring condition. The mixture was stirred for 24 h. The obtained green-black product was collected by centrifugation at 7000 rpm. The product was washed with 50:50 ratio of ethanol and deionized water mixture to remove the unreacted monomer and other impurities. The product is dried under vacuum and kept in a desiccator.

2.4 Synthesis of GaN–poly(3,4-ethyledioxythiphene)–polypyrrole nanocomposite

In a typical synthesis, a mixture of 10 mM of GaN nanoparticles, 5 mM of Pyrrole, 5 mM of 3,4-ethylenedioxythiophene, and 10 mM of CSA was taken in a 250 mL three neck round bottom flask containing 180 mL of deionized water. The above solution was ice cooled with magnetic stirring. Then, 20 mL of ammonium persulfate solution (14 mM) was added dropwise to the flask under magnetic stirring. The mixture was stirred for 24 h. The obtained green–yellow product was collected by centrifugation at 7000 rpm. The product was washed with 75:25 ratio of ethanol and deionized water mixture to remove the unreacted monomer and other impurities. The product is dried under vacuum and kept in a desiccator (see Scheme 1).

Close

Scheme 1

Synthesis of GaN/PEDOT–PPY.

Export to PPT

2.5 Instrumentation

UV–Visible absorption spectrum of the product was recorded from 200 to 800 nm using a Perkin–Elmer UV–Visible spectrophotometer. Infrared spectroscopic analysis in the range of 400–4000 cm⁻¹ was measured by using a Perkin–Elmer FTIR spectrophotometer. The X-ray diffraction (XRD) was carried out using a Rich Siefert 3000 diffractometer with Cu K_α1, radiation ( $\lambda$  = 1.5406 Å). The X-ray Photoelectron Spectroscopy (XPS) measurements were performed with Omicron nanotechnology; Germany XM 1000-monochromator with Al K_α radiation of 1483 eV operated at 300 W (20 mA emission current, 15 kV) and a base pressure of 5 × 10⁻⁵ mbar. The survey scan was performed with a step size of 0.5 eV along with 50 eV as the pass energy. The high resolution scan was done with 0.03 eV as the step size, and 20 eV as the pass energy with three sweep segments. The morphology and size of the sample were analyzed by FE-SEM and HR-TEM using a HITACHI SU6600 field emission-scanning electron microscopy and PHILIPS CM200 transmission electron microscopy. AC impedance measurements were carried out in the frequency range of 500 Hz–1000 MHz using Solatron-1260 impedance analyzer. The electrochemical experiments were performed on a CHI 1103A electrochemical instrument using the modified electrode and bare glassy carbon electrode (GCE, surface area, 0.0707 cm²) as working electrode; a platinum wire was the counter electrode, and saturated calomel electrode (SCE) was the reference electrode. The GaN/PEDOT–PPY nanocomposite modified GCE was fabricated by the following procedure. The 5 mg of nanocomposite was dispersed in 5 mL of acetone by ultrasonication. 1 μL of the dispersion was dropped onto a highly polished GCE and then dried at room temperature. The background electrolyte is phosphate buffer solution.

3.1 UV–Visible absorption spectroscopy

The UV–Visible absorption spectrum of PEDOT–PPY nanoparticles is shown in Fig. 1a. Three bands are observed at 498 nm, 661 nm and above 800 nm. The broadband at 498 nm was attributed to the π–π^∗ transition in PEDOT–PPY nanoparticles (Massonnet et al., 2015). The peak appearing at 661 nm corresponds to the formation of conducting phase of the transition of neutral or doped PEDOT–PPY. Band at inter band transition above 800 was attributed to the charge transition band (Kim et al., 2013). UV–Vis spectrum of GaN (Fig. 1b) nanoparticles shows a characteristic absorption band at 361 nm. Fig. 1c shows absorption spectrum of GaN/PEDOT–PPY nanocomposite which exhibits four characteristic bands at 373, 525, 702 and 863 nm. The absorption intensity of bands at 373 and 525 nm is increased, indicating the existence of interaction between GaN and PEDOT–PPY.

Close

Figure 1

The UV–Visible spectra of (a) PEDOT–PPY, (b) GaN and (c) GaN/PEDOT–PPY nanocomposite.

Export to PPT

3.2 FT-IR Spectroscopy

The FT-IR spectrum of PEDOT–PPY is shown in Fig. 2a. The peaks appear at 1440 cm⁻¹ and 1549 cm⁻¹ are due to the symmetric stretching of the thiophene ring and asymmetric stretching of C⚌C vibration of PEDOT. The peaks at 1368 cm⁻¹ and 1271 cm⁻¹ are due to C⚌C stretching and C—C stretching vibration of inter link band stretching. The peak at 1287 cm⁻¹ corresponds to the O⚌S⚌O stretching vibration corresponding to CSA doped with PEDOT–PPY. The 1184 cm⁻¹ and 1039 cm⁻¹ stretching is due to the C—O—C vibration of the ethylenedioxy group (Xiano et al., 2012). The spectrum corresponding to the copolymer shows many important peaks, and the first centered at 788 cm⁻¹ due to stretching of ⚌C—H (wagging), 919 cm⁻¹ due to vibration of C—C out plane, 1077 cm⁻¹ due to vibration of ⚌C—H plane, 1202 cm⁻¹ due to vibration of N—C bending, 1440 cm⁻¹ due to vibration of the PPY ring, 1549 cm⁻¹ due to vibration of C⚌C bonding stretching, and 1706 cm⁻¹ due to vibration of C⚌N bond stretching are observed (Buitrago et al., 2013; Martin, 2013). The peak at 3265 cm⁻¹ is due to N—H stretching vibration of PPY. Fig. 3b shows the FTIR spectrum of the gallium nitride nanoparticles, the band at 951 cm⁻¹ in gallium nitride attributed stretching mode. Fig. 3c shows FT-IR spectrum of GaN/PEDOT–PPY nanocomposite, and the following bands were observed at 1435, 1543, 1359, 1032 and 946 cm⁻¹. The vibration bands at 1435 cm⁻¹ and 1543 cm⁻¹ appeared as two sharp bands belong to the symmetry and asymmetric stretching vibration. These abovementioned peaks could be seen in the spectrum of the composite. The FTIR result confirms all the bands of composite and all the band stretching modes have been shifted to lower wave number in the composite because of strong interaction between gallium nitride and PEDOT–PPY nanocomposite. This suggests that gallium nitride was successfully deposited on the surface of the PEDOT–PPY.

Close

Figure 2

FT-IR spectra of (a) PEDOT–PPY, (b) GaN, and (c) GaN/PEDOT–PPY nanocomposite.

Export to PPT

Close

Figure 3

XRD of (a) PEDOT–PPY, (b) GaN, and (c) GaN/PEDOT–PPY nanocomposite.

Export to PPT

3.3 Micro Raman spectroscopy

The Raman spectra of GaN, PEDOT–PPY, and GaN/PEDOT–PPY nanocomposite are shown in Fig. S1(a–c). Fig. S1a shown in spectrum of PEDOT–PPY presents the following bands corresponding to the conducting phase of the polymer nanoparticles. The two characteristic bands at 1422 and 1560 cm⁻¹ are assigned to symmetric and asymmetric stretching PEDOT–PPY rings. The band at 1347 cm⁻¹ attributed to C—N⁺ stretching mode of the polaron radical cation. The presence of band at 1238 cm⁻¹ is assigned to SO₃ doped state of PEDOT–PPY nanoparticles and the band at 1042 cm⁻¹ can be attributed to C—H bending of PEDOT–PPY rings (De Kok et al., 2004). Fig. S1b shows the spectrum of GaN, and a prominent peak observed at 709 cm⁻¹ corresponds to the phonon mode of quartzite. The peak at 563 cm⁻¹ is attributed to the phonon mode of GaN. The observed broadness in the spectrum of GaN is due to the disorders in GaN (Tutuncu and Srivastava, 2000; Jung and Ki, 2012). The spectrum of GaN/PEDOT–PPY nanocomposite is shown in Fig. S1c. The two strong bands at 1331 cm⁻¹ are attributed to the symmetric vibration of C—N bond in PEDOT–PPY. The appearance of bands at 1558 and 1418 cm⁻¹ is attributed to stretching mode of symmetric and asymmetric ring bonds of PEDOT–PPY. The bands of GaN/PEDOT–PPY can be clearly observed in response to 696 and 850 cm⁻¹ in the presence of GaN. In fact, the stretching modes have been shifted to lower wave numbers in the composite because of strong interaction between GaN/PEDOT–PPY. The Raman spectrum of nanocomposite shows the bands attributed to GaN and PEDOT–PPY nanoparticles confirming the occurrence of all these components in the composite.

3.4 XRD analysis

XRD patterns of GaN, PEDOT–PPY and GaN/PEDOT–PPY nanocomposite are shown in Fig. 3a–c. Fig. 3a shows the XRD pattern of PEDOT–PPY nanoparticles, and a medium broad peak centered at 2θ-25° is observed, which is a characteristic of the diffraction by an amorphous PEDOT–PPY. The observed diffraction at 2θ peaks corresponds to the interchain planar-stacking distance. This indicates that efficient doping of CSA increases the order of PEDOT–PPY as well as the strength of molecular interaction between the stacked rings. It results in more ordered backbone chains of PEDOT–PPY along the axis which will enhance the conductivity of polymers nanoparticles. XRD analysis has been performed in order to examine the crystal structure and phase purity of nanostructure growth that was sintered at 900 °C for 9 h under super critical ammonia method. Fig. 3b shows XRD pattern of GaN, which exhibits hexagonal phase with reflections of 100, 002, 101, 102, 110, 103, 112 and the corresponding peaks were observed at 2θ = 32.49, 34.62, 36.82, 48.15, 57.69, and 19 respectively. The observed (1 0 0) and (1 0 1) planes act as basal plane for the hexagonal phase of gallium nitride (Kumar et al., 2014). These values are in good agreement with those listed in standard JCPDS no-89-8624. Although, the XRD pattern of GaN nanoparticles shows higher intensity and narrow full width at half maximum than others, Fig. 3c shows the XRD pattern of GaN/PEDOT–PPY nanocomposite, which exhibits the peaks corresponding to both PEDOT–PPY and GaN hexagonal phase reflections at 100, 002, 101, 102, 110, 103 and 112 planes with corresponding peaks at 2θ = 32.01, 34.25, 36.48, 47.68, 57.42, 63.35 and 68.94. It is to be noted that 100, 002, and 102 planes are slightly shifted and the intensity of the peaks is also lowered when compared to GaN nanoparticles and PEDOT–PPY nanoparticles. The XRD pattern of composite confirms the occurrence of both PEDOT–PPY and GaN. The GaN is an n-type semiconductor and PEDOT–PPY is a conducting p-type semiconductor. Thus the GaN/PEDOT–PPY nanocomposite exhibits p–n interaction which signifies the promising role of the present system. The organic and inorganic counterparts can be composite material with enhanced conductivity. Hence these can be effectively applied in innovative electrode materials for, photocatalyst, sensors and DSSC.

3.5 Morphology-formation mechanism of nanocomposite by FESEM and HRTEM analysis

The morphology of the as synthesized GaN/PEDOT–PPY nanocomposite was investigated by FESEM. Fig. 4(a–d) shows the FESEM images of the as prepared GaN/PEDOT–PPY composite with a well defined nanospike like structure. FESEM image reveals the uniform growth of GaN nanoparticles on the surface of PEDOT–PPY nanospike. The average length and diameter of nanocomposite are found to be in the range of 200 and 400 nm. The images of FESEM nanospike having clear fragment surface with an enlarged view are shown in the range of 200 nm. These images clearly indicate that donor–acceptor interaction has a considerable effect on the morphology of GaN/PEDOT–PPY nanocomposite. Further, HRTEM was used for more clear identification of the synthesized nanocomposite. Fig. 5(a–d) shows the HRTEM images of GaN/PEDOT–PPY, which reveals the clear control between the edges and the central part indicating the formation of GaN nanoparticles with the average size of 8 nm on the surface of PEDOT–PPY nanoparticles. The formation of GaN/PEDOT–PPY nanospike is possible through interaction of π–π^∗ stacking, electrostatic interaction of hydrogen bonds and donor–acceptor interaction. The use of low concentration of dopant of CSA affords the formation of nanospike with diameter as low as 20 nm and further ranging from 50 nm to 20 nm. The EDAX and SAED profile of the synthesized nanocomposite indicates the polycrystalline nature of the composite.

Close

Figure 4

FE-SEM image of the (a–d) GaN/PEDOT–PPY nanocomposite.

Export to PPT

Close

Figure 5

HR-TEM images (a–d) of GaN/PEDOT–PPY nanocomposite.

Export to PPT

3.6 XPS analysis

The binding energy of the as synthesized GaN/PEDOT–PPY nanocomposite was investigated by XPS. Fig. 6a shows the XPS survey spectrum of the as prepared GaN/PEDOT–PPY composite with a well defined survey spectrum of carbon, nitrogen, gallium, sulfur and oxygen. Fig. 6b shows the N core level spectrum of GaN/PEDOT–PPY by the appearance of peaks at 396.42, 399.58 and 400.0 eV. The peak at 396.42 eV is assigned to nitrogen binding energy of GaN, which confirms the formation of bond between gallium and nitrogen (Luo et al., 2013) and the peak at 399.58 eV is assigned for the binding energy of N—H. The peak appeared at 400 eV corresponds to the binding energy of N—C. The core level spectrum of Ga is shown in Fig. 6c. The peaks at 1118.72 and 1145.89 eV correspond to ²P_3/2 and ²P_1/2 of Ga (Wang et al., 2013). The core level spectrum of sulfur shown in Fig. 6d reveals that it has been successfully doped in PEDOT–PPY nanoparticles. The binding energy of sulfur at 163.80 and 168.85 eV corresponds to ²P_3/2 and ²P_1/2. Highest percent component is carbon which significantly originates from PEDOT–PPY backbone. The deconvoluted profile of XPS for carbon is shown in Fig. 6e. Three peaks are observed for the presence of C—C at 285.58 eV, for C⚌C at 284.86 and for C—N at 288.82 eV. Fig. 6f shows core level spectrum of oxygen, and the peak at 532.44 eV corresponds to ¹S.

Close

Figure 6

Xps core spectrum of (a) survey, (b) nitrogen, (c) gallium, (d) sulfur, (e) carbon and (f) oxygen in GaN/PEDOT–PPY nanocomposite.

Export to PPT

3.7 Photoluminescence spectroscopy

Fig. S2a–c shows the photoluminescence spectra of PEDOT–PPY (a), GaN (b) and GaN/PEDOT–PPY (c) nanocomposite. Fig. S2a PL reveals that the intensity of the PEDOT–PPY is lower than GaN. Fig. S2b shows the PL intensity of GaN nanoparticles is greater than PEDOT–PPY. Fig. S2c shows GaN/PEDOT–PPY nanocomposite is excitation wavelength of 373 nm due to the formation of more electrons and holes as compared to GaN, PEDOT–PPY. The presence of higher n-derivative quenches the luminescence of GaN and p-derivative quenches the luminescence of PEDOT–PPY nanoparticles. Mainly, two dominant peaks centered at 395 and 454 nm have been observed. The pronounced peak at 454 nm corresponds to the near band gap emission of GaN. The emission band at 454 nm is red shifted relative to that of GaN/PEDOT–PPY. This red shift peak may be due to the GaN/PEDOT–PPY nanocomposite where the carriers are confined leading to the decreased band gap. There was an oxidation of the surface leading to the formation of surface trap state that may also contribute this observed red shift. The causes for the emission of GaN/PEDOT–PPY are attributed to the donor–acceptor pair recombination.

3.8 Impedance spectroscopy

Electrochemical impedance spectroscopy is an evaluation of physical and interfacial properties of modified electrode. ESI measurements were carried out at GaN, PEDOT–PPY, and GaN/PEDOT–PPY nanocomposite and the result is shown in Fig. 7(a–c). The supporting electrolyte used was 0.1 M KCl and 5 mm Fe [CN]₆^3−/4−. The measurements were taken in the frequency range of 100–1000 kHz. Generally a semi-circle portion observed at higher frequencies in a plot corresponds to electron-transfer-limited process and diameter of the semicircle corresponds to interfacial electron-transfer resistance. Linear part of the spectrum is characteristic of lower frequency range and represents diffusion limited process .On the other hand, the plot of PEDOT–PPY(a), GaN(b) and GaN/PEDOT–PPY(c) nanocomposite exhibited much depressed semicircles. Their Ret values are 2794, 2181 and 678 Ω’ respectively. Among all the abovementioned smallest semicircles diameter with much lower Ret value has been observed at the composite which ultimately reveals a radical electron transfer process. Thus incorporation of GaN with PEDOT–PPY nanoparticles leads to a good conductivity of rapid electron shutting between the composite and underlying electrode surface.

Close

Figure 7

Impedance spectra of (a) PEDOT–PPY, (b) GaN, and (c) GaN/PEDOT–PPY nanocomposite.

Export to PPT

3.9 Thermo gravimetric analysis (TGA) and DTA

A comparative differential thermal analysis curve of pure GaN (a), PEDOT–PPY (b) and GaN/PEDOT–PPY (c) nanocomposite is shown in Fig. 8(a–c). Fig. 8a shows the TGA curve of PEDOT–PPY by the weight loss of three peaks at 100, 245 and 600 °C. The weight loss of the first step 100 °C, is due to the loss of water molecules from solvent by the decomposition of PEDOT–PPY. The second step 245 °C is due to loss of sulfur molecules from PEDOT–PPY. The third step 600 °C is due to loss of carbons from PEDOT–PPY. Fig. 8b shows the TGA curve of GaN by the weight loss of one peak at 500 °C is due to decomposition of GaN. Fig. 8c shows the TGA curve of GaN/PEDOT–PPY nanocomposite by weight loss of three peaks at 100, 200 and 460 °C. The weight loss of the first step 100 °C, is due to the loss of water molecules from solvent by the decomposition of GaN/PEDOT–PPY. The second step 240 °C is due to loss of sulfur and carbon from PEDOT–PPY. The third step at 550 °C is due to the decomposition of GaN. The weight loss continues to decrease slowly, the curve does not show a specific ending and this could be credited to the subsequent decomposition of GaN chased by a phase transformation. This is confirmed by the TGA curves of GaN/PEDOT–PPY nanocomposite.

Close

Figure 8

TGA of (a) PEDOT–PPY, (b) GaN, and (c) GaN/PEDOT–PPY nanocomposite.

Export to PPT

Comparative differential thermal analysis curves of PEDOT–PPY (a), GaN (b) and GaN/PEDOT–PPY (c) nanocomposite are shown in Fig. S3(a–c). Fig. S3a shows the PEDOT–PPY is bond breaking rewarded by the generated heat from the exothermic peaks at 127, 156, 297, 300, 410, 524 and 620 °C. The DTA curve shows the PEDOT–PPY total decomposition in air at high temperature. Fig. S3b shows the GaN is not formation of endothermic and exothermic bond. Finally Fig. S3c shows the GaN/PEDOT–PPY nanocomposite generated heat from the bond formation, thus showing exothermic peaks at 112, 307, 502 and 610 °C. The great shift of these peaks to higher temperatures indicates an interaction between GaN/PEDOT–PPY nanocomposites.

3.10 Electrochemical catalytic property

3.10.1

3.10.1 Electrochemical sensing of mebendazole by GaN/PEDOT–PPY

The electrocatalytic response to the oxidation of mebendazole was detected via cyclic voltammetry based on their good biocompatibility, high conductivity and excellent electrochemical behavior, and GaN/PEDOT–PPY nanocomposites were immobilized onto the surface of GCE to be developed as a sensor and applied for the detection of mebendazole. In order to study the effect of pH on sensor response to the solutions of 1 mM mebendazole with different pH varying from 2 to 7 were analyzed by CV for GaN/PEDOT–PPY modified GCE and the result is shown in Fig. S4. It has been observed that oxidation of mebendazole has very low potential with increased peak current at pH 2. The effect on these observations, the phosphate buffer solution with (pH 2) was chosen as the optimum supporting electrolyte and used in further studies.

Fig. 9 depicts the cyclic voltammograms in 1 mM mebendazole in 0.1 M PBS (pH 2) recorded at 50 mV/s: (a) PBS (pH 2), (b) Mebendazole, (c) GaN/PEDOT–PPY/GCE. Fig. 9 shows CV of GaN/PEDOT–PPY/GCE in the presence of 1 mM mebendazole in 0.1 M PBS buffer (pH 2) at scan rate 50 mV/s. It shows an irreversible behavior of mebendazole at bare GCE with the anodic peak potential at E_pa = 1.06 V, with an anodic peaks current I_pc = 4.41 μA. The oxidation of mebendazole is bare GCE which is generally believed to be totally irreversible and requires high over potential. However the modified GaN/PEDOT–PPY/GCE exhibits well-known oxidized mebendazole anodic peak potential at E_pa = 1.12 V_, with anodic peak current I_pa = 5.79 μA, and the oxidation peak current is enhanced compared to bare mebendazole/GCE with positive shift of 0.6 mV. The result suggests that the GaN/PEDOT–PPY/GCE has improved the electron transfer kinetics due to mebendazole.

Close

Figure 9

Cyclic voltammograms of (a) bare/GCE, (b) mebendazole/GCE and (c) GaN/PEDOT–PPY/GCE in 1 mM mebendazole (0.1 M PBS, pH = 2).

Export to PPT

3.10.2

3.10.2 Scan rate effect

Fig. 10 shows the differential pulse voltammograms of mebendazole at GaN/PEDOT–PPY/GCE in scan rate 50–130 mV/s. A consecutive scan rate of mebendazole produces a significant increase in the current. The calibration plots were found to be linear and the correlation equation of I (μA) = 2.513 mV/s + 0.013 (R² = 0.989). The calibration plot of GaN/PEDOT–PPY/GCE is shown as inset in Fig. 10b. The linear response is in the range from 50 to 130 mV/s. On the basis of our results, we expect that it is possible to use the GaN/PEDOT–PPY which have incredible advantages for constructing electrochemical sensor toward the mebendazole. Hence, GaN/PEDOT–PPY nanocomposite modified GCE has improved the electron transfer kinetics. The anodic peak potential shifted to positive direction slightly indicates the electrocatalytic ability surface area of GaN/PEDOT–PPY nanocomposite modified GCE for the detection of mebendazole. The number of electron transfer of mebendazole was detected via equation. $$I=\mathit{nFQV}/4\mathit{RT}$$ where

Close

Figure 10

Cyclic voltammograms of scan rate 50–130 mV/s GaN/PEDOT–PPY/GCE in 1 mM mebendazole (0.1 M PBS, pH 2).

Export to PPT

• n – Number electron involved mebendazole,

• F – Faraday constant, Q – Charge Transfer,

• V – Scan rate (mV/s)

• R – Gas constant

• T – Temperature −29 °C.

The result suggests that the GaN/PEDOT–PPY/GCE has two electron (n) transfer of kinetics due to mebendazole.

3.10.3

3.10.3 The differential pulse voltammetry

Fig. 11 shows the differential pulse voltammograms of mebendazole at GaN/PEDOT–PPY/GCE in different concentration. A consecutive addition of mebendazole to 0.1 M PBS (pH 2) produces a significant increase in the current with slight shift in peak potential. The calibration plots were found to be linear and the correlation equation of I (μA) = 0.699 + 0.059 c (R² = 0.997). The calibration plot of GaN/PEDOT–PPY/GCE is shown as inset in Fig. 11. The linear response in the range from 0.33 × 10⁻⁶ to 5.63 × 10⁻⁶M corresponding to a sensitivity of 0.059 μA/mM shows that the GaN/PEDOT–PPY/GCE is sensitive toward the mebendazole. The limit of detection is LOD = 3σ/s and LOQ = 10σ/s. where, σ is the standard deviation of the eight blanks, and S is the sensitivity. The LOD is found to be 2.44 × 10⁻⁶ M and LOQ = 16.16 × 10⁻⁶ M. On the basis of our results, we expect that it is possible to use the GaN/PEDOT–PPY which have incredible advantages for constructing electrochemical sensor toward such other analytes.

Close

Figure 11

Differential pulse voltammograms for concentrations (0.33 × 10⁻⁶–5.63 × 10⁻⁶ M) of mebendazole at GaN/PEDOT–PPY/GCE. Inset-Calibration plot.

Export to PPT

3.11 Photocatalytic degradation of Cresol red (CR)

Photocatalytic activity of the GaN/PEDOT–PPY was examined by using degradation of Cresol red (CR) as the model organic pollutant in Fig. S5 shows the absorption spectrum of 5 × 10⁻⁵ M CR solution during different time intervals in presence of GaN/PEDOT–PPY nanocomposite respectively. The UV–Visible light irradiation of aqueous dye solution in presence of GaN/PEDOT–PPY nanocomposite showed decrease in absorption maximum with shift in absorption maximum (λ = 434 nm). It suggests that the complete decolorization of CR solution was purely due to the photocatalytic degradation ability of GaN/PEDOT–PPY nanocomposite. Further, absorbance of GaN/PEDOT–PPY nanocomposite showed a maximum value of 0.77 before irradiation and decreased to a value after 180 min. It should be mentioned that the continued irradiation of visible light for another 180 min did not give any decrease in absorbance at 0.07 nm. Therefore, the photocatalytic efficiency and the rate constant values were calculated with that GaN/PEDOT–PPY value of absorbance and the degradation time of 180 min. It can be seen that the GaN/PEDOT–PPY nanocomposite exhibits maximum efficiency toward the photocatalytic degradation of CR.

3.11.1

3.11.1 Photodegradation mechanism of dye in GaN/PEDOT–PPY nanocomposite

From the obtained results, it can be observed that the degradation efficiency of GaN/PEDOT–PPY is higher than the GaN. When PEDOT–PPY was composited with GaN, the interface between the two phases may act as a rapid separation site for the photogenerated electrons and holes due to the difference in the energy level of their CB and VB. When GaN/PEDOT–PPY was irradiated with UV–Visible light, electrons in valence bands of GaN/PEDOT–PPY can be excited to their CB and left holes at their surface. Because the CB of GaN is higher than that of PEDOT–PPY, electrons in the conduction band of GaN are easily moved into the conduction band of PEDOT–PPY, and then the holes in the GaN are filled with the redox couples, which is advantageous for the efficient charge carrier separation. Moreover, GaN was coated with PEDOT–PPY partly; both GaN and PEDOT–PPY can contact the solution directly. The photogenerated holes in the GaN can be transferred up to the interface between PEDOT–PPY with the solution or penetrate the PEDOT–PPY; and then arrive up to the interface between GaN and the solution. Due to their strong oxidation ability, these photogenerated holes react with water to form OH radical. Similarly, photogenerated electrons were accepted by O₂ to form superoxide radical. In the case, the dye molecules are attacked by hydroxyl radicals and generate organic radicals or other intermediates (Scheme 2).

Close

Scheme 2

Photodegradation mechanism of dye in GaN/PEDOT–PPY nanocomposite.

Export to PPT

The dye adsorbed on the surface of photocatalyst can only be degraded photocatalytically which is due to the shorter lifetime of photoexcited electrons. The presence of GaN/PEDOT–PPY nanocomposite will favor the interaction of the dye (adsorption) on the surface of the samples and then undergo degradation during UV–Visible light irradiation. In this way, the GaN/PEDOT–PPY nanocomposite samples exhibit enhanced photodegradation to some extent. $$\text{GaN}/\text{PEDOT}''–''\text{PPY}+h\nu \to {\text{GaN}}^{+}/\text{PEDOT}''–''\text{PPY}+{\text{e}}_{\text{CB}}^{-}$$ $${\text{GaN}}^{+}/\text{PEDOT}''–''\text{PPY}\to \text{GaN}/\text{PEDOT}''–''\text{PPY}+h\nu {\text{B}}^{+}$$ $${\text{e}}_{\text{CB}}^{-}+{\text{O}}_2\to {\text{O}}_2^{\text{•}-}$$ $$h\nu {\text{B}}^{+}+{\text{H}}_2\text{O}\to ({\text{H}}^{+}+{\text{OH}}^{-})\to {\text{H}}^{+}+{\text{OH}}^{\text{•}}$$ $$\text{Dye}+{\text{OH}}^{\text{•}}\to \text{oxidation products.}$$