Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 31–37
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Photocatalytic detoxification of Acid Red 18 by modified ZnO catalyst under sunlight irradiation A. Senthilraja, B. Subash, P. Dhatshanamurthi, M. Swaminathan, M. Shanthi ⇑ Photocatalysis Laboratory, Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamil Nadu, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Hybrid structured Bi–Au–ZnO
O2•
composite was prepared. Bi–Au–ZnO heterojunction photocatalyst was more stable and reusable. Heterostructured Bi–Au–ZnO showed higher photocatalytic activity than other catalysts. A possible mechanism is proposed for the degradation of AR 18 dye under sun light.
Solar light
–
e-
CB hν
O2
O2•
–
Au
Solar light
e- CB ZnO
hν
3.2 eV Bi2O3 2.8 eV
h+
VB •
h + VB
OH H2O •
Dye / Dye* + HO
Mineral acids + CO2
•–
Mineral acids + CO2
Dye / Dye* + O2
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 30 July 2014 Received in revised form 26 October 2014 Accepted 5 November 2014 Available online 13 November 2014
In this work, hybrid structured Bi–Au–ZnO composite was prepared by precipitation–decomposition method. This method is mild, economical and efficient. Bi–Au–ZnO was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive spectrum (EDS), diffuse reflectance spectra (DRS), photoluminescence spectra (PL) and BET surface area measurements. Photocatalytic activity of Bi–Au–ZnO was evaluated by irradiating the Acid Red 18 (AR 18) dye solution under sun light. Heterostructured Bi–Au–ZnO photocatalyst showed higher photocatalytic activity than those of individual Bi–ZnO, Au–ZnO, bare ZnO, and TiO2-P25 at pH 11. The effects of operational parameters such as the amount of catalyst dosage, dye concentration, initial pH on photo mineralization of AR 18 dye have been analyzed. The mineralization of AR 18 has been confirmed by chemical oxygen demand (COD) measurements. A possible mechanism is proposed for the degradation of AR 18 under sun light. Finally, Bi–Au–ZnO heterojunction photocatalyst was more stable and could be easily recycled several times opening a new avenue for potential industrial applications. Ó 2014 Elsevier B.V. All rights reserved.
Keywords: Bi loaded Au–ZnO Nano-composite Sun light Acid Red 18 Dye mineralization
Introduction Environmental pollution and the energy crisis have become two biggest problems for human society and seriously intimidate the quality of life. Photocatalysts have consistently drawn much more ⇑ Corresponding author. Tel./fax: +91 4144 237386. E-mail Shanthi).
addresses:
[email protected],
http://dx.doi.org/10.1016/j.saa.2014.11.006 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
+ H2O + H2O
[email protected]
(M.
attention for many of the environmental challenges facing the modern world since they can provide an easy way to use light for environmental remediation and organic transformations. Semiconductor oxides TiO2, ZnO, Bi2O3, WO3 and Nb2O5 have been engaged as photocatalysts in pollutant degradation and water splitting reactions [1–5]. Among them TiO2 and ZnO are significant candidates for use in multiple applications because of wide band gap (3.37 eV) and the large exciton binding energy of 60 meV [6,7]. But, TiO2 and ZnO can be stimulated only by ultraviolet
A. Senthilraja et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 31–37
(UV) light which occupies only 4% in the solar spectra, which greatly impeded the photocatalytic activity of these catalysts in the direct use of solar light. Further the photocatalytic efficiency of ZnO is still insufficient for the practical applications due to the fast recombination of electron and holes in nanoseconds [8]. Doping or loading of transition metal (or) inner transition metals on to the surface of the ZnO semiconductor traps the electron and it could enhance the light absorption of ZnO in UV region to entire visible region [9–16]. Coupling of two semiconductors with different band gap materials could progress the stability necessary for practical applications. Bi2O3 is a semiconductor with several properties of interest, such as high oxygen ion conductivity, very high refraction index or good photoconductive response, which enable the application of this material in optoelectronics, gas sensing, catalysis and other fields [17–19]. Bi2O3 has four main polymorphs [20,21], but only the monoclinic a-phase and the face centred cubic d-phase are stable. Bi2O3 is a promising candidate because of its small band gap (2.85 eV), high oxidation power of valence hole (+3.13 V versus NHE), and non-toxic property as TiO2 [22]. But Bi2O3 shows poor photocatalytic efficiency for the destruction of organic waste due to a hasty recombination of the photoinduced electron and hole pairs [23]. Hameed et al. synthesised of Bi2O3/Bi2O4x nanocomposites and studied their superior photocatalytic activity under visible light irradiation [24]. They also synthesized NiO–Bi2O3 and ZnO–Bi2O3 composites for photocatalytic applications [25,26]. Nowadays, several research works were reported on the efficient visible-light-driven photocatalysis using noble metal-deposited on Bi2O3 [27,28]. The influences of the loaded Au and the electron transport paths, decide the photocatalytic activity of Au/Bi2O3. This promoted our interest in the development of a noble metal doped semiconductor oxide. In the present study we reported the synthesis and characterization of a heterostructured of Bi loaded Au–ZnO and its photocatalytic activity on Acid Red 18 dye degradation under sun light illumination. Experimental
1.7 HO
NaO 3 S
N
N
NaO 3 S
Absorbance
32
SO3Na
0.8
0.0 200
500
800
Wavelength (nm) Fig. 1. The structure and UV–visible spectrum of AR 18.
oxalate was homogeneous in dimension. Bi–Au–zinc oxalate precipitate was washed several times with distilled water, air-dried overnight and dried at 100 °C for 5 h. It was calcined in the muffle furnace at the rate of 20 °C min1 to reach the decomposition temperature of zinc oxalate (450 °C). After 12 h, the furnace was allowed to cool down to room temperature. The Bi loaded Au–ZnO catalyst was collected and used for further analysis. This catalyst contained 4 wt% of Bi (related to ZnO). Catalysts with 1, 2, 3 and 5 wt% of Bi were prepared with the same procedure. The bare ZnO was prepared without addition of Bi(NO3)3 and AuCl3. Bi–ZnO and Au–ZnO were prepared by the same procedure with respective precursors.
Materials The commercial AR 18 dye, (molecular formula = C20H11N2Na3O10S3; molecular weight 604.47 and dye content 80%) obtained from s.d. fine chemicals was used as such. The chemical structure of the dye and its UV–vis spectrum are given in Fig. 1. Oxalic acid dihydrate (99%) and zinc nitrate hexahydrate (99%) were obtained from Himedia chemicals. Bi(NO3)3 (Merck), ZnO (Himedia) and AuCl3 (Sigma Aldrich) were used as received. A gift sample of Degussa TiO2-P25 was obtained from Evonik (Germany). It is a 80:20 mixture of anatase and rutile phase. It has a particle size of 30 nm and surface area of 50 m2 g1. The double distilled water was used to prepare experimental solutions. The pH of the solutions prior to irradiation was adjusted using H2SO4 or NaOH. Preparation of Bi loaded Au–ZnO Bi loaded Au–ZnO was prepared by precipitation–decomposition method. Aqueous solutions of 100 mL of 0.4 M zinc nitrate hexahydrate and 100 mL of 0.6 M oxalic acid in deionized water were brought to their boiling points separately. About 0.041 g of AuCl3 in 5 ml of water was added to zinc nitrate solution. Solutions of zinc nitrate and AuCl3 were mixed with oxalic acid solution. Precipitation of zinc oxalate with Au occurred (1 wt% Au related to ZnO). To this 0.251 g of Bi(NO3)3 in 5 ml of water was added (few drops of concentrated nitric acid was also added to get a clear solution of bismuth nitrate) and stirred for 3 h. Precipitate of Bi with Au–zinc
Analytical methods Powder X-ray diffraction patterns were obtained using X’Per PRO diffractometer equipped with a Cu Ka radiation (wavelength 1.5406 Å) at 2.2 kW Max. Peak positions were compared with the standard files to identity the crystalline phase. The transmission electron microscopic (TEM) observation was carried out on a Hitachi H-7000, Japan, with an acceleration voltage of 100 kV. The specimens were prepared by depositing a drop of the suspension of sample powder, which was ultrasonically dispersed in acetone for 10 min, on a carbon-coated copper grid, followed by drying at room temperature. On an observation with a JEOL JSM-6500F cold field emission scanning electron microscope (FE-SEM), the samples were mounted on a gold platform placed in chamber. Energy dispersive X-ray spectrum (EDS) was examined using a JEOL-JSM 5610 LV. Diffuse reflectance spectra (DRS) were recorded using Shimadzu UV-2450. Photoluminescence (PL) spectra at room temperature were recorded using a Perkin Elmer LS 55 fluorescence spectrometer. The nanoparticles were dispersed in carbon tetrachloride and excited using light of wavelength 300 nm. The specific surface areas of the samples were determined through nitrogen adsorption at 77 K on the basis of BET equation using a Micromeritics ASAP 2020 V3.00 H. UV spectral measurements were done using Hitachi-U-2001 spectrometer.
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Photodegradation experiments (b)
∗ Bi O 2
∗
COD ¼
Characterization of catalyst Primary analysis of photocatalytic degradation of AR 18 with different wt% Bi loaded Au–ZnO a catalyst was carried out. Pseudo-first order rate constants determined for 1, 2, 3, 4 and 5 wt% Bi loading were 0.0139, 0.0151, 0.0155, 0.0193 and 0.0187 min1, respectively. The catalyst loaded with 4 wt% of Bi was found to be the most efficient. For this reason, 4 wt% of Bi was taken as optimal percentage of Bi on Au–ZnO, and this catalyst was characterized by XRD, FE-SEM, EDS, TEM, DRS, PL and BET surface area measurements. The structure and phase identification were determined by powder X-ray diffraction. Fig. 2 shows the XRD patterns of the bare ZnO and Bi–Au–ZnO. Fig. 2a exhibit all of the diffraction peaks from the catalyst matches with ZnO pattern of hexagonal wurtzite structure. The relative intensities of most of the predominant peaks are in good agreement with those in the JCPDS file for wurtzite-structure ZnO (JCPDS. 89-0511), the peaks at 31.84°, 34.48°, 36.32° and 56.66° correspond to (1 0 0), (0 0 2), (1 0 1) and (1 1 0) planes of wurtzite ZnO. No other diffraction peaks are detected in the bare ZnO. In the Bi–Au–ZnO system (Fig. 2b), in addition to the bare ZnO peaks, there are two new peaks at 2h values of 27.8° (2 0 1) and 31.5° (0 0 2), corresponding to Bi2O3 [29]. This confirms the loading of Bi. The crystalline size of bare ZnO and Bi–Au–ZnO was determined using Debye–Scherrer eqn. The average crystalline sizes of bare ZnO and Bi–Au–ZnO are found to be 4.15 and 3.9 nm, respectively.
(100)
(a)
40 50 Position [°2Theta]
(202)
(004)
(103) (200)
60
(112) (201)
(110) (102)
30
70
Fig. 2. XRD patterns of (a) bare ZnO and (b) Bi–Au–ZnO.
The surface morphology of the prepared Bi–Au–ZnO was characterized by FE-SEM and TEM analysis. Fig. 3 represents the FESEM images of Bi–Au–ZnO exhibiting ‘‘nanochain’’ structure. EDS reveals that presence of Bi, Au, O and Zn element in the prepared
ðBlank titre value dye sample titre valueÞ normality of FAS 8 1000 Volume of sample
Results and discussion
∗
(002)
20
Chemical oxygen demand (COD) measurements COD was determined using the following procedure. Sample was refluxed with HgSO4, known volume of standard K2Cr2O7, Ag2SO4 and H2SO4 for 2 h and titrated with standard ferrous ammonium sulfate (FAS) using ferroin as indicator. A blank titration was carried out with distilled water instead of dye sample. COD was determined using the following Eq. (1).
3
(101)
Counts (a.u)
The solar photodegradation experiments were carried out under similar conditions on sunny days between 11 a.m. and 2 p.m. An open borosilicate glass tube of 50 mL capacity, 40 cm height, and 20 mm diameter was used as the reaction vessel. Fifty milliliters of AR 18 (5 104 M) with the suitable amount of catalyst was stirred for 30 min in the dark prior to illumination in order to achieve maximum adsorption of dye onto the catalyst surface. Irradiation was carried out in the open air with continuous aeration by a pump to provide oxygen and for complete mixing of the reaction solution. During the illumination time no volatility of the solvent was noted. The temperature of the experimental solution is 30 °C, in all cases, 50 mL of reaction mixture was irradiated. At specific time intervals, 2–3 mL of the sample was withdrawn and centrifuged to separate the catalyst. One milliliter of the sample was suitably diluted, and the dye concentration was determined from the absorbance at the analytical wavelength (AR 18, 246 nm). Solar intensity (1250 100 ± 100 lx) was almost constant during the experiments.
ð1Þ
Bi–Au–ZnO material (Fig. 4). TEM analysis was performed to examine the size, shape and orientation of individual nanocrystallites. TEM images of Bi–Au–ZnO (Fig. 5) exhibit ‘‘Hexagonal huddle’’ as well as ‘‘chain like’’ structures. The optical absorption of as-prepared bare ZnO and Bi–Au–ZnO samples were measured by diffuse reflectance spectra (DRS), and shown in Fig. 6. It reveals that bare ZnO and Bi–Au–ZnO have no significant absorbance change in UV region. But in visible region (420–800 nm) Bi–Au–ZnO has higher absorbance when compared to the bare ZnO. This may increase the solar catalytic activity to a small extent. Fig. 7a and b represent the photoluminescence (PL) spectra of the bare ZnO and Bi–Au–ZnO. The luminescence occurs due to the recombination of electron and hole. Its intensity is proportional to the rate of electron–hole recombination. ZnO exhibits two emissions at 420 and 480 nm. Bi–Au–ZnO also gave two emissions at same wavelengths. Bi and Au loading on ZnO does not shift the emission, but PL intensity of Bi–Au–ZnO is lower when compared to the bare ZnO. This is because of suppression of electron hole recombination by loaded Bi and Au on ZnO. This enhances the catalytic activity of the Bi–Au–ZnO material. In general the surface area of the catalyst is the most important factor influencing the catalytic activity. The surface area and pore volume were measured for bare ZnO and Bi–Au–ZnO. The BET surface area analysis data are summarized in Table 1. It shows that Bi– Au ZnO has higher surface area as well as pore volume than that of the prepared ZnO. The increase in BET surface area and pore volume favor high adsorption of dye molecules on the Bi–Au ZnO catalyst surface. This will favor for the photocatalytic activity.
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(a)
(a)
(b)
(b)
Fig. 5. TEM images of Bi–Au–ZnO (a) 150 k and (b) 200 k.
Fig. 3. FE-SEM images of Bi–Au–ZnO (a) 100 nm and (b) 100 nm.
120
(a) 100
% of reflectance
(b) 80 60 40 20 0 200
400
600
800
Wavelength (nm) Fig. 6. DRS of (a) bare ZnO and (b) Bi–Au–ZnO. Fig. 4. EDS of Bi–Au–ZnO.
Photodegradability of AR 18 Fig. 8 shows the percentage of AR 18 on irradiation of an aqueous solution of AR 18 (5 104 M) in the presence of different photocatalysts under solar light. When the dye was irradiated without catalyst there was negligible degradation and for the same
experiment when performed in the absence of solar light with Bi– Au–ZnO, there was a decrease of 1.7% in the dye concentration (curve b). Furthermore it has been observed that 97.7% degradation of AR 18 took place at the time of 150 min with Bi–Au–ZnO (curve a) under solar light. These observations reveal that both solar light and photocatalyst are needed for effective degradation of AR 18 dye. When the photocatalyst of Bi–ZnO, Au–ZnO, bare ZnO, commercial ZnO and TiO2-P25 were used under same conditions,
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300
250
of operational parameters had been carried out to find out the optimum conditions. The photocatalytic degradation of AR 18 dye containing Bi–Au– ZnO obeys pseudo-first order kinetics. At low initial dye concentration the rate expression is given by
(a)
PL intensity
200
0
d½C=dt ¼ k ½C
ð2Þ
0
where k is the pseudo-first order rate constant. The dye is adsorbed onto the Bi–Au–ZnO surface and the adsorption–desorption equilibrium is reached. After adsorption, the equilibrium concentration of the dye solution is determined and it is taken as the initial dye concentration for kinetic analysis. Integration of Eq. (2) (with the limit of C = C0 at t = 0) gives Eq. (3)
150
(b) 100
50
0
lnðC 0 =CÞ ¼ k t
0 250
350
450
550
650
Wavelength (nm) Fig. 7. Photoluminescence spectra of (a) bare ZnO and (b) Bi–Au–ZnO.
Table 1 Surface properties of the catalysts. Properties
ZnO bare
Bi–Au–ZnO
BET surface area Total pore volume (single point)
14.9 (m2 g1) 0.12 (cm3 g1)
28.2 (m2 g1) 0.25 (cm3 g1)
ð3Þ
where C0 is the equilibrium concentration of dye and C is the concentration at time t. As depicted in Fig. 9, the maximum absorption peaks of AR 18 at 246 nm diminished gradually and disappeared completely under solar light irradiation for 150 min in the presence of the Bi–Au– ZnO. The color of the solution changed gradually, suggesting that the chromophoric structure was decomposed. There is no significant change in the shape of spectrum during irradiation and the intensity at 246 and 508 nm decreases gradually during the degradation. This reveals that the intermediates do not absorb at the analytical wavelength of 246 nm. Influence of operational parameters Effect of solution pH The solution pH plays an important role in the photocatalytic degradation process of various pollutants [30,31]. The effect of pH on the photo degradation of AR 18 was studied in the pH range of 3–12 (Fig. S1, see Supplementary data). The pseudo-first order rate constants for Bi–Au–ZnO at pH 3, 5, 7, 9, 11 and 12 are 0.0082,
100
80
(g)
1.7 60
(c)
40
(a)
(e)
(f)
(d)
20
(a)
0 0
50
100
150
200
Absorbance
% of AR 18 remaining
(b)
0.8
(f)
Time (min) Bi-Au-ZnO (a)
Bi-Au-ZnO/dark (b)
Bi-ZnO (c)
Au-ZnO (d)
Commerical ZnO (f)
TiO2-P25 (g)
Bare ZnO (e)
Fig. 8. Photodegradability of AR 18; [AR 18] = 5 104 M, catalyst suspended = 4 gL1, pH = 11, airflow rate = 8.1 mL s1, Isolar = (1250 100) ± 100 lx.
0.0 200
55.3 (curve c), 92.0 (curve d), 71.8 (curve e), 93.8 (curve f) and 24.7% (curve g) degradations occurred, respectively. This clearly exhibits that the solar/Bi–Au–ZnO catalyst is most efficient in AR 18 degradation than other prepared and commercial catalysts. Since the degradation was effective with Bi–Au–ZnO, the influence
500
800
Wavelength (nm) Fig. 9. The changes in UV–vis spectra of AR 18 on irradiation with Bi–Au–ZnO under solar light, [AR 18] = 5 104 M; pH = 11; Bi–Au–ZnO suspended = 4 g L1; airflow rate = 8.1 mL s1, Isolar = (1250 100) ± 100 lx, (a) 0 min, (b) 30 min, (c) 60 min, (d) 90 min, (e) 120 min and (f) 150 min.
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0.0096, 0.0096, 0.0130, 0.0193 and 0.0109 min1, 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. It is well known than at acidic pH the removal efficiency is low and it is due to dissolution of ZnO material. Since the photocatalytic efficiency depends on the adsorption of the dye on the surface of the catalyst, the adsorption of the dye under different pH values was investigated. 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%, respectively after attainment of adsorption equilibrium (30 min). As the adsorption is high at pH 11, the degradation is also efficient at this pH. Effect of catalyst loading Catalyst loading in slurry photocatalytic processes is a significant factor that can strongly manipulate dye degradation. Experiments performed with different amounts of Bi–Au–ZnO showed that the photodegradation efficiency increased with an increase in amount up to 4 g L1 and then slightly decreased (Fig. S2, see Supplementary data). The pseudo-first order rate constants are 0.0086, 0.0105, 0.0143, 0.0193, and 0.0098 min1 for Bi–Au–ZnO at catalyst loadings of 1, 2, 3, 4 and 5 g L1, respectively. Enhancement of removal rate is due to (i) The increase in the amount of catalyst which increases the number of dye molecules adsorbed. (ii) The increase in the density of catalyst particles in the area of illumination. The decrease in the removal efficiency of AR 18 at higher amount (above 4 g L1) is due to the light reflectance by catalyst particles. Similar results have been reported for the photodegradation of dyes by ZnO [32,33]. Effect of initial dye concentration The effect of various initial dye concentrations on the degradation of AR 18 on Bi–Au–ZnO surface has been investigated. Increase of dye concentration from 2 to 6 104 M decreases the rate constant from 0.0288 to 0.0077 min1 in solar light (Fig. S3, see Supplementary data). The rate of degradation relates to the OH (hydroxyl radical) formation on catalyst surface and probability of OH (hydroxyl radical) reacting with dye molecule. Catalyst reusability Heterogeneous catalyst is used for any type of degradation as it has the main advantage of reusability. The reusability of Bi–Au– ZnO was tested by four successive cycles for the degradation of AR 18 under solar light and the results are shown in Fig. 10. Although the degradation efficiency of Bi–Au–ZnO is slightly decreased after each cycle, the catalyst exhibited 93% activity at fourth cycle under the solar irradiation. These results clearly exhibit that the prepared catalyst is found to be reusable.
100
% of AR 18 remaining
36
80
60
40
20
0 0
150
I Run
IV Run
III Run
II Run
Fig. 10. Catalyst reusability: pH = 11, 4 wt% Bi–Au–ZnO suspended = 4 g L1, airflow rate = 8.1 mL s1, Isolar = (1250 100) ± 100 lx.
Table 2 COD measurements. Time (min)
% COD reduction
60 120 150
37.9 79.6 98.7
[AR 18] = 5 104 M; 4 wt% Bi–Au–ZnO suspended = 4 g L1; pH = 11; airflow rate = 8.1 mL s1; Isolar = (1250 100) ± 100 lx.
composite (Scheme 1). The conduction band of Bi2O3 is lower than that of ZnO so, it can act as a sink for photogenerated electrons in the mixed semiconductor. Thus, photoinduced electrons on the ZnO surface could transfer to Bi2O3 via interface. 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 holes in the ZnO surface, resulting in enhanced separation efficiency for photogenerated electron and holes, which would have a positive effect on the photocatalytic performance [34]. In addition to that, the presence of ‘‘Au’’ traps the electron from both
O2• Solar light
–
e-
CB hν
O2•
O2
ZnO
–
Au
Solar light
e-
COD analysis To confirm the mineralization of AR 18, the degradation was also analyzed by COD measurements under solar light. The % of COD reductions are given in Table 2. After 150 min irradiation with Bi–Au–ZnO, 98.7% of COD reduction was obtained. This indicates the mineralization of AR 18 dye.
600
450
300
Time (min)
CB hν
3.2 eV Bi2O3 2.8 eV
h
VB •
+
h + VB
OH H2O
Mechanism of degradation On the basis of the above results, a mechanism can be proposed to explain the enhanced solar photocatalytic activity of Bi–Au–ZnO
•
Dye / Dye* + HO
Mineral acids + CO2
•– Dye / Dye* + O2
Mineral acids + CO2
+ H2O + H2O
Scheme 1. Mechanism of degradation of AR 18 by Bi–Au–ZnO.
A. Senthilraja et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 31–37
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 [35]. The trapped electrons produce large numbers of superoxide radical anion (O 2 ) 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 AR 18 dye. Conclusions In summary, Bi–Au–ZnO photocatalyst was successfully synthesized by a precipitation–decomposition method. The highest photocatalytic activity of Bi–Au–ZnO is due to the combined effect of efficient electron–hole separation and superior textural properties compared to the undoped ZnO. COD measurements confirm the complete mineralization of AR 18 molecule. The studies described herein demonstrate that Bi–Au–ZnO nanoparticles can be utilized for efficient sunlight-driven photocatalytic application. Thus, an ideal construction of Bi–Au–ZnO composites could be employed not only in photocatalysis, but also used in effluent treatment as well as in highly toxic chemicals degradation. Acknowledgements One of the authors (M. Swaminathan) is thankful to CSIR, New Delhi, India for financial support through research Grant No. 21 (0799)/10/EMR-II. One of the authors (M. Shanthi) is highly thankful to UGC, New Delhi, India for financial support through research project F. No. 41-288/2012 (SR).
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Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2014.11.006. References [1] X. Chen, S. Shen, L. Guo, S. Mao, Chem. Rev. 110 (2010) 6503–6570. [2] I. Nowak, M. Ziolek, Chem. Rev. 99 (1999) 3603–3624. [3] W.J. Youngblood, S.H.A. Lee, K. Maeda, T.E. Mallouk, Acc. Chem. Res. 42 (2009) 1966–1973.
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37
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Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Photocatalytic detoxification of Acid Red 18 by modified ZnO catalyst under sunlight irradiation A. Senthilraja, B. Subash, P. Dhatshanamurthi, M. Swaminathan, M. Shanthi ⇑ Photocatalysis Laboratory, Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamil Nadu, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Hybrid structured Bi–Au–ZnO
O2•
composite was prepared. Bi–Au–ZnO heterojunction photocatalyst was more stable and reusable. Heterostructured Bi–Au–ZnO showed higher photocatalytic activity than other catalysts. A possible mechanism is proposed for the degradation of AR 18 dye under sun light.
Solar light
–
e-
CB hν
O2
O2•
–
Au
Solar light
e- CB ZnO
hν
3.2 eV Bi2O3 2.8 eV
h+
VB •
h + VB
OH H2O •
Dye / Dye* + HO
Mineral acids + CO2
•–
Mineral acids + CO2
Dye / Dye* + O2
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 30 July 2014 Received in revised form 26 October 2014 Accepted 5 November 2014 Available online 13 November 2014
In this work, hybrid structured Bi–Au–ZnO composite was prepared by precipitation–decomposition method. This method is mild, economical and efficient. Bi–Au–ZnO was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive spectrum (EDS), diffuse reflectance spectra (DRS), photoluminescence spectra (PL) and BET surface area measurements. Photocatalytic activity of Bi–Au–ZnO was evaluated by irradiating the Acid Red 18 (AR 18) dye solution under sun light. Heterostructured Bi–Au–ZnO photocatalyst showed higher photocatalytic activity than those of individual Bi–ZnO, Au–ZnO, bare ZnO, and TiO2-P25 at pH 11. The effects of operational parameters such as the amount of catalyst dosage, dye concentration, initial pH on photo mineralization of AR 18 dye have been analyzed. The mineralization of AR 18 has been confirmed by chemical oxygen demand (COD) measurements. A possible mechanism is proposed for the degradation of AR 18 under sun light. Finally, Bi–Au–ZnO heterojunction photocatalyst was more stable and could be easily recycled several times opening a new avenue for potential industrial applications. Ó 2014 Elsevier B.V. All rights reserved.
Keywords: Bi loaded Au–ZnO Nano-composite Sun light Acid Red 18 Dye mineralization
Introduction Environmental pollution and the energy crisis have become two biggest problems for human society and seriously intimidate the quality of life. Photocatalysts have consistently drawn much more ⇑ Corresponding author. Tel./fax: +91 4144 237386. E-mail Shanthi).
addresses:
[email protected],
http://dx.doi.org/10.1016/j.saa.2014.11.006 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
+ H2O + H2O
[email protected]
(M.
attention for many of the environmental challenges facing the modern world since they can provide an easy way to use light for environmental remediation and organic transformations. Semiconductor oxides TiO2, ZnO, Bi2O3, WO3 and Nb2O5 have been engaged as photocatalysts in pollutant degradation and water splitting reactions [1–5]. Among them TiO2 and ZnO are significant candidates for use in multiple applications because of wide band gap (3.37 eV) and the large exciton binding energy of 60 meV [6,7]. But, TiO2 and ZnO can be stimulated only by ultraviolet
A. Senthilraja et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 31–37
(UV) light which occupies only 4% in the solar spectra, which greatly impeded the photocatalytic activity of these catalysts in the direct use of solar light. Further the photocatalytic efficiency of ZnO is still insufficient for the practical applications due to the fast recombination of electron and holes in nanoseconds [8]. Doping or loading of transition metal (or) inner transition metals on to the surface of the ZnO semiconductor traps the electron and it could enhance the light absorption of ZnO in UV region to entire visible region [9–16]. Coupling of two semiconductors with different band gap materials could progress the stability necessary for practical applications. Bi2O3 is a semiconductor with several properties of interest, such as high oxygen ion conductivity, very high refraction index or good photoconductive response, which enable the application of this material in optoelectronics, gas sensing, catalysis and other fields [17–19]. Bi2O3 has four main polymorphs [20,21], but only the monoclinic a-phase and the face centred cubic d-phase are stable. Bi2O3 is a promising candidate because of its small band gap (2.85 eV), high oxidation power of valence hole (+3.13 V versus NHE), and non-toxic property as TiO2 [22]. But Bi2O3 shows poor photocatalytic efficiency for the destruction of organic waste due to a hasty recombination of the photoinduced electron and hole pairs [23]. Hameed et al. synthesised of Bi2O3/Bi2O4x nanocomposites and studied their superior photocatalytic activity under visible light irradiation [24]. They also synthesized NiO–Bi2O3 and ZnO–Bi2O3 composites for photocatalytic applications [25,26]. Nowadays, several research works were reported on the efficient visible-light-driven photocatalysis using noble metal-deposited on Bi2O3 [27,28]. The influences of the loaded Au and the electron transport paths, decide the photocatalytic activity of Au/Bi2O3. This promoted our interest in the development of a noble metal doped semiconductor oxide. In the present study we reported the synthesis and characterization of a heterostructured of Bi loaded Au–ZnO and its photocatalytic activity on Acid Red 18 dye degradation under sun light illumination. Experimental
1.7 HO
NaO 3 S
N
N
NaO 3 S
Absorbance
32
SO3Na
0.8
0.0 200
500
800
Wavelength (nm) Fig. 1. The structure and UV–visible spectrum of AR 18.
oxalate was homogeneous in dimension. Bi–Au–zinc oxalate precipitate was washed several times with distilled water, air-dried overnight and dried at 100 °C for 5 h. It was calcined in the muffle furnace at the rate of 20 °C min1 to reach the decomposition temperature of zinc oxalate (450 °C). After 12 h, the furnace was allowed to cool down to room temperature. The Bi loaded Au–ZnO catalyst was collected and used for further analysis. This catalyst contained 4 wt% of Bi (related to ZnO). Catalysts with 1, 2, 3 and 5 wt% of Bi were prepared with the same procedure. The bare ZnO was prepared without addition of Bi(NO3)3 and AuCl3. Bi–ZnO and Au–ZnO were prepared by the same procedure with respective precursors.
Materials The commercial AR 18 dye, (molecular formula = C20H11N2Na3O10S3; molecular weight 604.47 and dye content 80%) obtained from s.d. fine chemicals was used as such. The chemical structure of the dye and its UV–vis spectrum are given in Fig. 1. Oxalic acid dihydrate (99%) and zinc nitrate hexahydrate (99%) were obtained from Himedia chemicals. Bi(NO3)3 (Merck), ZnO (Himedia) and AuCl3 (Sigma Aldrich) were used as received. A gift sample of Degussa TiO2-P25 was obtained from Evonik (Germany). It is a 80:20 mixture of anatase and rutile phase. It has a particle size of 30 nm and surface area of 50 m2 g1. The double distilled water was used to prepare experimental solutions. The pH of the solutions prior to irradiation was adjusted using H2SO4 or NaOH. Preparation of Bi loaded Au–ZnO Bi loaded Au–ZnO was prepared by precipitation–decomposition method. Aqueous solutions of 100 mL of 0.4 M zinc nitrate hexahydrate and 100 mL of 0.6 M oxalic acid in deionized water were brought to their boiling points separately. About 0.041 g of AuCl3 in 5 ml of water was added to zinc nitrate solution. Solutions of zinc nitrate and AuCl3 were mixed with oxalic acid solution. Precipitation of zinc oxalate with Au occurred (1 wt% Au related to ZnO). To this 0.251 g of Bi(NO3)3 in 5 ml of water was added (few drops of concentrated nitric acid was also added to get a clear solution of bismuth nitrate) and stirred for 3 h. Precipitate of Bi with Au–zinc
Analytical methods Powder X-ray diffraction patterns were obtained using X’Per PRO diffractometer equipped with a Cu Ka radiation (wavelength 1.5406 Å) at 2.2 kW Max. Peak positions were compared with the standard files to identity the crystalline phase. The transmission electron microscopic (TEM) observation was carried out on a Hitachi H-7000, Japan, with an acceleration voltage of 100 kV. The specimens were prepared by depositing a drop of the suspension of sample powder, which was ultrasonically dispersed in acetone for 10 min, on a carbon-coated copper grid, followed by drying at room temperature. On an observation with a JEOL JSM-6500F cold field emission scanning electron microscope (FE-SEM), the samples were mounted on a gold platform placed in chamber. Energy dispersive X-ray spectrum (EDS) was examined using a JEOL-JSM 5610 LV. Diffuse reflectance spectra (DRS) were recorded using Shimadzu UV-2450. Photoluminescence (PL) spectra at room temperature were recorded using a Perkin Elmer LS 55 fluorescence spectrometer. The nanoparticles were dispersed in carbon tetrachloride and excited using light of wavelength 300 nm. The specific surface areas of the samples were determined through nitrogen adsorption at 77 K on the basis of BET equation using a Micromeritics ASAP 2020 V3.00 H. UV spectral measurements were done using Hitachi-U-2001 spectrometer.
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Photodegradation experiments (b)
∗ Bi O 2
∗
COD ¼
Characterization of catalyst Primary analysis of photocatalytic degradation of AR 18 with different wt% Bi loaded Au–ZnO a catalyst was carried out. Pseudo-first order rate constants determined for 1, 2, 3, 4 and 5 wt% Bi loading were 0.0139, 0.0151, 0.0155, 0.0193 and 0.0187 min1, respectively. The catalyst loaded with 4 wt% of Bi was found to be the most efficient. For this reason, 4 wt% of Bi was taken as optimal percentage of Bi on Au–ZnO, and this catalyst was characterized by XRD, FE-SEM, EDS, TEM, DRS, PL and BET surface area measurements. The structure and phase identification were determined by powder X-ray diffraction. Fig. 2 shows the XRD patterns of the bare ZnO and Bi–Au–ZnO. Fig. 2a exhibit all of the diffraction peaks from the catalyst matches with ZnO pattern of hexagonal wurtzite structure. The relative intensities of most of the predominant peaks are in good agreement with those in the JCPDS file for wurtzite-structure ZnO (JCPDS. 89-0511), the peaks at 31.84°, 34.48°, 36.32° and 56.66° correspond to (1 0 0), (0 0 2), (1 0 1) and (1 1 0) planes of wurtzite ZnO. No other diffraction peaks are detected in the bare ZnO. In the Bi–Au–ZnO system (Fig. 2b), in addition to the bare ZnO peaks, there are two new peaks at 2h values of 27.8° (2 0 1) and 31.5° (0 0 2), corresponding to Bi2O3 [29]. This confirms the loading of Bi. The crystalline size of bare ZnO and Bi–Au–ZnO was determined using Debye–Scherrer eqn. The average crystalline sizes of bare ZnO and Bi–Au–ZnO are found to be 4.15 and 3.9 nm, respectively.
(100)
(a)
40 50 Position [°2Theta]
(202)
(004)
(103) (200)
60
(112) (201)
(110) (102)
30
70
Fig. 2. XRD patterns of (a) bare ZnO and (b) Bi–Au–ZnO.
The surface morphology of the prepared Bi–Au–ZnO was characterized by FE-SEM and TEM analysis. Fig. 3 represents the FESEM images of Bi–Au–ZnO exhibiting ‘‘nanochain’’ structure. EDS reveals that presence of Bi, Au, O and Zn element in the prepared
ðBlank titre value dye sample titre valueÞ normality of FAS 8 1000 Volume of sample
Results and discussion
∗
(002)
20
Chemical oxygen demand (COD) measurements COD was determined using the following procedure. Sample was refluxed with HgSO4, known volume of standard K2Cr2O7, Ag2SO4 and H2SO4 for 2 h and titrated with standard ferrous ammonium sulfate (FAS) using ferroin as indicator. A blank titration was carried out with distilled water instead of dye sample. COD was determined using the following Eq. (1).
3
(101)
Counts (a.u)
The solar photodegradation experiments were carried out under similar conditions on sunny days between 11 a.m. and 2 p.m. An open borosilicate glass tube of 50 mL capacity, 40 cm height, and 20 mm diameter was used as the reaction vessel. Fifty milliliters of AR 18 (5 104 M) with the suitable amount of catalyst was stirred for 30 min in the dark prior to illumination in order to achieve maximum adsorption of dye onto the catalyst surface. Irradiation was carried out in the open air with continuous aeration by a pump to provide oxygen and for complete mixing of the reaction solution. During the illumination time no volatility of the solvent was noted. The temperature of the experimental solution is 30 °C, in all cases, 50 mL of reaction mixture was irradiated. At specific time intervals, 2–3 mL of the sample was withdrawn and centrifuged to separate the catalyst. One milliliter of the sample was suitably diluted, and the dye concentration was determined from the absorbance at the analytical wavelength (AR 18, 246 nm). Solar intensity (1250 100 ± 100 lx) was almost constant during the experiments.
ð1Þ
Bi–Au–ZnO material (Fig. 4). TEM analysis was performed to examine the size, shape and orientation of individual nanocrystallites. TEM images of Bi–Au–ZnO (Fig. 5) exhibit ‘‘Hexagonal huddle’’ as well as ‘‘chain like’’ structures. The optical absorption of as-prepared bare ZnO and Bi–Au–ZnO samples were measured by diffuse reflectance spectra (DRS), and shown in Fig. 6. It reveals that bare ZnO and Bi–Au–ZnO have no significant absorbance change in UV region. But in visible region (420–800 nm) Bi–Au–ZnO has higher absorbance when compared to the bare ZnO. This may increase the solar catalytic activity to a small extent. Fig. 7a and b represent the photoluminescence (PL) spectra of the bare ZnO and Bi–Au–ZnO. The luminescence occurs due to the recombination of electron and hole. Its intensity is proportional to the rate of electron–hole recombination. ZnO exhibits two emissions at 420 and 480 nm. Bi–Au–ZnO also gave two emissions at same wavelengths. Bi and Au loading on ZnO does not shift the emission, but PL intensity of Bi–Au–ZnO is lower when compared to the bare ZnO. This is because of suppression of electron hole recombination by loaded Bi and Au on ZnO. This enhances the catalytic activity of the Bi–Au–ZnO material. In general the surface area of the catalyst is the most important factor influencing the catalytic activity. The surface area and pore volume were measured for bare ZnO and Bi–Au–ZnO. The BET surface area analysis data are summarized in Table 1. It shows that Bi– Au ZnO has higher surface area as well as pore volume than that of the prepared ZnO. The increase in BET surface area and pore volume favor high adsorption of dye molecules on the Bi–Au ZnO catalyst surface. This will favor for the photocatalytic activity.
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(a)
(a)
(b)
(b)
Fig. 5. TEM images of Bi–Au–ZnO (a) 150 k and (b) 200 k.
Fig. 3. FE-SEM images of Bi–Au–ZnO (a) 100 nm and (b) 100 nm.
120
(a) 100
% of reflectance
(b) 80 60 40 20 0 200
400
600
800
Wavelength (nm) Fig. 6. DRS of (a) bare ZnO and (b) Bi–Au–ZnO. Fig. 4. EDS of Bi–Au–ZnO.
Photodegradability of AR 18 Fig. 8 shows the percentage of AR 18 on irradiation of an aqueous solution of AR 18 (5 104 M) in the presence of different photocatalysts under solar light. When the dye was irradiated without catalyst there was negligible degradation and for the same
experiment when performed in the absence of solar light with Bi– Au–ZnO, there was a decrease of 1.7% in the dye concentration (curve b). Furthermore it has been observed that 97.7% degradation of AR 18 took place at the time of 150 min with Bi–Au–ZnO (curve a) under solar light. These observations reveal that both solar light and photocatalyst are needed for effective degradation of AR 18 dye. When the photocatalyst of Bi–ZnO, Au–ZnO, bare ZnO, commercial ZnO and TiO2-P25 were used under same conditions,
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300
250
of operational parameters had been carried out to find out the optimum conditions. The photocatalytic degradation of AR 18 dye containing Bi–Au– ZnO obeys pseudo-first order kinetics. At low initial dye concentration the rate expression is given by
(a)
PL intensity
200
0
d½C=dt ¼ k ½C
ð2Þ
0
where k is the pseudo-first order rate constant. The dye is adsorbed onto the Bi–Au–ZnO surface and the adsorption–desorption equilibrium is reached. After adsorption, the equilibrium concentration of the dye solution is determined and it is taken as the initial dye concentration for kinetic analysis. Integration of Eq. (2) (with the limit of C = C0 at t = 0) gives Eq. (3)
150
(b) 100
50
0
lnðC 0 =CÞ ¼ k t
0 250
350
450
550
650
Wavelength (nm) Fig. 7. Photoluminescence spectra of (a) bare ZnO and (b) Bi–Au–ZnO.
Table 1 Surface properties of the catalysts. Properties
ZnO bare
Bi–Au–ZnO
BET surface area Total pore volume (single point)
14.9 (m2 g1) 0.12 (cm3 g1)
28.2 (m2 g1) 0.25 (cm3 g1)
ð3Þ
where C0 is the equilibrium concentration of dye and C is the concentration at time t. As depicted in Fig. 9, the maximum absorption peaks of AR 18 at 246 nm diminished gradually and disappeared completely under solar light irradiation for 150 min in the presence of the Bi–Au– ZnO. The color of the solution changed gradually, suggesting that the chromophoric structure was decomposed. There is no significant change in the shape of spectrum during irradiation and the intensity at 246 and 508 nm decreases gradually during the degradation. This reveals that the intermediates do not absorb at the analytical wavelength of 246 nm. Influence of operational parameters Effect of solution pH The solution pH plays an important role in the photocatalytic degradation process of various pollutants [30,31]. The effect of pH on the photo degradation of AR 18 was studied in the pH range of 3–12 (Fig. S1, see Supplementary data). The pseudo-first order rate constants for Bi–Au–ZnO at pH 3, 5, 7, 9, 11 and 12 are 0.0082,
100
80
(g)
1.7 60
(c)
40
(a)
(e)
(f)
(d)
20
(a)
0 0
50
100
150
200
Absorbance
% of AR 18 remaining
(b)
0.8
(f)
Time (min) Bi-Au-ZnO (a)
Bi-Au-ZnO/dark (b)
Bi-ZnO (c)
Au-ZnO (d)
Commerical ZnO (f)
TiO2-P25 (g)
Bare ZnO (e)
Fig. 8. Photodegradability of AR 18; [AR 18] = 5 104 M, catalyst suspended = 4 gL1, pH = 11, airflow rate = 8.1 mL s1, Isolar = (1250 100) ± 100 lx.
0.0 200
55.3 (curve c), 92.0 (curve d), 71.8 (curve e), 93.8 (curve f) and 24.7% (curve g) degradations occurred, respectively. This clearly exhibits that the solar/Bi–Au–ZnO catalyst is most efficient in AR 18 degradation than other prepared and commercial catalysts. Since the degradation was effective with Bi–Au–ZnO, the influence
500
800
Wavelength (nm) Fig. 9. The changes in UV–vis spectra of AR 18 on irradiation with Bi–Au–ZnO under solar light, [AR 18] = 5 104 M; pH = 11; Bi–Au–ZnO suspended = 4 g L1; airflow rate = 8.1 mL s1, Isolar = (1250 100) ± 100 lx, (a) 0 min, (b) 30 min, (c) 60 min, (d) 90 min, (e) 120 min and (f) 150 min.
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0.0096, 0.0096, 0.0130, 0.0193 and 0.0109 min1, 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. It is well known than at acidic pH the removal efficiency is low and it is due to dissolution of ZnO material. Since the photocatalytic efficiency depends on the adsorption of the dye on the surface of the catalyst, the adsorption of the dye under different pH values was investigated. 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%, respectively after attainment of adsorption equilibrium (30 min). As the adsorption is high at pH 11, the degradation is also efficient at this pH. Effect of catalyst loading Catalyst loading in slurry photocatalytic processes is a significant factor that can strongly manipulate dye degradation. Experiments performed with different amounts of Bi–Au–ZnO showed that the photodegradation efficiency increased with an increase in amount up to 4 g L1 and then slightly decreased (Fig. S2, see Supplementary data). The pseudo-first order rate constants are 0.0086, 0.0105, 0.0143, 0.0193, and 0.0098 min1 for Bi–Au–ZnO at catalyst loadings of 1, 2, 3, 4 and 5 g L1, respectively. Enhancement of removal rate is due to (i) The increase in the amount of catalyst which increases the number of dye molecules adsorbed. (ii) The increase in the density of catalyst particles in the area of illumination. The decrease in the removal efficiency of AR 18 at higher amount (above 4 g L1) is due to the light reflectance by catalyst particles. Similar results have been reported for the photodegradation of dyes by ZnO [32,33]. Effect of initial dye concentration The effect of various initial dye concentrations on the degradation of AR 18 on Bi–Au–ZnO surface has been investigated. Increase of dye concentration from 2 to 6 104 M decreases the rate constant from 0.0288 to 0.0077 min1 in solar light (Fig. S3, see Supplementary data). The rate of degradation relates to the OH (hydroxyl radical) formation on catalyst surface and probability of OH (hydroxyl radical) reacting with dye molecule. Catalyst reusability Heterogeneous catalyst is used for any type of degradation as it has the main advantage of reusability. The reusability of Bi–Au– ZnO was tested by four successive cycles for the degradation of AR 18 under solar light and the results are shown in Fig. 10. Although the degradation efficiency of Bi–Au–ZnO is slightly decreased after each cycle, the catalyst exhibited 93% activity at fourth cycle under the solar irradiation. These results clearly exhibit that the prepared catalyst is found to be reusable.
100
% of AR 18 remaining
36
80
60
40
20
0 0
150
I Run
IV Run
III Run
II Run
Fig. 10. Catalyst reusability: pH = 11, 4 wt% Bi–Au–ZnO suspended = 4 g L1, airflow rate = 8.1 mL s1, Isolar = (1250 100) ± 100 lx.
Table 2 COD measurements. Time (min)
% COD reduction
60 120 150
37.9 79.6 98.7
[AR 18] = 5 104 M; 4 wt% Bi–Au–ZnO suspended = 4 g L1; pH = 11; airflow rate = 8.1 mL s1; Isolar = (1250 100) ± 100 lx.
composite (Scheme 1). The conduction band of Bi2O3 is lower than that of ZnO so, it can act as a sink for photogenerated electrons in the mixed semiconductor. Thus, photoinduced electrons on the ZnO surface could transfer to Bi2O3 via interface. 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 holes in the ZnO surface, resulting in enhanced separation efficiency for photogenerated electron and holes, which would have a positive effect on the photocatalytic performance [34]. In addition to that, the presence of ‘‘Au’’ traps the electron from both
O2• Solar light
–
e-
CB hν
O2•
O2
ZnO
–
Au
Solar light
e-
COD analysis To confirm the mineralization of AR 18, the degradation was also analyzed by COD measurements under solar light. The % of COD reductions are given in Table 2. After 150 min irradiation with Bi–Au–ZnO, 98.7% of COD reduction was obtained. This indicates the mineralization of AR 18 dye.
600
450
300
Time (min)
CB hν
3.2 eV Bi2O3 2.8 eV
h
VB •
+
h + VB
OH H2O
Mechanism of degradation On the basis of the above results, a mechanism can be proposed to explain the enhanced solar photocatalytic activity of Bi–Au–ZnO
•
Dye / Dye* + HO
Mineral acids + CO2
•– Dye / Dye* + O2
Mineral acids + CO2
+ H2O + H2O
Scheme 1. Mechanism of degradation of AR 18 by Bi–Au–ZnO.
A. Senthilraja et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 31–37
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 [35]. The trapped electrons produce large numbers of superoxide radical anion (O 2 ) 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 AR 18 dye. Conclusions In summary, Bi–Au–ZnO photocatalyst was successfully synthesized by a precipitation–decomposition method. The highest photocatalytic activity of Bi–Au–ZnO is due to the combined effect of efficient electron–hole separation and superior textural properties compared to the undoped ZnO. COD measurements confirm the complete mineralization of AR 18 molecule. The studies described herein demonstrate that Bi–Au–ZnO nanoparticles can be utilized for efficient sunlight-driven photocatalytic application. Thus, an ideal construction of Bi–Au–ZnO composites could be employed not only in photocatalysis, but also used in effluent treatment as well as in highly toxic chemicals degradation. Acknowledgements One of the authors (M. Swaminathan) is thankful to CSIR, New Delhi, India for financial support through research Grant No. 21 (0799)/10/EMR-II. One of the authors (M. Shanthi) is highly thankful to UGC, New Delhi, India for financial support through research project F. No. 41-288/2012 (SR).
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