CHAPTER 3. PHOTOCATALYTIC DEGRADATION OF METHYLENE BLUE AND ACID RED 18 DYES BY Bi-Au-ZnO

Transcription

1 CHAPTER 3 PHOTOCATALYTIC DEGRADATION OF METHYLENE BLUE AND ACID RED 18 DYES BY Bi-Au-ZnO In this chapter, characterization of Bi-Au-ZnO and its photocatalytic activity on the degradation of Methylene Blue (MB) and Acid Red 18 (AR 18) under various conditions have been discussed. Structure and UV spectra of the dyes are given in Figs. 3.1 and 3.2. Analytical wavelength used for concentration measurements is given in figure caption. 1.5 Absorbance 1.0 Absorption maxima 293 and 663 nm Wavelength (nm) Fig Structure of MB and its UV spectrum: Analytical wavelength λ max = 293 nm

2 HO NaO 3 S N N NaO 3 S Absorbance 0.8 SO 3 Na Absorption maxima 246 and Wavelength (nm) Fig Structure of AR 18 and its UV spectrum: Analytical wavelength λ max = PHOTOCATALYTIC DEGRADATION OF MB AND AR 18 WITH Bi-Au-ZnO CATALYSTS In this section, photocatalytic activities of Bi-Au-ZnO catalysts with different Bi content on the degradation of MB and AR 18 using UV and solar light have been tested to find out the optimum concentration of Bi. Bi-Au-ZnO was prepared by precipitationdecomposition method (section 2.2.1) With UV light The photodegradation efficiencies of the Bi-Au-ZnO catalysts with 1, 2, 3, 4 and 5 wt % of Bi loading were evaluated by the

3 40 degradation of MB and AR 18 under UV light and the results are given in Table As the concentration of Bi is increased from 1 to 4 wt %, the pseudo-first-order rate constants of degradation increases from to min 1 in MB degradation and to min 1 in AR 18 degradation, respectively. Catalyst loaded with 4 wt % of Bi shows a higher degradation in both the dyes. Further increase of Bi (5 wt %) content decreases the rate constant of degradation. Hence, 4 wt % of Bi was taken as the optimum concentration of Bi on Au-ZnO. Table Rate constants with different wt % Bi on Au-ZnO for the degradation of MB and AR 18 under UV light Wt % of Bi Rate constants (k) min -1 MB AR [MB] = M, catalyst suspended = 4 g L 1, ph = 7, irradiation time = 20 min, [AR 18] = M, catalyst suspended = 4 g L 1, ph = 11, irradiation time = 90 min, airflow rate = 8.1 ml s -1, IUV = einstein L 1 s With solar light The photocatalytic activities of the Bi-Au-ZnO catalysts with 1, 2, 3, 4 and 5 wt % of Bi loading were evaluated by the degradation of MB and AR 18 under solar light and the results are given in Table The trend is similar to that of UV degradation. Catalyst loaded with 4 wt % of Bi shows a higher degradation in both the dyes. Hence, 4 wt % of Bi in the catalyst was found to be the optimum concentration.

4 41 Table Rate constants with different wt % Bi on Au-ZnO for the degradation of MB and AR 18 under solar light Wt % of Bi Rate constants (k) min -1 MB AR [MB] = M, catalyst suspended = 4 g L 1, ph = 7, irradiation time = 40 min [AR 18] = M, catalyst suspended = 4 g L 1, ph = 11, irradiation time = 120 min, airflow rate = 8.1 ml s -1, Isolar = ( ) ± 100 lux. Since 4 wt % Bi-Au-ZnO was found to be most efficient in UV and solar light for the degradation of both dyes, this catalyst was further characterized CHARACTERIZATION OF 4 wt % Bi-Au-ZnO The catalyst 4 wt % Bi-Au-ZnO was characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) surface area measurements, energy dispersive spectroscopy (EDS), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), diffuse reflectance spectra (DRS), photoluminescence (PL) spectra and X-ray photoelectron spectroscopy (XPS) XRD analysis XRD patterns of the bare ZnO, Bi-Au-ZnO and Au-ZnO are shown in Fig The diffraction peaks of bare ZnO (Figure 3.2.1a) at 31.68, 34.36, and 56.56, correspond to (100), (002), (101) and (110) planes of wurtzite ZnO (JCPDS

5 ). The diffraction pattern of Bi-Au-ZnO is different from that of bare ZnO as shown in Fig b. In the Bi-Au-ZnO system (Fig. 1b), there are two new peaks at 2θ values of 27.8 and 31.5, corresponding to Bi in the form of Bi2O3 (Cao et al., 2014). This confirms the loading of Bi. Due to very low concentration Au could not be detected by XRD (Fig c). To confirm the loading of Au, wt % Au is increased from 1 to 5 wt % and its XRD is given as Fig d. In addition, the weak characteristic peaks of Au heterostructure corresponding to face centered- cubic (fcc) metallic Au were noticed at 2 = 38.2, 44.4, and Such results indicate that the extent of Au on the ZnO surface is relatively scarce (Pei-Kuan Chen et al., 2012). However, EDS shows the presence of Au in the catalyst. If the gold and bismuth are substituted in place of Zn, a corresponding peak shift is expected in XRD. Lack of such shifts in the XRD of Bi-Au-ZnO indicates the presence of Au and Bi on the surface of ZnO. In addition, the doping possibility of both metals is unlikely because of the difference in their ionic radii (Zn 2+ - (0.72 Å), Au + - (0.85 Å) and Bi 3+ - (1.03 Å)). Broadening of Bi-Au- ZnO peaks indicates that the reduction of size of the particle when compared to bare ZnO. The crystallite sizes of bare ZnO and Bi-Au- ZnO were determined using Debye-Scherrer equation (eqn 3.1). D K cos. (3.1) where D is the crystal size of the catalyst, K is dimensionless constant (0.9), is the wavelength of X-ray, is the full width at half-maximum (FWHM) of the diffraction peak and is the diffraction angle. The average crystalline size of Bi-Au-ZnO is found to be 3.9 nm which is less than the size of bare ZnO (4.2 nm).

6 BET surface area analysis In general the surface area of the catalyst is the most important factor influencing the catalytic activity. The surface area of Bi-Au-ZnO was determined using the nitrogen gas adsorption method. N2 adsorption desorption isotherms of bare ZnO and Bi-Au-ZnO are presented in Figs 3.2.2a and b, respectively. The isotherms of bare ZnO and Bi-Au-ZnO reveal type II hysteresis loop. The pore size distribution of the bare ZnO and Bi-Au-ZnO are given in inset of Figs a and b, respectively. The BET surface and pore volume of bare ZnO and Bi-Au-ZnO are given in Table BET surface area of Bi-Au-ZnO (28.2 m 2 g 1 ) is higher than bare ZnO (14.9 m 2 g 1 ). Table Surface properties of bare ZnO and Bi-Au-ZnO Properties Bare ZnO Bi-Au-ZnO BET surface area 14.9 (m 2 g -1 ) 28.2 (m 2 g -1 ) Total pore volume (single point) 0.12 (cm 3 g -1 ) 0.25 (cm 3 g -1 ) EDS analysis The EDS of Bi-Au-ZnO is displayed in Fig It shows the presence of Bi, Au, O and Zn in the catalyst FE-SEM analysis The texture and morphology of bare ZnO and Bi-Au-ZnO samples are very important parameters and might influence the photocatalytic activity. FE-SEM images of bare ZnO (Fig a), and Bi-Au-ZnO (Figs b-e) are shown in Fig The FE- SEM images of Bi-Au-ZnO at two different magnifications in two different locations are given in Figs 3.2.4b-e.

7 44 (d) Au 0 (c) Bi3+ (002) (100) (101) (a) (102) (110) (103) (200) (112) (201) (004) (202) Counts (a.u) (b) Position [ 2Theta] Fig XRD patterns of a) bare ZnO, b) Bi-Au-ZnO c) 1 wt % Au-ZnO and d) 5 wt % Au-ZnO

8 45 Quantity adsorbed (cm 2 /g STP) Pore volume (cm 3 /g) Pore radius (A ) Adsorption Desorption (b) Relative pressure (P/P 0 ) 1.0 Quantity adsorbed (cm 2 /g STP) Pore volume (cm 3 /g) Pore radius (A ) Adsorption Desorption (a) Relative pressure (P/P 0 ) 1.0 Fig N 2 adsorption-desorption isotherms and BJH pore size distribution (inset) of a) bare ZnO and b) Bi-Au-ZnO

9 Fig EDS of Bi-Au-ZnO 46

10 47 At higher magnifications, nanochain structure of Bi-Au-ZnO is clearly seen (Figs d and e). Moreover structure of ZnO has roughly spherical and hexagonal shape in all cases TEM analysis The surface morphology of Bi-Au-ZnO has been analyzed by TEM images. Fig shows the TEM images of bare ZnO, (Fig a) and Bi-Au-ZnO (Figs b-e) with different magnifications at different location. TEM images of bare ZnO and Bi-Au-ZnO (Fig ) exhibit Hexagonal huddle as well as chain like structures. Moreover, the distinction of Au and Bi in Bi-Au- ZnO was impossible due to low concentration of both species DRS analysis The diffuse reflectance spectra of ZnO and Bi-Au-ZnO are depicted in Fig.3.2.6a and Fig b, and it reveals that the bare ZnO and Bi-Au-ZnO displayed no significant absorbance change in UV range. But in visible region (400 to 800 nm) Bi-Au-ZnO has higher absorbance when compared to the bare ZnO Kubelka-Munk function In addition, UV-vis spectra in the diffuse reflectance mode (R) were transformed to the Kubelka-Munk function F(R) to separate the extent of light absorption from scattering. The band gap energy was obtained from the plot of the modified Kubelka-Munk function (F(R)E) 1/2 versus the energy of the absorbed light E (eqn 3.2) 1/2 (1-R) 2 F (R) E 1/2 = 2R x hv (3.2)

11 48 (a) (b) (c) (d) (e) Fig FE-SEM images of a) bare ZnO (100 nm), b) Bi-Au-ZnO (1 m), c) Bi-Au-ZnO (1 m), d) Bi-Au-ZnO (100 nm) and e) Bi-AuZnO (100 nm).

12 49 (a) (b) (c) (d) (e) Fig TEM images of a) bare ZnO (200 k), b) Bi-Au-ZnO (50 k), c) Bi -Au-ZnO (100 k), d) Bi-Au-ZnO (400 k) and e) Bi-Au-ZnO (400 k).

13 50 % of reflectance (a) (b) Wavelength (nm) Fig DRS of a) bare ZnO and b) Bi-Au -ZnO

14 51 The band gap energies of bare ZnO and Bi-Au-ZnO are found to be 3.13 and 2.90 ev, respectively (Fig.3.2.7) PL spectral analysis Photoluminescence spectra of bare ZnO and Bi-Au-ZnO are shown in Figs a and 3.2.8b, respectively. As the photoluminescence occurs due to electron-hole recombination, its intensity is directly proportional to the rate of electron-hole recombination. The bare ZnO gave two emissions at 420 and 500 nm. The doping of Bi and Au with ZnO do not shift the emission wavelength of ZnO but the intensity of PL emission is less when compared to bare ZnO. This is because of suppression of recombination of electron-hole pairs by loaded Bi and Au, which enhanced the photocatalytic activity of the catalyst. The photocatalytic activities of the materials are governed by the efficiency of the separation of the photogenerated electrons and holes (Wang et al., 2011) XPS analysis In order to know the chemical state of Bi and Au present in this catalyst, the XPS of this sample was taken. The survey spectrum (Fig a) of the Bi-Au-ZnO indicates the peaks of elements Zn, O, Au and Bi. The carbon peak is attributed to the residual carbon from the sample and adventitious hydrocarbon from XPS instrument itself and is not indicated. Figs (b-e) show the binding energy peaks of Bi, Au, O and Zn, respectively. The binding energy peaks of Bi at and ev (Fig b) correspond to 4f7/2 and 4f5/2 of Bi 3+, respectively (Hai-Ying Jiang et al., 2012). Fig c shows the binding energies of Au 4f7/2 (89.2 ev) and Au 4f5/2 (92.2 ev) and these binding energies indicate that gold is present in metallic state (Zhang et al., (2012).

15 52 6 (b) K-M Photon energy (ev) 6 7 (a) 5 K-M Photon energy (ev) Fig Plot of transferred Kubelka-Munk function versus energy of the light absorbed for a) bare ZnO and b) Bi-Au-ZnO.

16 PL intensity (a) (b) Wavelength (nm) Fig Photoluminescence spectra of a) bare ZnO and b) Bi-Au-ZnO

17 54 In Fig d, the O1s peak at ev indicates oxygen species in the sample (Ren et al., 2010). The peaks appearing in Fig e centered at and ev are attributed to the Zn 2p3/2 and Zn 2p1/2, respectively (Krishnakumar et al., 2012) PHOTODEGRADATION OF MB UNDER UV LIGHT Primary analysis The photodegradability of MB with different photocatalysts under UV light irradiation is shown in Fig Almost complete degradation of the dye takes place at the time of 40 min with Bi-Au-ZnO (curve a) under UV light. 8.7% decrease in dye concentration occurred for the same experiment performed with Bi-Au-ZnO in the absence of UV light (curve b). This may be due to adsorption of the dye on the surface of the catalyst. Dye is resistant to self photolysis (curve c). By these observations, we can say that both UV light and catalyst are needed for effective degradation of MB dye. When Bi-ZnO, Au-ZnO, bare ZnO, commercial ZnO and TiO2-P25 were used under same conditions 76.2 (curve d), 80.2 (curve e), 74.0 (curve f), 77.2 (curve g) and 49.5 (curve h) percentages of degradation occurred, respectively. This shows that Bi-Au-ZnO is more efficient in MB degradation than other catalysts Effect of solution ph The ph is an important parameter in photocatalytic degradation as it determines the surface charge properties of ZnO, the size of aggregates formed, the charge of dye molecules, adsorption of dye onto catalyst surface and the concentration of hydroxyl radicals (Sakthivel et al., 2003; Aguedach et al., 2005,

18 55 Intensity (cps) Au4f 92.2 Au4f 89.2 Bi4f Bi4f O1S Zn2P Zn2P (a) Binding energy (ev) Intensity (a.u) Bi 4f (b) Intensity (a.u) Au 4f (c) Binding energy (ev) Binding energy (ev) Intensity (a.u) O 1S (d) Intensity (a.u) Zn 2P (e) Binding energy (ev) Binding energy (ev) Fig XPS of Bi-Au-ZnO a) survey spectrum, b) Bi4f peak, c) Au4f peak, d) O1s peak and e) Zn2p peak.

19 (c) (b) 80 % of MB remaining (g) (d) (h) (f) (e) 0 Bi-Au-ZnO (a) Bi-ZnO (d) Time (min) Commercial ZnO (g) Bi-Au-ZnO/dark (b) Without catalysts (c) Au-ZnO (e) Bare ZnO (f) TiO 2 -P25 (h) Fig Photodegradability of MB: [MB] = M, ph = 7, catalyst suspended = 4 g L 1, airflow rate = 8.1 ml s 1, I UV = einstein L 1 s 1. (a)

20 57 Krishnakumar et al., 2012). The wastewater from textile industries usually has a wide range of ph. Fig shows MB degradation as a function of irradiation time under different ph. The pseudo-first order rate constants for Bi-Au-ZnO at ph 3, 5, 7 and 9 are , , and min -1, respectively. It is observed that the increase in ph from 3 increases the removal efficiency of MB up to ph 7 and then decreases. The optimum ph for efficient MB removal on Bi-Au-ZnO is 7. At acidic ph range the removal efficiency is less and it is due to the dissolution of ZnO. ZnO can react with acids to produce the corresponding salt at low acidic ph values. At high ph value Bi-Au-ZnO surface is negatively charged by means of adsorbed OH ions, which may decreases the adsorption of anionic dye molecules. Degaradation efficiency of a catalyst depends on the adsorption of dye molecules. An experiment to verify dark adsorption of MB under different ph was carried out. The percentages of adsorption at ph 3, 5, 7 and 9 were found to be 7.6, 7.8, 8.7 and 8.1% after the attainment of adsorption equilibrium (30 min). The adsorption is high at ph 7 and hence, the degradation is most efficient at this ph Effect of catalyst loading Catalyst loading in slurry photocatalytic processes is an important factor that can strongly influence the dye degradation. Experiments performed with different amounts of Bi-Au-ZnO shows that the photodegradation efficiency increases with an increase the catalyst amount up to 4 g L 1 and then decreases as shown in Fig The pseudo-first order rate constants are , , , and min -1 for Bi-Au-ZnO at catalyst loading of 1, 2, 3, 4 and 5 g L 1, respectively. The total active surface area increases with increasing catalyst dosage. But with excess dosage there is a decrease in light penetration due to increased light

21 (k) min Initial ph Fig Effect of solution ph: [MB] = M, Bi-Au-ZnO suspended = 4 g L 1, airflow rate = 8.1 ml s 1, I UV = einstein L 1 s 1, irradiation time = 20 min.

22 (k) min Catalyst loading (g L -1 ) Fig Effect of catalyst loading: [MB] = M, ph = 7, airflow rate = 8.1 ml s 1, I UV = einstein L 1 s 1, irradiation time = 20 min.

23 60 scattering effect by catalyst particles (Herramann, 1999; Kartal et al., 2001, Velmurugan et al., 2011). Additionally, it is important to keep the treatment expenses low for industrial use. So we use 4 g L 1 as the optimal catalyst amount in our work Effect of initial dye concentration It is important from an application point of view to study the dependence of degradation on the initial concentration of dyes. Fig shows that the increase of dye concentration from 1 to M decreases the rate constant from to min -1. The rate of degradation relates to the OH (hydroxyl radical) formation on catalyst surface and probability of OH reacting with dye molecule. As the initial concentration of the dye increases, the path length of the photons entering the solution decreases. Thus the photocatalytic degradation efficiency decreases (Krishnakumar et al., 2011; Jothivel et al., 2011). At low concentration the reverse effect is observed, thereby increasing the photon absorption by the catalyst. The large amount of adsorbed dye may also have a competing effect on the adsorption of oxygen and OH onto the surface of catalyst Reusability of the catalyst The reusability of Bi-Au-ZnO catalyst was investigated by the repeated use of the catalyst, after the each run, the catalyst was separated by centrifugation, washed with distilled water and dried at C for 12 h. The catalyst was efficient at first run for MB degradation; however, the catalyst lost its efficiency at second run for the same dye degradation. This may be due to strong adsorption of dye blocking the active sites of the catalyst. Hence this catalyst could not be reused. But this dye adsorbed catalyst can be used as a visible active catalyst for the degradation of other toxic organic compounds as reported earlier (Chatterjee and Mahata, 2001).

24 (k) min Initial dye concentration ( 10-4 M) Fig Effect of intial dye concentration: Bi-Au -ZnO suspended = 4 g L -1, ph = 7, airflow rate = 8.1 ml s 1, I UV = einstein L 1 s 1, irradiation time = 20 min.

25 PHOTODEGRADATION OF MB WITH SOLAR LIGHT Primary analysis The photodegradability of MB with the different photocatalysts under solar light irradiation is shown in Fig Almost complete degradation of the dye takes place at the time of 60 min with Bi-Au-ZnO (curve a) under solar light. 8.7% decrease in dye concentration occurred for the same experiment performed with Bi-Au-ZnO in the absence of solar light (curve b). Dye is resistant to self photolysis (curve c). These observations reveal that both solar light and catalyst are needed for effective degradation of MB. When the photocatalysts of Bi-ZnO, Au-ZnO, bare ZnO, commercial ZnO and TiO2-P25 were used under the same conditions, 93.4 (curve d), 94.6 (curve e), 91.1 (curve f), 87.3 (curve g) and 86.8 (curve h) percentages of degradations occurred, respectively. This shows that Bi-Au-ZnO is more efficient than other catalysts in MB degradation. Since the degradation was highly effective with Bi-Au-ZnO, the influence of operational parameters was investigated to identify the optimum conditions Effect of solution ph The effect of ph on the photodegradation of MB was studied in the ph range of 3 to 9. The pseudo-first order rate constants were , , and min 1 at ph 3, 5, 7 and 9, respectively. Degradation efficiency increases upto ph 7 and then decreases (Fig ). It is found that the optimum ph for efficient MB removal is 7. Reason for this effect has been discussed earlier (section 3.3.2).

26 (c) (b) 80 % of MB remaining (f) (d) (h) (e) (g) 0 (a) Time (min) 80 Bi-Au-ZnO (a) Bi-ZnO (d) Commerical ZnO (g) Bi-Au-ZnO/dark (b) Without catalysts (c) Au-ZnO (e) Bare ZnO (f) TiO 2 -P25 (h) Fig Photodegradability of MB: [MB] = M, ph = 7, catalyst suspended = 4 g L 1, airflow rate = 8.1 ml s 1, I solar = ( ) ± 100 lux,

27 (k) min Initial ph Fig Effect of solution ph: [MB] = M, Bi-Au-ZnO suspended = 4 g L 1, airflow rate = 8.1 ml s 1, I solar = ( ) ± 100 lux, irradiation time = 40 min.

28 Effect of catalyst loading The influence of the photocatalyst dosage on the degradation of MB has been investigated by employing different amounts of Bi-Au-ZnO. The results are presented in Fig The increase of catalyst dosage from 1 to 4 g L -1 increases the degradation rate appreciably and further increase of catalyst dosage above 4 g L -1, decreases the degradation rate. The pseudo-first order rate constant values for 1, 2, 3, 4 and 5 g L -1 of Bi-Au-ZnO are , , , and min -1, respectively. Hence, 4 g L -1 of Bi-Au-ZnO catalyst amount is found to be the optimum dosage. Reason for this effect has been discussed earlier (section 3.3.3) Effect of initial dye concentration Fig shows that the increase of dye concentration from 1 to M decreases the rate constant from to min -1. Reason for this effect has been discussed earlier (section 3.3.4) Reusability of the catalyst The reusability of Bi-Au-ZnO catalyst was investigated by the repeated use of the catalyst, after the each run, the catalyst was separated by centrifugation, washed with distilled water and dried at 110 C for 12 h. The catalyst was efficient at first run for MB degradation; however, the catalyst lost its efficiency at second run for the same dye degradation under solar light. Reason for this effect has been discussed earlier in sec PHOTODEGRADATION OF AR 18 WITH UV LIGHT Primary analysis The photodegradability of AR 18 with different photocatalysts under UV light irradiation is shown in Fig Almost complete degradation of the dye takes place at a time of 120 min with Bi-Au-ZnO (curve a) under UV light. 1.7% decrease in dye concentration occurred due to adsorption for the same experiment performed with Bi-Au-ZnO in the absence of UV light (curve b).

29 (k) min Catalyst loading (g L -1 ) Fig Effect of catalyst loading: [MB] = M, ph = 7, airflow rate = 8.1 ml s 1, I solar = ( ) ± 100 lux, irradiation time = 40 min.

30 (k) min Initial dye concentration ( 10-4 M) Fig Effect of initial dye concentration: Bi-Au-ZnO suspended = 4 g L -1, ph = 7, airflow rate = 8.1 ml s 1, I solar = ( ) ± 100 lux, irradiation time = 40 min.

31 68 Dye is resistant to self photolysis (curve c). By these observations, we can say that both UV light and catalyst are needed for effective degradation of the AR 18. When Bi-ZnO, Au-ZnO, bare ZnO, commercial ZnO and TiO2-P25, were used under the same conditions, 49.7 (curve d), 91.6 (curve e), 86.3 (curve f), 97.4 (curve g) and 87.0 (curve h) percentages of degradation occurred, respectively. This shows that Bi-Au-ZnO is more efficient in AR 18 degradation than other catalysts Effect of solution ph The effect of ph on the photodegradation of AR 18 was studied in the ph range of 3 to 12. The pseudo-first order rate constants for degradation were , , , and min -1 at ph 3, 5, 7, 9, 11 and 12, respectively. It is observed that increase in ph from 3 to 11 increases the removal efficiency of AR 18 and then decreases. The optimum ph for efficient AR 18 removal on Bi-Au-ZnO is 11 (Fig ). In order to find out the reason for this ph effect an experiment was carried out on dark adsorption of AR 18 under different ph. The percentages of adsorption at ph 3, 5, 7, 9, 11 and 12 were found to be 0, 0, 0.5, 1.3, 1.7 and 0.1% after the attainment of adsorption equilibrium. This reveals that the higher degradation efficiency at ph 11 is due to the higher adsorption at this ph Effect of catalyst loading The influence of the photocatalyst dosage on the degradation of AR 18 has been investigated by employing different amounts of Bi-Au-ZnO. The results are presented in Fig The increase of catalyst dosage from 1 to 4 g L -1 increases the degradation rate appreciably and further increase of catalyst amount above 4 g L -1,

32 (c) (b) 80 % of AR 18 remaining (g) (h) (f) (d) 0 (e) (a) Time (min) 150 Bi-Au-ZnO (a) Bi-ZnO (d) Commerical ZnO (g) Bi-Au-ZnO/dark (b) Au-ZnO (e) Without catalysts (c) Bare ZnO (f) TiO 2 -P25 (h) Fig Photodegradability of AR 18: [AR 18] = M, ph = 11, catalyst suspended = 4 g L -1, airflow rate = 8.1 ml s -1, I UV = einstein L 1 s 1.

33 (k) min Initial ph Fig Effect of solution ph: [AR 18] = M, Bi-Au-ZnO suspended = 4 g L 1, airflow rate = 8.1 ml s 1, I UV = einstein L 1 s 1, irradiation time = 90 min.

34 71 decreases the degradation rate. The pseudo-first order rate constant values for 1, 2, 3, 4 and 5 g L -1 are , , , and min 1, respectively. Hence, 4 g L -1 of Bi-Au-ZnO catalyst amount is found to be the optimum dosage. In all other experiments, 4 g L -1 of catalyst was used. Reason for this effect has been discussed earlier (section 3.3.3) Effect of initial dye concentration Fig shows that the increase of dye concentration from 2 to M decreases the rate constant from to min ). Reason for this effect has been discussed earlier (section Reusability of the catalyst The reusability of Bi-Au-ZnO catalyst was investigated by the repeated use of the catalyst for four runs under the same conditions. After each run, the catalyst was separated by centrifugation and dried at 110 C for 12 h. The results are displayed in Fig The catalyst showed an efficiency of 93.2 % even at 4 th run. These results show that this catalyst is found to be stable and reusable under UV light PHOTODEGRADATION OF AR 18 WITH SOLAR LIGHT Primary analysis The photodegradability of AR 18 with different photocatalysts under solar light irradiation is shown in Figure % degradation of the dye takes place at a time of 150 min with Bi-Au-ZnO (curve a) under solar light. 1.7% decrease in dye concentration occurred due to adsorption for the same experiment performed with Bi-Au-ZnO in the absence of solar light (curve b). Dye is resistant to

35 (k) min Catalysts of loading (g L -1 ) Fig Effect of catalyst loading: [AR 18] = M, ph = 11, airflow rate = 8.1 ml s 1, I UV = einstein L 1 s 1, irradiation time = 90 min.

36 (k) min Initial dye concentration ( 10-4 M) Fig Effect of initial dye concentration: Bi-Au-ZnO catalyst suspended = 4 g L -1, ph = 11, airflow rate = 8.1 ml s 1, I UV = einstein L 1 s 1, irradiation time = 90 min.

37 % of AR 18 remaining Time (min) I Run II Run III Run IV Run Fig Reusability of the catalyst: [AR 18] = M, ph = 11, Bi-Au-ZnO suspended = 4 g L 1, airflow rate = 8.1 ml s 1, I UV = einstein L 1 s 1.

38 75 self photolysis (curve c). By these observations we can say that both solar light and catalyst are needed for effective degradation of the AR 18. When Bi-ZnO, Au-ZnO, bare ZnO, commercial ZnO, and TiO2-P25 were used under the same conditions, 55.3 (curve d), 92.0 (curve e), 71.8 (curve f), 93.8 (curve g) and 24.7 (curve h) percentages of degradation occurred, respectively. This shows that Bi-Au-ZnO is more efficient in AR 18 degradation than other catalysts Effect of solution ph The effect of ph on the photodegradation of AR 18 was studied in the ph range of 3 to 12. The pseudo-first order rate constants were , , , , and min 1 at ph 3, 5, 7, 9, 11 and 12, respectively. Degradation efficiency increases up to ph 11 and then decreases (Fig ). Hence, the optimum ph is 11. Reason for this effect has been discussed earlier (section 3.5.2) Effect of catalyst loading The influence of the photocatalyst dosage on the degradation of AR 18 has been investigated by employing different amounts of Bi-Au-ZnO (Fig.3.6.3). The increase of catalyst dosage from 1 to 4 g L -1 increases the degradation rate appreciably and further increase of catalyst dosage above 4 g L -1, decreases the degradation rate. The pseudo-first order rate constant values for 1, 2, 3, 4 and 5 g L 1 of Bi-Au-ZnO are , , , and min -1, respectively. Hence 4 g L -1 of Bi-Au-ZnO catalyst amount is the optimum dosage. Reason for this effect has been discussed earlier (section 3.3.3).

39 (c) (b) % of AR 18 remaining (g) (h) (d) (f) (e) 0 (a) Time (min) 200 Bi-Au-ZnO (a) Bi-ZnO (d) Commerical ZnO (g) Bi-Au-ZnO/dark (b) Without catalysts (c) Au-ZnO (e) Bare ZnO (f) TiO 2 -P25 (h) Fig Photodegradability of AR 18: [AR 18] = M, ph = 11, catalyst suspended = 4 g L 1, airflow rate = 8.1 ml s 1, I solar = ( ) ± 100 lux.

40 (k) min Initial ph Fig Effect of solution ph: [AR 18] = M, Bi-Au-ZnO suspended = 4 g L 1, airflow rate = 8.1 ml s 1, I solar = ( ) ± 100 lux, irradiation time = 120 min.

41 (k) min Catalysis loading (g L -1 ) Fig Effect of catalyst loading: [AR 18] = M, ph = 11, airflow rate = 8.1 ml s 1, I solar = ( ± 100 lux), irradiation time = 120 min.

42 Effect of initial dye concentration Fig shows that the increase of dye concentration from 2 to M decreases the rate constant from to min -1. Reason for this effect has been discussed earlier (section 3.3.4) Reusability of the catalyst The reusability of Bi-Au-ZnO catalyst was investigated by the repeated use of the catalyst for four runs under the same conditions. After each run, the catalyst was separated by centrifugation, washed with distilled water and dried at C for 12 h. The results are displayed in Fig The catalyst was efficient even at 4 th run (92.9%). These results show that this catalyst is found to stable and reusable under solar light MINERALIZATION STUDIES CHEMICAL OXYGEN DEMAND (COD) MEASUREMENTS In order to confirm the mineralization of dye, COD measurements were made for the degradation of MB and AR 18 with Bi-Au-ZnO catalyst under optimum conditions. For M of MB, the COD value of 12,185.0 (100%) ppm gradually decreases to ppm (52.8%) and ppm (6.3%) after 20 and 40 min of UV irradiation (UV), respectively. This indicates 93.7% mineralization of MB in 40 min. The COD value of (100%) ppm for M of AR 18 concentration gradually decreases to ppm (46.2%) and 11.8 ppm (1.1%) after 60 and 120 min of irradiation (UV), respectively. This indicates 98.9% mineralization of AR 18 in 120 min.

43 (k) min Initial dye concentration ( 10-4 M) Fig Effect of initial dye concentration: Bi-Au-ZnO catalyst suspended = 4 g L -1, ph = 11, airflow rate = 8.1 ml s 1, I solar = ( ) ± 100 lux, irradiation time = 120 min.

44 % of AR 18 remaining Time (min) I Run II Run III Run IV Run Fig Reusability of the catalyst: [AR 18] = M, ph = 11, Bi-Au-ZnO, suspended = 4 g L 1, airflow rate = 8.1 ml s 1, I solar = ( ) ± 100 lux.

45 82 In solar light, 95.2% COD reduction was observed for MB dye at 60 min while 98.7% COD reduction was observed for AR 18 dye at 150 min Mechanism of the degradation On the basis of the above results, a mechanism can be proposed to explain the enhanced UV/solar light photocatalytic activity of Bi-Au-ZnO composite (Scheme 3.1). The conduction band of Bi2O3 (+0.33 ev) is lower than (Zhang et al., 2014) that of ZnO ( ev) so, it can act as a sink for photogenerated electrons in the mixed semiconductors. Thus, photoinduced electrons on the ZnO surface could transfer to Bi2O3 via interfaces. Similarly, photoinduced holes on the Bi2O3 surface could migrate to ZnO. Therefore, there would be a greater number of electrons on the Bi2O3 surface as well as hole in the ZnO surface, resulting in enhanced separation efficiency for photogenerated electron and holes, which would have a positive effect on the photocatalytic performance [Xu et al., 2008]. In addition to that, the presence of Au traps the electron from both the CB of ZnO and Bi2O3, which suppresses the electron-hole recombination. It is well established that Au traps the electrons from CB of ZnO [Subramanian et al., 2003]. The trapped electrons produce large numbers of superoxide radical anion (O2 ) and at the same time hole in the VB of ZnO react with water to generate highly reactive hydroxyl ( OH) radical. These superoxide radical anion and hydroxyl radical are mainly used for the destruction of dye.

46 83 UV-A/solar light Energy (ev) (Vs) NHE (ev) -1 O2 O2 hν hν CB e- O2 Au e- 0 ZnO (3.2 ev) 1 CB Bi2O3 (2.8 ev) UV-A/solar light hν 2 VB h 3 HO + h+ VB H2O Scheme 3.1. Mechanism of degradation of AR 18 by Bi2O3-Au-ZnO.