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Uniform deposition of silver dots on sheet like BiVO4 nanomaterials for efficient visible light active photocatalyst towards methylene blue degradation
Journal Pre-proofs Uniform deposition of silver dots on sheet like BiVO4 nanomaterials for efficient visible light active photocatalyst towards methylene blue degradation K.R. Basavalingaiah, Udayabhanu, S. Harishkumar, G. Nagaraju, Chikkahanumantharayappa PII: DOI: Reference:
S2452-2627(19)30089-3 https://doi.org/10.1016/j.flatc.2019.100142 FLATC 100142
To appear in:
FlatChem
Received Date: Revised Date: Accepted Date:
9 April 2019 25 September 2019 30 October 2019
Please cite this article as: K.R. Basavalingaiah, Udayabhanu, S. Harishkumar, G. Nagaraju, Chikkahanumantharayappa, Uniform deposition of silver dots on sheet like BiVO4 nanomaterials for efficient visible light active photocatalyst towards methylene blue degradation, FlatChem (2019), doi: https://doi.org/ 10.1016/j.flatc.2019.100142
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Uniform deposition of silver dots on sheet like BiVO4 nanomaterials for efficient visible light active photocatalyst towards methylene blue degradation Basavalingaiah K Ra,b, Udayabhanuc, Harishkumar Sc,d, Nagaraju Gc, Chikkahanumantharayappaa* aDepartment
of Physics, Vivekananda degree college, Bengaluru, Karnataka, India-560055 of Science, Government Polytechnic, Tumkur, Karnataka, India-572103 cEnergy Materials Research Laboratory, Department of Chemistry, Siddaganga Institute of Technology, Tumkur, Karnataka, India-572103 dDepartment of Pharmaceutical Chemistry, Kuvempu University, Post-Graduate Centre, Kadur, Karnataka, India-572103 bDepartment
Corresponding Email: [email protected] Abstract BiVO4 and Ag-BiVO4 Nanoparticles were prepared using Azadirachta indica gum as a fuel via solution combustion synthesis (SCS) at 500 ºC. From the PXRD, FTIR, UV-Visible DRS studies the synthesized NPs were characterized. The morphologies of the prepared NPs were studied by SEM and TEM analysis. The synthesized NPs were tested for photocatalytic and photoluminescence studies. The PXRD data indicated that the synthesized nanoparticles belong to monoclinic phase structure. The SEM data revealed that bead like structure were obtained. BiVO4 and Ag-BiVO4 Nps were taken to determine the photocatalytic activity on methylene blue dye. The results indicated that Ag-BiVO4 NPs exhibited promising photocatalytic activity due to the occurrence of Ag particles on the BiVO4 material, which makes the catalyst more sensitive and reduce the electron-hole recombination. Furthermore, the photoluminescence study reveals that Ag-BiVO4 nano particles shown blue light emission.
Keywords: Green synthesis, Azadirachta indica, Ag-BiVO4, Photocatalytic, Photoluminescence.
1. Introduction Dye impurities from printing, textiles, production and many other industries play an important role in environment polluting. These wastes enter the aquatic ecosystem and cause a huge environmental and health risks. Coagulation, adsorption, osmotic pressure, etc. have been used to remove dyes from the river, but each method has unique advantages and limitations. Photocatalytic treatment provides a relatively inexpensive and eco-friendly way to solve this problem [1]. In recent years, BiVO4 has been used most often for the degradation of many organic pollutants [2]. Nanoscale semiconductors have been reported as an effective photocatalytic agent, because of its large surface area or the quantum confinement effects of charge carriers on the degradation of organic pollutants in water under ultraviolet light and sunlight [3-5]. When the photocatalyst opens to light with energy that is higher than its bandgap energy, the electron-hole pair generated spreads on the surface of the photocatalyst. These reactive intermediates can participate in chemical reactions with electron acceptors and donors. These free holes and electrons convert adjacent water and oxygen molecules to OH. (Hydroxyl radicals) and superoxide radicals (O22-.), and this process is transformed into a powerful oxidizing agent for organic color degeneration [6]. Among the various metallic vanadate nanoparticles, the extensive effort to synthesize and characterize BiVO4 because of its unique characteristics of ferroelasticity, optical, and conductivity. It is used as a solid-state electrolyte, gas sensor, and electrodes for batteries. It is also used for decomposition of water, organic pollutants and O2 evolutionary degradation [7-10]. Nowadays, BiVO4 is considered to be the most visible light-driven photocatalysts. The monoclinic BiVO4 medium band gap (2.4 EV) displays excellent photodegradation for pollutants such as organic dyes with the presence of visible light radiation [11]. Various methods are
employed for the preparation of BiVO4 nanoparticles, such as hydrothermal, direct precipitation, sol–gel and solvothermal methods [12-15]. These methods need costliest starting material, lengthy procedures, sophisticated apparatus and more time required. But the neem gum assisted combustion method was constructed as it is an easier, less energy and time consuming method than among all the methods. The major advantages of green synthesis can be easily reduce or zero pollution to the globe and useful materials can be synthesized easily, eco-friendly and reasonable quantities. There are several methods were reported for the synthesis and electrochemical properties of metal vanadate nanoparticles. Nevertheless, we have chosen a convenient thermal decomposition method to preparation BiVO4 NPs, and investigated the photocatalytic activity of nanoparticles. Neem Gum, a distinctive plant gum that originates from the tree (Melia Azidirachta, maliaceae) is a complex polysaccharide acid salt. It has been medicinally used in India for many centuries [16]. Neem gum on hydrolysis yields D-galactose, D-glucuronic acid, L-fucose and L-arabinose (Fig.1.). The aldobiuronic acid is the ingredient of neem gum derived from graded hydrolysis is shown to be 4-O-(D-glucopyranosyluronic acid)-D-galactopyranose. The synthesized BiVO4 and Ag-BiVO4 nanoparticles exhibit sheet like structure and Ag was distributed uniformly on sheet. BiVO4 and Ag-BiVO4 nanoparticles can be considered as a promising photocatalyst over Methylene blue dye [17-22]. The present work reports the synthesis of BiVO4 and Ag-BiVO4 NPs using neem gum as fuel. The prepared NPs were characterized using PXRD (powder X-ray diffraction), FTIR (Fourier transform infrared analysis), UVDRS (UV-diffused reflectant spectrum), SEM (scanning electron microscopy) and TEM (transmission electron microscopy) of the BiVO4 NPs.
Photocatalytic activity of BiVO4 was examined by photodegradation of methylene blue (MB) under visible light illumination and photoluminescence (PL) study was also carried out. OH
O
O OH
OH
OH OH
HO
OH
HO
L-arabinose
OH
L-fructose HO
HO
O
OH HO
O
H OH
HO
O HO
D-Galactose
OH
HO
OH
D-glucuronic acid
Fig.1. Major compounds present in Neem gum 2. Experimental 2.1. Synthesis of BiVO4 nanoparticles To synthesize BiVO4 and Ag-BiVO4 NPs through solution combustion method, bismuth nitrate, ammonium vanadate and silver nitrate were used as precursors and Neem gum was used as fuel. Stoichiometric ratios of bismuth nitrate and ammonium vanadate was mixed with different concentrations (120 and 240 mg) of neem gum. The obtained mixture was stirred until form a homogeneous solution. This mixture was taken in a crucible and transferred into preheated muffle furnace maintained at 500 ºC, the smoldering type of the combustion reaction occurs and nanocrystalline BiVO4 was formed in 5 minutes. The synthesized nanoparticles were named as (B1) for pure BiVO4 using 120 mg neem gum, (B2) for pure BiVO4 using 240 mg neem gum, For silver doping, the optimized fuel concentration i.e. 120 mg of neem gum. Silver was doped to BiVO4 at different molar concentrations and named 5 % Ag doped BiVO4 into AgB.
2.2. Characterization The synthesized nanoparticles were characterized by using powder X-ray diffractometer (Rigaku smart Lab). FTIR (Bruker-alpha) to analyze metal oxide bond stretching frequencies. The UVDRS spectrum of the NPs was measured using UV-Visible spectrophotometer (Lab IndiaDiffuse reflectance spectra and Cary 60 Agilent technologies-Absorbance spectra). The morphologies of the prepared nanoparticles were scanned using Scanning Electron Microscopy (Hitachi-7000 Table top). Shape and size of the BiVO4 crystallites was determined using transmission electron microscope (JEOL 3010). Photoluminescence studies were recorded using fluorescence spectrophotometer Agilent technology Cary Eclipse). 2.3. Photocatalytic activity Prepared BiVO4 and Ag doped BiVO4 nanoparticles were taken for degradation of methylene blue (MB) using 120 W light source for radiation. 5, 10, 15, and 20 mg of photocatalyst were added to 100 mL of various concentrations of MB solutions (5, 10, 15, and 20 ppm) in a HEBER photo reactor. The gap between the mercury lamp and the quartz tube containing pollutant was 6 cm. The uniform distribution of catalytic particles throughout the solution was achieved using air pump [23]. The solution was allowed for constantly bubbled in the dark for 30 minutes to ensure the organization of an adsorption-desorption equilibrium between the photocatalyst and MB previous to irradiation of light source. The suspension 2 mL was withdrawn from the reactor over 30 minutes intervals. Centrifuge the solution using spinwin microcentrifuger to remove the BiVO4 NPs from the mixture. The dye concentration of left over aqueous solution was measured using UV-Vis spectrophotometer at 664 nm. The % degradation of the MB is calculated using the equation (i)
Where Ci initial and Cf final dye concentrations in ppm. The photocatalytic experiment was repeated by changing different parameters such as concentration of dye, catalytic load, pH variation, catalyst recycling, etc. 3. Result and discussion 3.1. PXRD study The PXRD patterns of BiVO4 nanoparticles are represented in Fig.2. From the XRD data indicated that BiVO4 NPs has monoclinic phase structure resemble with the standard JCPDS card number 14-688 [24]. The XRD pattern of Ag-BiVO4 Nps displays an additional peak at 39° compared to undoped BiVO4 Nps corresponding to cubic phase and space group (Fm-3m) silver with standard JCPDS card number 4-783. No extra peaks were found in the pattern which indicates that purity of the samples. The average crystallite size was measured to be 21.9 nm for BiVO4 and 26.9 nm for Ag-BiVO4, respectively.
Ag * (JCPDS card No. 14-688)
Fig.2. Powder XRD patterns of the BiVO4 nanopowders prepared (B1) 120 mg (a), (B2) 240 mg (b) of Neem gum. 5% is (AgB) is (c) Ag-BiVO4 synthesized using 120mg of Neem gum. 3.2. FTIR Study The FTIR spectra of BiVO4 and Ag- BiVO4 nanomaterials were recorded between the ranges of 4000 to 400 cm-1 as shown in Fig 3. Additionally, characteristic bands of BiVO4, which include the V-O symmetric and asymmetric stretching vibrations at 736 cm-1 and 824 cm-1 were also observed [25]. The broad peak at 3429 and 1630 cm-1 is due to the stretching and bending vibrations of –OH, which is due to the absorbed moisture on the surface of the catalyst [26, 27].
BiVO4 Ag-BiVO4
75
50
1630
25
3 (VO ) 1 (VO )
0
4
824 736
3429
% Transmittance
100
4
3500
3000
2500
2000
1500
1000
500
Wavelength (nm) Fig.3. FTIR spectra of BiVO4 and Ag-BiVO4 NPs synthesized by Neem gum. 3.3. UV-Visible Diffuse reflectance studies: (UV-DRS) Fig. 4 shows the UV–Visible spectra of BiVO4 and Ag-BiVO4 NPs synthesized by Neem gum. The spectra demonstrate with absorption band shown nearly at 460 to 480 nm. Due to electron transfer from valence band to conduction band. The calculated Eg values were found to be 2.59 eV and 2.7 eV for BiVO4 and Ag-BiVO4 NPs respectively [28].
BiVO4 Ag-BiVO4
F(R)2
Reflectance (%)
BiVO4 Ag-BIvo4
2.59 eV 400
500
600
Wavelength (nm)
700
1.8
2.0
2.2
2.4
2.7 eV 2.6
2.8
Energy (eV)
Fig.4. UV-DRS spectrum of BiVO4 and Ag-BiVO4 NPs synthesized by Neem gum
3.0
3.4. Scanning Electron Microscopy (SEM) studies Fig. 5 (a, b, c, d) represents SEM images of the compound BiVO4. The particles are looking like a small bead like structures, which are formed by the agglomeration of NPs. The small pores also visible from the SEM images. These pores were created during the synthesis due to the escaping of gases at higher temperatures. The pores were uniformly formed and looks like a bead like structures.
(a)
(b)
(c)
(d)
(e)
Fig.5. SEM images of BiVO4 nanomaterials (a) and (b), and (c) and (d), EDX spectrum of Ag-BiVO4 (e).
3.5. Transmission Electron Microscopy (TEM) studies Figure 6 shows the TEM images, HR-TEM images and SAED pattern of synthesized Ag-BiVO4 NPs using Azardichta indica gum as a fuel by combustion method. From the TEM images (Fig. 6a,b,c) we can clearly observe the fine distribution of silver (Ag) on Sheet like BiVO4 nanomaterials. The distributed silver on BiVO4 acquired spherical shape with size of 5 to 15 nm. The sizes of BiVO4 sheets were gotten the height of about 144 nm and width of about 66 nm. HR-TEM images (Fig. 6d, e) give the clear information about the d-spacing values which are different for BiVO4 and Ag materials. For Ag, (Fig. 6d) the d-spacing value is 0.236 nm, which belongs to (111) plane. Presence of Ag on the BiVO4 material can be easily marked as a dark spots. For BiVO4, (Fig. 6e) the d-spacing value is 0.31 nm, which belongs to (121) plane. From the SAED pattern, (Fig. 6f) it is confirmed as a polycrystalline in nature due to the diffracted spots are combined together to form a fringes of circles and these circles are well matched with the d-spacing values calculated from XRD data. The planes at (110), (121) and (040) planes having highest intensity in XRD and which are matched with bright circular fringes in SAED pattern and HR-TEM images.
(a)
(d)
d= 0.236 (111) Ag
(b)
(c)
(e)
(e) (f)
d= 0.31 (121) BiVO4
110 121 040
Fig.6. TEM images (a,b,c), HR-TEM images (d,e) and SAED pattern(f) of Ag-BiVO4 nanomaterials
4. Photocatalytic degradation of BiVO4 and Ag-BiVO4 NPs Under light radiation, the semiconductor absorbs photons of energy greater than the band gap of semiconductors and then creates electrons and holes in the conduction and valence band. If the charge carriers don’t accompanied, then they’ll travel on the surface, where the free electrons form the reduction of oxygen and form the peroxides and superoxide radicals. The holes in the valance band oxidize the water and form the OH·; these generated species are extremely reactive, unstable and ultimately cause the degradation of organic dyes. Many factors are influencing the Photocatalytic action on dyes, i.e., band gap, surface area, crystallinity, phase composition, surface hydroxyl density, size distribution, morphology, and particle size of the photocatalyst [29]. Prepared BiVO4 NPs were taken as photocatalyst to check
the methylene blue dye degradation under UV-light. In every 30 minutes, 2mL of the aliquots sample solution was withdrawn and it was centrifuged and absorption of the samples was recorded. By measuring the change in intensity, we can calculate the degradation rate of dye at λ max-664 nm. The Ag dopant was found to be optimum concentration. When the % of Ag higher than the optimal level, the rate of dye degradation was decreased due to metallic Ag act as recombination centers which is caused by the electrostatic attraction of negatively charged electrons on Ag to positively charged holes. Furthermore excessive Ag reduces the number of photons absorbed by the photocatalyst because of light-filtered effect [30]. In addition to this, Ag acts as electron acceptor, which reduce the electron-hole recombination and hence increases the photocatalytic activity. 4.1. Effect of Dye Concentration The concentration of the dye is playing a crucial role in photocatalytic activity of BiVO4 nanomaterials. Hence, the optimum concentration of dye for promising photocatalytic activity was carried out by changing the dye concentration of 5-20 ppm under catalytic loading constant (100 mg) and UV light as in Fig.7. This clearly shows that as the dye concentration of 5-20 ppm increases, the photocatalytic degradation is reduced by 80-20% and 5 ppm is the most effective concentration for dye degradation. Generally, as increases the dye concentration, the number of dye molecules is adsorbed on the surface of BiVO4 nanoparticles, so the degradation rate decreases. Higher the dye concentration, the smaller the penetration power of light, which means that the photocatalytic decomposition is less at higher concentrations with less occurrence of hydroxyl groups and superoxide radicals [31].
5ppm 10ppm 15ppm 20ppm
100
% Degradation
80
60
40
20
0 120
90
60
30
0
Time(min)
Fig.7. Degradation MB with varying concentration of dye and constant catalyst load
4.2. Effect of Catalytic Load Figure 8 represents the graph of MB degradation with varying catalyst load (10-20 mg) by maintaining the constant concentration of dye (100 mL of 5 ppm). The data clearly reveals that as the catalyst load increases over 180 minutes, the rate of dye degradation increases from 80% to 100%. This is because more active sites are available due to increased catalytic load [31]. 10mg 20mg 30mg 40mg 50mg
100
% degradation
80
60
40
20
0 0
30
60
90
120
Time (min)
Fig.8. Degradation of Methylene Blue with varying catalyst load and constant dye concentration
4.3. Effect of pH In order to see the optimum pH for the photolysis of the MB dye, experiments were conducted at completely different pH (2 to 12) by maintaining the constant catalyst (100 mg) and the dye concentration (5 ppm), and the result represented in Figure 9. This clearly shows that the MB degradation was effective in the basic medium [32, 33] with the very best degradation rate at pH 12. The presence of an large number of OH- ions on the surface of the catalyst can produced less range of OH• radicals, that act as primary oxidizing agents and are responsible for the degradation of MB dye [34-38].
PH02 PH04 PH06 PH08 PH10 PH12
100
% degradation
80
60
40
20
0 0
30
60
90
120
Time(min)
Fig.9. Degradation of MB with varying pH of the solution and keeping the catalyst load and constant dye constant 4.4. Kinetics of photocatalytic degradation The obtained results of photocatalytic activity of BiVO4 and Ag-BiVO4 NPs were explained through kinetic study using Langmuir–Hinshelwood model. The photodegradation of MB dye as presented in the following relation ln(C0/Ct) = kt ------------------(ii)
where, C0 is the initial concentration of dye (mg/L) at t = 0, Ct is the concentration after irradiation in the selected time interval (30 min), k is the rate constant. The plots of ln(Co/Ct) as a function of reaction time which is approximate. From Fig 10, the slopes of the straight lines from the plot of ln(Co/Ct) vs reaction time are the rate constants (k) for photocatalytic degradation of MB for BiVO4 and Ag-BiVO4 NPs. The rate constant k has the highest value for Ag-BiVO4 NPs was 0.7 × 10−2 min−1 compared to BiVO4 which was 0.48 × 10−2 min−1 under visible light illumination [39].
Fig.10. Photocatalytic kinetic studies of BiVO4 and Ag-BiVO4 NPs 4.4. Catalyst Recycling To evaluate the stability of the photocatalyst, a recycling experiment was carried out to degradation the methylene blue dye (Fig. 11). The experiment was carried out with 100 mg of catalyst and 100 mL of 5 ppm dye. The decomposition potency of MB was nearly an equivalent for six cycles. This figure clearly shows a reduction in potency of nearly 80% altogether six cycles.
100
% Degradation
75
50
25
0 1
2
3
4
5
Recycling no.
Fig.11. Recycling of 10 mg of catalyst and 100 ml of 5 ppm dye 4.5. Effect of Scavengers Photocatalytic reaction is mainly involved in charge separation, photoinduced species and surface redox process. To build the photocatalytic mechanism by nanoparticles for the degradation of organic dyes, it is important to identify which reactive species are mainly involved to break down the organic molecules. In the time of methylene blue photodegradation with Ag-BiVO4 nanoparticles, the O2•-, HO•, electrons (e-) and h+ are produced and these can be trapped by the addition of radical scavengers like benzoquinone [40], benzoic acid [41], AgNO3 [42] and EDTA [43] respectively, into the degradation reaction. The percentage of degradation of dye in the absence and presence of scavengers are shown in Figure 12. These result reveals that the percentage of dye degradation was suppressed by the addition of h+, O2•- and HO• scavengers compared to without addition of scavengers. However, the addition of e- scavenger increases the degradation efficiency by the consumption of photoinduced electrons could which successively increases the e-/h+ pairs separation efficiency. These results confirmed that the degradation of methylene blue decreases by adding h+, O2•- and HO• scavengers meanwhile degradation efficiency was increases by adding e- scavenger.
100
% of Degradation
No Scavenger
AgNO3
80
60
Benzoquinone EDTA
40
Benzoic acid
20
0 1
2
3
4
5
Different Scavengers
Fig.12. Effect of different scavengers during the degradation of MB dye. 4.6. Mechanism. Scheme: A schematic representation of the degradation of MB using BiVO4 Nps is shown below. During irradiation, there is a transition of an electron from valence band (VB) to conduction band (CB) of the material and generating e−/h+ pairs. These e−/h+ pairs are more advantageous during degradation of dye, if which cannot be recombined easily. In a VB, the holes (h+) generate OH. radicals by oxidizing the water molecules; these generated species are unstable and highly reactive [44-48]. In the conduction band electron (e−) reducing the oxygen molecule to O2·− radicals (super oxide oxygen radicals) are degrade the organic dye molecules into CO2, H2O and inorganic mineral acids. BiVO4 + hν → BiVO4 (h+vb + e-cb) OH−ads + h+vb → OH•ads (in basic medium) MB + OH•ads → dye degradation As it is well-known, Ag nanoparticles can trap the excited electrons from BiVO4 nanoparticles and leave the holes for the degradation reaction of organic pollutants, improving the charge carrier separation [49].
Visible light E[NHE](eV) Bi-Ag -1.0 H2
/H+
CB = -0.79
0
MB*
O2
Ag
Ag
2.3eV
O2
CB .O 2
.O 2
+1.0 VB = 1.51
+2.0
MB
.OH
MB
H2 O O2/.O2-(-0.28eV) .OH/H O (+2.27eV) 2
2H2O + 4 h+ = 4H+ + O2
VB
Degradation product
BiVO4
Electrons migrated from CB of BiVO4 Holes left in VB of BiVO4
Mechanism.1. Graphical representation for the mechanism of BiVO4 under the irradiation of UV light 4.7. Detection of OH· Radicals OH• radicals are the most reactive species throughout photocatalytic degradation reactions. The rate of OH• formation and detection can be measured with straightforward, sensitive and quick PL (photoluminescence) techniques using coumarin as the probe molecule. OH• reacts with coumarin to produce a fluorescent compound 7-hydroxyl coumarin at 456 nm. In this method, BiVO4 (130 mg) was dispersed in 1 mM coumarin (50 mL) aqueous solution was taken in borosil jar. The solution mixture was allowed to stand for 10 minutes to attain adsorption-desorption equilibrium between BiVO4, water and coumarin before irradiation. This reaction was irradiated with ultraviolet light of 60 W/m2 as a light source. Aliquots of 2 mL were collected each ten minutes and the PL spectra were measured using an Agilent Technologies Cary Eclipse spectrophotometer. Figure 12 shows that the PL intensity at 531 nm will increase linearly with time and clearly indicates the formation of OH. is directly proportional to the
irradiation time [50]. It shows that formation of OH• increases with an increase in time. This OH• is liable for the decomposition of organic dyes.
PL intensity (a.u.)
Coumarin 10 min 20 min 30 min 40 min
400
500
600
700
Wavelength Fig.13. Concentration of hydroxyl radicals against irradiation time Cyclic voltammetry (CV) Cyclic voltammetry (CV) experiments were carried out in a three-electrode system with SCE as the reference electrode, Platinum (Pt) wire as the counter electrode and surface modified glassy carbon as a working electrode at a scan rate of 50 mV/s. The electrolyte used was a mixture of 0.25 mM K3[Fe(CN)6]/K4[Fe(CN)6] in a 1:1 molar ratio with 0.1 M KCl solution. Figure shows clear anodic and cathodic peaks for each sample. The peak at positive potentials on the anodic (forward) sweep around 0.25 V vs. SCE represents the oxidation of ferrocyanide to ferricyanide with the loss of one electron. From the figure 14, it is clear that anodic and cathodic peak intensities are enhanced in case of Ag-BiVO4 modified glassy carbon electrode than that of BiVO4 modified glassy carbon electrode, from this it is cleared that the rate of electron transfer is more in case of Ag/BiVO4 composite than bare BiVO4 nanoparticles.
BiVO4 Ag-BiVO4
Current (mA)
0.18
0.00
-0.18
-0.36 -0.5
0.0
0.5
1.0
Potential (V)
Fig.14. Cyclic voltammograms of BiVO4 and Ag-BiVO4 nanoparticles in 0.25 mM K3[Fe(CN)6]/K4[Fe(CN)6] in 0.1 M KCl solution. 50 mVs-1scan rate was used. Photoluminescence Studies PL study is one among the helpful technique for notice the potency of charge carrier separation within the semiconductor [51]. The PL emission spectrum of BiVO4 and Ag-BiVO4 NPs were recorded in room temperature with an excitation wavelength of 260 nm represented in Fig.15b. The pure BiVO4 Nps provides a strong ultraviolet emission peak at 531 nm and weak emission peak at 487 nm. In case of Ag-BiVO4 Nps, the obtained PL emission intensity decreased and the resultant datas are in good agreement with the Stern-Volmer quenching. This indicates the efficiency of high charge carrier separation in Ag-BiVO4 NPS. The emission peaks in visible region can be ascribed to bound excitons and defect states positioned at surface of nanostructured. The PL excitation spectrum with emission at 531 nm [52] is represented in Fig. 15a. Chromaticity coordinates are used to find luminous coloured objects and it can be estimated by Commission International De I’Eclairage (CIE) system. CIE chromaticity diagram
of BiVO4 and Ag- BiVO4 Nps represented in Fig. 15c reveals that both the material emit blue light region.
(b)
240
250
260
270
280
290
300
350
400
Wavelength (nm)
0.9
BiVO4
487 nm
PL intensity (a.u.)
PL intensity (a.u.)
230
Ag-BiVO4
531 nm
emi = 531 nm
260 nm
350 nm
(a)
450
500
550
600
Wavelength (nm)
(c)
Ag-BiVO4 Nanoparticles exi= 260 nm
Y
0.6
0.3
0.0 0.0
0.2
Compounds
X
Y
BiVO4
0.18524
0.29049
Ag-BiVO4
0.19287
0.35192
0.4
0.6
0.8
X
Fig.15. PL spectrum of BiVO4 and Ag- BiVO4 (a) excitation spectra (b) emission spectra (c) CIE diagram
(c) CIE diagram
650
Conclusion In this study, BiVO4 and Ag-BiVO4 nanoparticles were prepared by employing solution combustion method using neem gum as a fuel. The XRD pattern shows that BiVO4 and AgBiVO4 nanoparticles belong to a monoclinic phase with an average crystallite size of 21.9 nm and 26.5 nm respectively. The SEM image clearly shows that the morphology depends heavily on the pathway to add the green surfactant. The TEM image shows that the particles are virtually agglomerated and crystalline. The prepared BiVO4 and Ag-BiVO4 NPs display promising photocatalytic activity against the photo degradation of methylene blue. The photoluminescence study reveals that Ag-BiVO4 nano particles shown blue emission.
Acknowledgement Mr. Udayabhanu gratefully acknowledge to CSIR-SRF (09/1204(0001)2018-EMR-I). Dr. GN thanks to DST-Nanomission (SR/NM/NS-1262/2013) for financial support. Mr. Basavalingaiah thanks SIT for providing lab facilities.
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Graphical Abstract
OH
O
O
OH
OH
HO
OH
L-fructose HO O
H OH
HO
O HO
OH
HO
2.59 eV 1.8
2.0
2.2
2.4
2.7 eV 2.6
2.8
3.0
Energy (eV)
OH
D-glucuronic acid (110)
D-Galactose
(b)
(040) (002) (200) (150) (060) (222) (161)
OH HO
O
(121)
L-arabinose
HO
BiVO4 Ag-BIvo4
OH
HO
F(R)2
OH
Intensity (a.u)
OH
20
30
40
(a)
50
60
2 (Degree)
BiVO4 NPs
Visible light E[NHE](eV) Bi-Ag -1.0
CB = -0.79
H2/H+ 0
MB*
O2
Ag
Ag
O2
CB
2.3eV
.O 2
.O 2
+1.0 VB = 1.51
+2.0
MB
.OH
MB
H2 O O2/.O2-(-0.28eV) .OH/H O (+2.27eV) 2
2H2O + 4 h+ = 4H+ + O2
Degradation product
VB BiVO4
Electrons migrated from CB of BiVO4 Holes left in VB of BiVO4 0.9
Ag-BiVO4 Nanoparticles exi= 260 nm
Y
0.6
0.3
0.0 0.0
0.2
Compounds
X
Y
BiVO4
0.18524
0.29049
Ag-BiVO4
0.19287
0.35192
0.4
0.6
0.8
X
54.
Research highlights
For the first time Neem gum assisted BiVO4 and Ag-BiVO4 nanoparticles were prepared.
The morphology of the obtained nanostructures resembles the bead like structure. And the fine distributions of silver (Ag) on Sheet like BiVO4 nanomaterials. Super structured BiVO4 NPs displays strong blue emission for UV excitation. Synthesized BiVO4 NPs exhibited good photocatalytic properties on Methylene Blue dye over 180 minutes, the rate of dye degradation increases from 80% to 100%.
55.
S2452-2627(19)30089-3 https://doi.org/10.1016/j.flatc.2019.100142 FLATC 100142
To appear in:
FlatChem
Received Date: Revised Date: Accepted Date:
9 April 2019 25 September 2019 30 October 2019
Please cite this article as: K.R. Basavalingaiah, Udayabhanu, S. Harishkumar, G. Nagaraju, Chikkahanumantharayappa, Uniform deposition of silver dots on sheet like BiVO4 nanomaterials for efficient visible light active photocatalyst towards methylene blue degradation, FlatChem (2019), doi: https://doi.org/ 10.1016/j.flatc.2019.100142
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© 2019 Published by Elsevier B.V.
Uniform deposition of silver dots on sheet like BiVO4 nanomaterials for efficient visible light active photocatalyst towards methylene blue degradation Basavalingaiah K Ra,b, Udayabhanuc, Harishkumar Sc,d, Nagaraju Gc, Chikkahanumantharayappaa* aDepartment
of Physics, Vivekananda degree college, Bengaluru, Karnataka, India-560055 of Science, Government Polytechnic, Tumkur, Karnataka, India-572103 cEnergy Materials Research Laboratory, Department of Chemistry, Siddaganga Institute of Technology, Tumkur, Karnataka, India-572103 dDepartment of Pharmaceutical Chemistry, Kuvempu University, Post-Graduate Centre, Kadur, Karnataka, India-572103 bDepartment
Corresponding Email: [email protected] Abstract BiVO4 and Ag-BiVO4 Nanoparticles were prepared using Azadirachta indica gum as a fuel via solution combustion synthesis (SCS) at 500 ºC. From the PXRD, FTIR, UV-Visible DRS studies the synthesized NPs were characterized. The morphologies of the prepared NPs were studied by SEM and TEM analysis. The synthesized NPs were tested for photocatalytic and photoluminescence studies. The PXRD data indicated that the synthesized nanoparticles belong to monoclinic phase structure. The SEM data revealed that bead like structure were obtained. BiVO4 and Ag-BiVO4 Nps were taken to determine the photocatalytic activity on methylene blue dye. The results indicated that Ag-BiVO4 NPs exhibited promising photocatalytic activity due to the occurrence of Ag particles on the BiVO4 material, which makes the catalyst more sensitive and reduce the electron-hole recombination. Furthermore, the photoluminescence study reveals that Ag-BiVO4 nano particles shown blue light emission.
Keywords: Green synthesis, Azadirachta indica, Ag-BiVO4, Photocatalytic, Photoluminescence.
1. Introduction Dye impurities from printing, textiles, production and many other industries play an important role in environment polluting. These wastes enter the aquatic ecosystem and cause a huge environmental and health risks. Coagulation, adsorption, osmotic pressure, etc. have been used to remove dyes from the river, but each method has unique advantages and limitations. Photocatalytic treatment provides a relatively inexpensive and eco-friendly way to solve this problem [1]. In recent years, BiVO4 has been used most often for the degradation of many organic pollutants [2]. Nanoscale semiconductors have been reported as an effective photocatalytic agent, because of its large surface area or the quantum confinement effects of charge carriers on the degradation of organic pollutants in water under ultraviolet light and sunlight [3-5]. When the photocatalyst opens to light with energy that is higher than its bandgap energy, the electron-hole pair generated spreads on the surface of the photocatalyst. These reactive intermediates can participate in chemical reactions with electron acceptors and donors. These free holes and electrons convert adjacent water and oxygen molecules to OH. (Hydroxyl radicals) and superoxide radicals (O22-.), and this process is transformed into a powerful oxidizing agent for organic color degeneration [6]. Among the various metallic vanadate nanoparticles, the extensive effort to synthesize and characterize BiVO4 because of its unique characteristics of ferroelasticity, optical, and conductivity. It is used as a solid-state electrolyte, gas sensor, and electrodes for batteries. It is also used for decomposition of water, organic pollutants and O2 evolutionary degradation [7-10]. Nowadays, BiVO4 is considered to be the most visible light-driven photocatalysts. The monoclinic BiVO4 medium band gap (2.4 EV) displays excellent photodegradation for pollutants such as organic dyes with the presence of visible light radiation [11]. Various methods are
employed for the preparation of BiVO4 nanoparticles, such as hydrothermal, direct precipitation, sol–gel and solvothermal methods [12-15]. These methods need costliest starting material, lengthy procedures, sophisticated apparatus and more time required. But the neem gum assisted combustion method was constructed as it is an easier, less energy and time consuming method than among all the methods. The major advantages of green synthesis can be easily reduce or zero pollution to the globe and useful materials can be synthesized easily, eco-friendly and reasonable quantities. There are several methods were reported for the synthesis and electrochemical properties of metal vanadate nanoparticles. Nevertheless, we have chosen a convenient thermal decomposition method to preparation BiVO4 NPs, and investigated the photocatalytic activity of nanoparticles. Neem Gum, a distinctive plant gum that originates from the tree (Melia Azidirachta, maliaceae) is a complex polysaccharide acid salt. It has been medicinally used in India for many centuries [16]. Neem gum on hydrolysis yields D-galactose, D-glucuronic acid, L-fucose and L-arabinose (Fig.1.). The aldobiuronic acid is the ingredient of neem gum derived from graded hydrolysis is shown to be 4-O-(D-glucopyranosyluronic acid)-D-galactopyranose. The synthesized BiVO4 and Ag-BiVO4 nanoparticles exhibit sheet like structure and Ag was distributed uniformly on sheet. BiVO4 and Ag-BiVO4 nanoparticles can be considered as a promising photocatalyst over Methylene blue dye [17-22]. The present work reports the synthesis of BiVO4 and Ag-BiVO4 NPs using neem gum as fuel. The prepared NPs were characterized using PXRD (powder X-ray diffraction), FTIR (Fourier transform infrared analysis), UVDRS (UV-diffused reflectant spectrum), SEM (scanning electron microscopy) and TEM (transmission electron microscopy) of the BiVO4 NPs.
Photocatalytic activity of BiVO4 was examined by photodegradation of methylene blue (MB) under visible light illumination and photoluminescence (PL) study was also carried out. OH
O
O OH
OH
OH OH
HO
OH
HO
L-arabinose
OH
L-fructose HO
HO
O
OH HO
O
H OH
HO
O HO
D-Galactose
OH
HO
OH
D-glucuronic acid
Fig.1. Major compounds present in Neem gum 2. Experimental 2.1. Synthesis of BiVO4 nanoparticles To synthesize BiVO4 and Ag-BiVO4 NPs through solution combustion method, bismuth nitrate, ammonium vanadate and silver nitrate were used as precursors and Neem gum was used as fuel. Stoichiometric ratios of bismuth nitrate and ammonium vanadate was mixed with different concentrations (120 and 240 mg) of neem gum. The obtained mixture was stirred until form a homogeneous solution. This mixture was taken in a crucible and transferred into preheated muffle furnace maintained at 500 ºC, the smoldering type of the combustion reaction occurs and nanocrystalline BiVO4 was formed in 5 minutes. The synthesized nanoparticles were named as (B1) for pure BiVO4 using 120 mg neem gum, (B2) for pure BiVO4 using 240 mg neem gum, For silver doping, the optimized fuel concentration i.e. 120 mg of neem gum. Silver was doped to BiVO4 at different molar concentrations and named 5 % Ag doped BiVO4 into AgB.
2.2. Characterization The synthesized nanoparticles were characterized by using powder X-ray diffractometer (Rigaku smart Lab). FTIR (Bruker-alpha) to analyze metal oxide bond stretching frequencies. The UVDRS spectrum of the NPs was measured using UV-Visible spectrophotometer (Lab IndiaDiffuse reflectance spectra and Cary 60 Agilent technologies-Absorbance spectra). The morphologies of the prepared nanoparticles were scanned using Scanning Electron Microscopy (Hitachi-7000 Table top). Shape and size of the BiVO4 crystallites was determined using transmission electron microscope (JEOL 3010). Photoluminescence studies were recorded using fluorescence spectrophotometer Agilent technology Cary Eclipse). 2.3. Photocatalytic activity Prepared BiVO4 and Ag doped BiVO4 nanoparticles were taken for degradation of methylene blue (MB) using 120 W light source for radiation. 5, 10, 15, and 20 mg of photocatalyst were added to 100 mL of various concentrations of MB solutions (5, 10, 15, and 20 ppm) in a HEBER photo reactor. The gap between the mercury lamp and the quartz tube containing pollutant was 6 cm. The uniform distribution of catalytic particles throughout the solution was achieved using air pump [23]. The solution was allowed for constantly bubbled in the dark for 30 minutes to ensure the organization of an adsorption-desorption equilibrium between the photocatalyst and MB previous to irradiation of light source. The suspension 2 mL was withdrawn from the reactor over 30 minutes intervals. Centrifuge the solution using spinwin microcentrifuger to remove the BiVO4 NPs from the mixture. The dye concentration of left over aqueous solution was measured using UV-Vis spectrophotometer at 664 nm. The % degradation of the MB is calculated using the equation (i)
Where Ci initial and Cf final dye concentrations in ppm. The photocatalytic experiment was repeated by changing different parameters such as concentration of dye, catalytic load, pH variation, catalyst recycling, etc. 3. Result and discussion 3.1. PXRD study The PXRD patterns of BiVO4 nanoparticles are represented in Fig.2. From the XRD data indicated that BiVO4 NPs has monoclinic phase structure resemble with the standard JCPDS card number 14-688 [24]. The XRD pattern of Ag-BiVO4 Nps displays an additional peak at 39° compared to undoped BiVO4 Nps corresponding to cubic phase and space group (Fm-3m) silver with standard JCPDS card number 4-783. No extra peaks were found in the pattern which indicates that purity of the samples. The average crystallite size was measured to be 21.9 nm for BiVO4 and 26.9 nm for Ag-BiVO4, respectively.
Ag * (JCPDS card No. 14-688)
Fig.2. Powder XRD patterns of the BiVO4 nanopowders prepared (B1) 120 mg (a), (B2) 240 mg (b) of Neem gum. 5% is (AgB) is (c) Ag-BiVO4 synthesized using 120mg of Neem gum. 3.2. FTIR Study The FTIR spectra of BiVO4 and Ag- BiVO4 nanomaterials were recorded between the ranges of 4000 to 400 cm-1 as shown in Fig 3. Additionally, characteristic bands of BiVO4, which include the V-O symmetric and asymmetric stretching vibrations at 736 cm-1 and 824 cm-1 were also observed [25]. The broad peak at 3429 and 1630 cm-1 is due to the stretching and bending vibrations of –OH, which is due to the absorbed moisture on the surface of the catalyst [26, 27].
BiVO4 Ag-BiVO4
75
50
1630
25
3 (VO ) 1 (VO )
0
4
824 736
3429
% Transmittance
100
4
3500
3000
2500
2000
1500
1000
500
Wavelength (nm) Fig.3. FTIR spectra of BiVO4 and Ag-BiVO4 NPs synthesized by Neem gum. 3.3. UV-Visible Diffuse reflectance studies: (UV-DRS) Fig. 4 shows the UV–Visible spectra of BiVO4 and Ag-BiVO4 NPs synthesized by Neem gum. The spectra demonstrate with absorption band shown nearly at 460 to 480 nm. Due to electron transfer from valence band to conduction band. The calculated Eg values were found to be 2.59 eV and 2.7 eV for BiVO4 and Ag-BiVO4 NPs respectively [28].
BiVO4 Ag-BiVO4
F(R)2
Reflectance (%)
BiVO4 Ag-BIvo4
2.59 eV 400
500
600
Wavelength (nm)
700
1.8
2.0
2.2
2.4
2.7 eV 2.6
2.8
Energy (eV)
Fig.4. UV-DRS spectrum of BiVO4 and Ag-BiVO4 NPs synthesized by Neem gum
3.0
3.4. Scanning Electron Microscopy (SEM) studies Fig. 5 (a, b, c, d) represents SEM images of the compound BiVO4. The particles are looking like a small bead like structures, which are formed by the agglomeration of NPs. The small pores also visible from the SEM images. These pores were created during the synthesis due to the escaping of gases at higher temperatures. The pores were uniformly formed and looks like a bead like structures.
(a)
(b)
(c)
(d)
(e)
Fig.5. SEM images of BiVO4 nanomaterials (a) and (b), and (c) and (d), EDX spectrum of Ag-BiVO4 (e).
3.5. Transmission Electron Microscopy (TEM) studies Figure 6 shows the TEM images, HR-TEM images and SAED pattern of synthesized Ag-BiVO4 NPs using Azardichta indica gum as a fuel by combustion method. From the TEM images (Fig. 6a,b,c) we can clearly observe the fine distribution of silver (Ag) on Sheet like BiVO4 nanomaterials. The distributed silver on BiVO4 acquired spherical shape with size of 5 to 15 nm. The sizes of BiVO4 sheets were gotten the height of about 144 nm and width of about 66 nm. HR-TEM images (Fig. 6d, e) give the clear information about the d-spacing values which are different for BiVO4 and Ag materials. For Ag, (Fig. 6d) the d-spacing value is 0.236 nm, which belongs to (111) plane. Presence of Ag on the BiVO4 material can be easily marked as a dark spots. For BiVO4, (Fig. 6e) the d-spacing value is 0.31 nm, which belongs to (121) plane. From the SAED pattern, (Fig. 6f) it is confirmed as a polycrystalline in nature due to the diffracted spots are combined together to form a fringes of circles and these circles are well matched with the d-spacing values calculated from XRD data. The planes at (110), (121) and (040) planes having highest intensity in XRD and which are matched with bright circular fringes in SAED pattern and HR-TEM images.
(a)
(d)
d= 0.236 (111) Ag
(b)
(c)
(e)
(e) (f)
d= 0.31 (121) BiVO4
110 121 040
Fig.6. TEM images (a,b,c), HR-TEM images (d,e) and SAED pattern(f) of Ag-BiVO4 nanomaterials
4. Photocatalytic degradation of BiVO4 and Ag-BiVO4 NPs Under light radiation, the semiconductor absorbs photons of energy greater than the band gap of semiconductors and then creates electrons and holes in the conduction and valence band. If the charge carriers don’t accompanied, then they’ll travel on the surface, where the free electrons form the reduction of oxygen and form the peroxides and superoxide radicals. The holes in the valance band oxidize the water and form the OH·; these generated species are extremely reactive, unstable and ultimately cause the degradation of organic dyes. Many factors are influencing the Photocatalytic action on dyes, i.e., band gap, surface area, crystallinity, phase composition, surface hydroxyl density, size distribution, morphology, and particle size of the photocatalyst [29]. Prepared BiVO4 NPs were taken as photocatalyst to check
the methylene blue dye degradation under UV-light. In every 30 minutes, 2mL of the aliquots sample solution was withdrawn and it was centrifuged and absorption of the samples was recorded. By measuring the change in intensity, we can calculate the degradation rate of dye at λ max-664 nm. The Ag dopant was found to be optimum concentration. When the % of Ag higher than the optimal level, the rate of dye degradation was decreased due to metallic Ag act as recombination centers which is caused by the electrostatic attraction of negatively charged electrons on Ag to positively charged holes. Furthermore excessive Ag reduces the number of photons absorbed by the photocatalyst because of light-filtered effect [30]. In addition to this, Ag acts as electron acceptor, which reduce the electron-hole recombination and hence increases the photocatalytic activity. 4.1. Effect of Dye Concentration The concentration of the dye is playing a crucial role in photocatalytic activity of BiVO4 nanomaterials. Hence, the optimum concentration of dye for promising photocatalytic activity was carried out by changing the dye concentration of 5-20 ppm under catalytic loading constant (100 mg) and UV light as in Fig.7. This clearly shows that as the dye concentration of 5-20 ppm increases, the photocatalytic degradation is reduced by 80-20% and 5 ppm is the most effective concentration for dye degradation. Generally, as increases the dye concentration, the number of dye molecules is adsorbed on the surface of BiVO4 nanoparticles, so the degradation rate decreases. Higher the dye concentration, the smaller the penetration power of light, which means that the photocatalytic decomposition is less at higher concentrations with less occurrence of hydroxyl groups and superoxide radicals [31].
5ppm 10ppm 15ppm 20ppm
100
% Degradation
80
60
40
20
0 120
90
60
30
0
Time(min)
Fig.7. Degradation MB with varying concentration of dye and constant catalyst load
4.2. Effect of Catalytic Load Figure 8 represents the graph of MB degradation with varying catalyst load (10-20 mg) by maintaining the constant concentration of dye (100 mL of 5 ppm). The data clearly reveals that as the catalyst load increases over 180 minutes, the rate of dye degradation increases from 80% to 100%. This is because more active sites are available due to increased catalytic load [31]. 10mg 20mg 30mg 40mg 50mg
100
% degradation
80
60
40
20
0 0
30
60
90
120
Time (min)
Fig.8. Degradation of Methylene Blue with varying catalyst load and constant dye concentration
4.3. Effect of pH In order to see the optimum pH for the photolysis of the MB dye, experiments were conducted at completely different pH (2 to 12) by maintaining the constant catalyst (100 mg) and the dye concentration (5 ppm), and the result represented in Figure 9. This clearly shows that the MB degradation was effective in the basic medium [32, 33] with the very best degradation rate at pH 12. The presence of an large number of OH- ions on the surface of the catalyst can produced less range of OH• radicals, that act as primary oxidizing agents and are responsible for the degradation of MB dye [34-38].
PH02 PH04 PH06 PH08 PH10 PH12
100
% degradation
80
60
40
20
0 0
30
60
90
120
Time(min)
Fig.9. Degradation of MB with varying pH of the solution and keeping the catalyst load and constant dye constant 4.4. Kinetics of photocatalytic degradation The obtained results of photocatalytic activity of BiVO4 and Ag-BiVO4 NPs were explained through kinetic study using Langmuir–Hinshelwood model. The photodegradation of MB dye as presented in the following relation ln(C0/Ct) = kt ------------------(ii)
where, C0 is the initial concentration of dye (mg/L) at t = 0, Ct is the concentration after irradiation in the selected time interval (30 min), k is the rate constant. The plots of ln(Co/Ct) as a function of reaction time which is approximate. From Fig 10, the slopes of the straight lines from the plot of ln(Co/Ct) vs reaction time are the rate constants (k) for photocatalytic degradation of MB for BiVO4 and Ag-BiVO4 NPs. The rate constant k has the highest value for Ag-BiVO4 NPs was 0.7 × 10−2 min−1 compared to BiVO4 which was 0.48 × 10−2 min−1 under visible light illumination [39].
Fig.10. Photocatalytic kinetic studies of BiVO4 and Ag-BiVO4 NPs 4.4. Catalyst Recycling To evaluate the stability of the photocatalyst, a recycling experiment was carried out to degradation the methylene blue dye (Fig. 11). The experiment was carried out with 100 mg of catalyst and 100 mL of 5 ppm dye. The decomposition potency of MB was nearly an equivalent for six cycles. This figure clearly shows a reduction in potency of nearly 80% altogether six cycles.
100
% Degradation
75
50
25
0 1
2
3
4
5
Recycling no.
Fig.11. Recycling of 10 mg of catalyst and 100 ml of 5 ppm dye 4.5. Effect of Scavengers Photocatalytic reaction is mainly involved in charge separation, photoinduced species and surface redox process. To build the photocatalytic mechanism by nanoparticles for the degradation of organic dyes, it is important to identify which reactive species are mainly involved to break down the organic molecules. In the time of methylene blue photodegradation with Ag-BiVO4 nanoparticles, the O2•-, HO•, electrons (e-) and h+ are produced and these can be trapped by the addition of radical scavengers like benzoquinone [40], benzoic acid [41], AgNO3 [42] and EDTA [43] respectively, into the degradation reaction. The percentage of degradation of dye in the absence and presence of scavengers are shown in Figure 12. These result reveals that the percentage of dye degradation was suppressed by the addition of h+, O2•- and HO• scavengers compared to without addition of scavengers. However, the addition of e- scavenger increases the degradation efficiency by the consumption of photoinduced electrons could which successively increases the e-/h+ pairs separation efficiency. These results confirmed that the degradation of methylene blue decreases by adding h+, O2•- and HO• scavengers meanwhile degradation efficiency was increases by adding e- scavenger.
100
% of Degradation
No Scavenger
AgNO3
80
60
Benzoquinone EDTA
40
Benzoic acid
20
0 1
2
3
4
5
Different Scavengers
Fig.12. Effect of different scavengers during the degradation of MB dye. 4.6. Mechanism. Scheme: A schematic representation of the degradation of MB using BiVO4 Nps is shown below. During irradiation, there is a transition of an electron from valence band (VB) to conduction band (CB) of the material and generating e−/h+ pairs. These e−/h+ pairs are more advantageous during degradation of dye, if which cannot be recombined easily. In a VB, the holes (h+) generate OH. radicals by oxidizing the water molecules; these generated species are unstable and highly reactive [44-48]. In the conduction band electron (e−) reducing the oxygen molecule to O2·− radicals (super oxide oxygen radicals) are degrade the organic dye molecules into CO2, H2O and inorganic mineral acids. BiVO4 + hν → BiVO4 (h+vb + e-cb) OH−ads + h+vb → OH•ads (in basic medium) MB + OH•ads → dye degradation As it is well-known, Ag nanoparticles can trap the excited electrons from BiVO4 nanoparticles and leave the holes for the degradation reaction of organic pollutants, improving the charge carrier separation [49].
Visible light E[NHE](eV) Bi-Ag -1.0 H2
/H+
CB = -0.79
0
MB*
O2
Ag
Ag
2.3eV
O2
CB .O 2
.O 2
+1.0 VB = 1.51
+2.0
MB
.OH
MB
H2 O O2/.O2-(-0.28eV) .OH/H O (+2.27eV) 2
2H2O + 4 h+ = 4H+ + O2
VB
Degradation product
BiVO4
Electrons migrated from CB of BiVO4 Holes left in VB of BiVO4
Mechanism.1. Graphical representation for the mechanism of BiVO4 under the irradiation of UV light 4.7. Detection of OH· Radicals OH• radicals are the most reactive species throughout photocatalytic degradation reactions. The rate of OH• formation and detection can be measured with straightforward, sensitive and quick PL (photoluminescence) techniques using coumarin as the probe molecule. OH• reacts with coumarin to produce a fluorescent compound 7-hydroxyl coumarin at 456 nm. In this method, BiVO4 (130 mg) was dispersed in 1 mM coumarin (50 mL) aqueous solution was taken in borosil jar. The solution mixture was allowed to stand for 10 minutes to attain adsorption-desorption equilibrium between BiVO4, water and coumarin before irradiation. This reaction was irradiated with ultraviolet light of 60 W/m2 as a light source. Aliquots of 2 mL were collected each ten minutes and the PL spectra were measured using an Agilent Technologies Cary Eclipse spectrophotometer. Figure 12 shows that the PL intensity at 531 nm will increase linearly with time and clearly indicates the formation of OH. is directly proportional to the
irradiation time [50]. It shows that formation of OH• increases with an increase in time. This OH• is liable for the decomposition of organic dyes.
PL intensity (a.u.)
Coumarin 10 min 20 min 30 min 40 min
400
500
600
700
Wavelength Fig.13. Concentration of hydroxyl radicals against irradiation time Cyclic voltammetry (CV) Cyclic voltammetry (CV) experiments were carried out in a three-electrode system with SCE as the reference electrode, Platinum (Pt) wire as the counter electrode and surface modified glassy carbon as a working electrode at a scan rate of 50 mV/s. The electrolyte used was a mixture of 0.25 mM K3[Fe(CN)6]/K4[Fe(CN)6] in a 1:1 molar ratio with 0.1 M KCl solution. Figure shows clear anodic and cathodic peaks for each sample. The peak at positive potentials on the anodic (forward) sweep around 0.25 V vs. SCE represents the oxidation of ferrocyanide to ferricyanide with the loss of one electron. From the figure 14, it is clear that anodic and cathodic peak intensities are enhanced in case of Ag-BiVO4 modified glassy carbon electrode than that of BiVO4 modified glassy carbon electrode, from this it is cleared that the rate of electron transfer is more in case of Ag/BiVO4 composite than bare BiVO4 nanoparticles.
BiVO4 Ag-BiVO4
Current (mA)
0.18
0.00
-0.18
-0.36 -0.5
0.0
0.5
1.0
Potential (V)
Fig.14. Cyclic voltammograms of BiVO4 and Ag-BiVO4 nanoparticles in 0.25 mM K3[Fe(CN)6]/K4[Fe(CN)6] in 0.1 M KCl solution. 50 mVs-1scan rate was used. Photoluminescence Studies PL study is one among the helpful technique for notice the potency of charge carrier separation within the semiconductor [51]. The PL emission spectrum of BiVO4 and Ag-BiVO4 NPs were recorded in room temperature with an excitation wavelength of 260 nm represented in Fig.15b. The pure BiVO4 Nps provides a strong ultraviolet emission peak at 531 nm and weak emission peak at 487 nm. In case of Ag-BiVO4 Nps, the obtained PL emission intensity decreased and the resultant datas are in good agreement with the Stern-Volmer quenching. This indicates the efficiency of high charge carrier separation in Ag-BiVO4 NPS. The emission peaks in visible region can be ascribed to bound excitons and defect states positioned at surface of nanostructured. The PL excitation spectrum with emission at 531 nm [52] is represented in Fig. 15a. Chromaticity coordinates are used to find luminous coloured objects and it can be estimated by Commission International De I’Eclairage (CIE) system. CIE chromaticity diagram
of BiVO4 and Ag- BiVO4 Nps represented in Fig. 15c reveals that both the material emit blue light region.
(b)
240
250
260
270
280
290
300
350
400
Wavelength (nm)
0.9
BiVO4
487 nm
PL intensity (a.u.)
PL intensity (a.u.)
230
Ag-BiVO4
531 nm
emi = 531 nm
260 nm
350 nm
(a)
450
500
550
600
Wavelength (nm)
(c)
Ag-BiVO4 Nanoparticles exi= 260 nm
Y
0.6
0.3
0.0 0.0
0.2
Compounds
X
Y
BiVO4
0.18524
0.29049
Ag-BiVO4
0.19287
0.35192
0.4
0.6
0.8
X
Fig.15. PL spectrum of BiVO4 and Ag- BiVO4 (a) excitation spectra (b) emission spectra (c) CIE diagram
(c) CIE diagram
650
Conclusion In this study, BiVO4 and Ag-BiVO4 nanoparticles were prepared by employing solution combustion method using neem gum as a fuel. The XRD pattern shows that BiVO4 and AgBiVO4 nanoparticles belong to a monoclinic phase with an average crystallite size of 21.9 nm and 26.5 nm respectively. The SEM image clearly shows that the morphology depends heavily on the pathway to add the green surfactant. The TEM image shows that the particles are virtually agglomerated and crystalline. The prepared BiVO4 and Ag-BiVO4 NPs display promising photocatalytic activity against the photo degradation of methylene blue. The photoluminescence study reveals that Ag-BiVO4 nano particles shown blue emission.
Acknowledgement Mr. Udayabhanu gratefully acknowledge to CSIR-SRF (09/1204(0001)2018-EMR-I). Dr. GN thanks to DST-Nanomission (SR/NM/NS-1262/2013) for financial support. Mr. Basavalingaiah thanks SIT for providing lab facilities.
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Graphical Abstract
OH
O
O
OH
OH
HO
OH
L-fructose HO O
H OH
HO
O HO
OH
HO
2.59 eV 1.8
2.0
2.2
2.4
2.7 eV 2.6
2.8
3.0
Energy (eV)
OH
D-glucuronic acid (110)
D-Galactose
(b)
(040) (002) (200) (150) (060) (222) (161)
OH HO
O
(121)
L-arabinose
HO
BiVO4 Ag-BIvo4
OH
HO
F(R)2
OH
Intensity (a.u)
OH
20
30
40
(a)
50
60
2 (Degree)
BiVO4 NPs
Visible light E[NHE](eV) Bi-Ag -1.0
CB = -0.79
H2/H+ 0
MB*
O2
Ag
Ag
O2
CB
2.3eV
.O 2
.O 2
+1.0 VB = 1.51
+2.0
MB
.OH
MB
H2 O O2/.O2-(-0.28eV) .OH/H O (+2.27eV) 2
2H2O + 4 h+ = 4H+ + O2
Degradation product
VB BiVO4
Electrons migrated from CB of BiVO4 Holes left in VB of BiVO4 0.9
Ag-BiVO4 Nanoparticles exi= 260 nm
Y
0.6
0.3
0.0 0.0
0.2
Compounds
X
Y
BiVO4
0.18524
0.29049
Ag-BiVO4
0.19287
0.35192
0.4
0.6
0.8
X
54.
Research highlights
For the first time Neem gum assisted BiVO4 and Ag-BiVO4 nanoparticles were prepared.
The morphology of the obtained nanostructures resembles the bead like structure. And the fine distributions of silver (Ag) on Sheet like BiVO4 nanomaterials. Super structured BiVO4 NPs displays strong blue emission for UV excitation. Synthesized BiVO4 NPs exhibited good photocatalytic properties on Methylene Blue dye over 180 minutes, the rate of dye degradation increases from 80% to 100%.
55.