Accepted Manuscript Original article Synthesis and characterization of TiO2 quantum dots for photocatalytic application Lalitha Gnanasekaran, R. Hemamalini, K. Ravichandran PII: DOI: Reference:
S1319-6103(15)00056-3 http://dx.doi.org/10.1016/j.jscs.2015.05.002 JSCS 732
To appear in:
Journal of Saudi Chemical Society
Received Date: Revised Date: Accepted Date:
27 February 2015 29 April 2015 5 May 2015
Please cite this article as: L. Gnanasekaran, R. Hemamalini, K. Ravichandran, Synthesis and characterization of TiO2 quantum dots for photocatalytic application, Journal of Saudi Chemical Society (2015), doi: http://dx.doi.org/ 10.1016/j.jscs.2015.05.002
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Synthesis and characterization of TiO2 quantum dots for photocatalytic application Lalitha Gnanasekaran,a* Hemamalini. R,a** K. Ravichandranb Email:
[email protected]*,
[email protected]**. a
b
Department of Physics, Queen Mary’s College, Chennai 600 004, India. Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai 600 025, India.
Abstract In the present study, TiO2 quantum dots (QDs) were effectively synthesized by sol-gel method and it was characterized by several techniques. The structure of the TiO2 QDs, before and after calcination, was analyzed by powder X-ray diffraction technique. The XRD results indicates that after calcination, TiO2 QDs has pure anatase phase with tetragonal structure without any other phases and impurities. The morphology and size of the TiO2 QDs were determined from FE-SEM and TEM analysis. The optical band gap of TiO2 QDs was estimated using Kubelka-Munk function from diffuse reflectance spectroscopy. The photocatalytic activity and recycling ability of the TiO2 QDs have been investigated. The TiO2 QDs used to degrade the model dyes such as methyl orange and methylene blue under UV light irradiation and their results have been discussed. A mechanism is proposed and discussed accordingly in detail.
Keywords: Quantum dots; TiO2; Sol-gel method; Photocatalytic activity; UV light; Recycle.
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1. Introduction In recent years, quantum dots (QDs) have attracted much attention due to its large surface area and high activity. The physical and chemical properties of QDs have been found to be different from that of bulk materials [1-3]. The efficiency of several applications such as solar cell, fuel cells, photocatalyst and antibacterial activity can be enhanced due to the surface property of QDs [1-3]. Titanium dioxide (TiO2) and zinc oxide (ZnO) are considered to be significant materials in the area of pollutant degradation due to their high photo catalytic efficiency [4-6]. TiO2 is a predominant semiconductor photocatalyst because of its stability, non-toxicity, low cost production and eco-friendly nature [4,5]. The efficiency of photo catalytic activity of TiO2 can be further enhanced by various physical and chemical methods. Since synthesis of TiO2 QDs uses sophisticated methods like chemical vapor deposition (CVD), Ultra high vacuum (UHV) etc., their synthesis by a cost effective method is essential and at the same time it remains a challenge. Water is the most important natural resource for living organisms. According to the World Health Organization, nowadays, it is uncertain about the quality of water and due to poor hygienic condition nearly 1.5 million children dies per year worldwide [7]. Increasing environmental pollution, especially water pollution, plays a key factor in destroying green environment. Various technologies have been developed to sustain green environment [8]. The heterogeneous catalytic activity has paid much attention in the treatment of waste water. Even under mild conditions of temperature and pressure the complete degradation of pollutants can be attained. The most significant feature of this process is its cost-effectiveness [9-16]. In this work, TiO2 QDs have been synthesized by a simple and low cost method. The synthesized TiO2 QDs were characterized by several techniques and the results have been discussed. The degradation of various model dyes were achieved by photo catalytic method
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in the presence of the prepared quantum dots under UV light irradiation. The recycling ability of the catalyst has also been verified.
2. Experimental details 2.1 Materials The required chemicals such as titanium (IV) isopropoxide (TTIP), isopropanol, cetyl trimethyl ammonium bromide (CTAB), methyl orange (MO) and methylene blue (MB) were procured from Sigma-Aldrich. P-25 TiO2 (20-30 nm) was obtained from Degussa and these chemicals were used without further purification. All the aqueous solution was prepared using double distilled water. 2.2 Preparation of quantum dots The procedure followed in the synthesis of TiO2 QDs is as follows: Initially 5.0 mL of TTIP was taken in a syringe and it was dissolved in 150 mL of isopropanol in a 500 mL beaker. Then, the mixed solution was stirred for more than 1 h. 0.01 mole of aqueous CTAB was added drop-wise into the mixed solution.
The resulting solution was stirred
mechanically for about 24 h. Finally, a white dry powder was obtained. This was removed from the beaker and transferred to the mortar for grinding. The finely grounded sample was transferred into an alumina boat and calcinated at 350 ºC for 30 minutes in a muffle furnace. 2.3 Photocatalytic activity The photocatalytic studies were carried out as reported by Saravanan et al. [6]. The synthesized TiO2 QDs were used as a catalyst for the degradation of MO and MB solution under UV light irradiation. At first, the photo stability of the dyes was tested without catalyst. The MO and MB aqueous solutions were magnetically stirred in dark condition for 2 h under UV light irradiation. The irradiated samples were later analyzed using UV-Visible
3
spectrophotometer and it was found that there was no change in the concentration of the solution. This implies that the dyes are enough stable under UV light. The source of UV light (8 W mercury vapor lamp of 365 nm wavelength) is sealed by cylindrical quartz glass tube to prevent direct contact with the dye solution. The photocatalytic suspension were prepared by taking 100 mg of TiO2 in 100 mL of dye (MO/MB) solution (initial concentration of 5× 10 -5 moles/L) in a 500 mL cylindrical beaker containing water jacket so as to maintain a constant temperature (25 ºC). Later, the suspension was magnetically stirred under dark condition for 20 min to establish adsorption/desorption equilibrium condition. The irradiated sample was collected at every 20 minutes and the sample was centrifuged and filtered. The concentrations of the irradiated MO and MB solution were determined by UV-Visible spectrophotometer to the corresponding
max
464 nm and 664 nm respectively. The degradation efficiencies of MO
and MB were calculated using the following formula:
η = 1−
C ×100 C0
(1)
where, C0 and C are the concentrations of the solution before illumination (t = 0) and after illumination of light for ‘t’ minutes respectively. 2.4 Characterization details
The powder X-ray diffraction techniques was used to find the phase, structure and crystallite size of the synthesized TiO2 QDs at room temperature (2 = 20º to 70º) using a D5000 (Siemens, USA) diffractometer with CuK
1
( = 1.5406 Å) radiation. The surface
morphology of the synthesized TiO2 QDs was analysed with (FEI quanta FEG 200, USA) Field Emission Scanning Electron Microscope. The size and chemical composition of the material was confirmed by TEM (Tecnai G2 20 TEM, FEI, Netherland) with EDX detector. The optical band gap of the quantum dots was determined from the UV-Vis diffuse
4
reflectance spectra which were recorded at room temperature using (VARIAN CARY 5E, USA) UV-VIS-NIR Spectrophotometer. The photocatalytic activity was measured by a UV– Visible spectrophotometer (Perkin Elmer Lambda 35, USA). 3. Results and discussion
In the present study, synthesis of TiO2 QDs was achieved using sol-gel method. The proposed method is simple and cost effective compared to other methods because it does not required any special equipment. The prepared TiO2 QDs effectively degraded the MO and MB under UV light irradiation. 3.1. Structural analysis
The X-ray diffraction pattern of the as synthesized TiO2 QDs are shown in Figure 1. Figure 1 (a) clearly shows that there are no peaks in the diffraction pattern which reveals that as synthesized TiO2 QDs is amorphous in nature. In contrast, after the calcination, several diffraction peaks were apparent which is evident from Figure 1(b). The result confirms that after calcination the synthesized TiO2 QDs becomes crystalline in nature. The scientific software XRDA 3.1 and Origin 8 were used to index the diffraction peaks. The peaks at the 2 values 25.3º, 38.1º, 47.4º, 54.3º, and 62.1º were fitted and their hkl planes were identified as (101), (112), (200), (105), and (213) respectively, which coincides with the JCPDS card no 84-1285. The 2 values and the planes reveal that the as synthesized TiO2 QDs is in pure anatase phase having tetragonal structure. The calculated lattice parameter values are, a = 3.826 Å and c = 9.403 Å. The XRD data clearly indicates that there are no other phases (Rutile and brookite) or impurities of any other kind. Using Scherrer formula, the average crystallite size of the as synthesized TiO2 QDSs was calculated as 4.8 nm. The Xray diffraction results hereby confirms that the synthesized TiO2 QDSs are pure anatase phase having tetragonal structure.
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Figure 1. The X-ray diffraction pattern of the as synthesized TiO2 QDs (a) before calcination, and, (b) after calcination. 3.2 Morphology, size and elemental analysis The surface morphology and size of the as synthesized TiO2 QDs were investigated using FE-SEM and TEM analysis. Figures 2 (a), (b) and (c) represent the FE-SEM images of as synthesized TiO2 QDs at different scales. It was observed from all the FE-SEM images that the spherical shaped particles are tightly bound. The agglomeration of the TiO2 limited the accurate measurement of the size of the TiO2 QDs. Therefore, the size of the as synthesized TiO2 QDs was measured using TEM.
(a)
(b)
500 nm
1 µm
(c)
300 nm
Figure 2. The FE-SEM images of the as synthesized QDs at different scales (a) 1 µm, (b) 500 nm, and (c) 300 nm. Figure 3(a) shows TEM image of as synthesized TiO2 QDs which clearly shows the random distributions of homogenous fine spherical shaped particles. Its equivalent selected
6
area electron diffraction (SAED) pattern is shown by Figure 3 (b). The SAED pattern illustrates that there are several electron diffraction rings. The d-spacing of the rings corresponding to the hkl planes (101), (105), (112) and (213) were identified. These hkl planes of pure anatase phase having tetragonal structure of TiO2 QDs were in agreement with the XRD data. From the TEM image, the particle size distribution plot of the TiO2 QDs was also measured which was found in the range of 3 – 7 nm and is shown by the Figure 3(c). Further the chemical composition of the synthesized TiO2 QDs were analyzed using EDS spectrum. Figure 3(d) shows EDS spectrum which confirms that the TiO2 QDs contains titanium and oxygen without any other impurities.
(b)
(a)
(105) (101) (112) (213)
(d)
(c)
Ti O
Figure 3. (a) The TEM image, (b) SAED pattern, (c) Size distribution plot, and (d) EDS spectrum of TiO2 QDs. 7
3.3 Band gap determination
The UV-Vis diffuse reflectance spectra (DRS) was used to find the optical band gap of the TiO2 QDs as shown by Figure 4 (a). The spectrum exhibits a strong reflection edge which is at around 340 nm. The band gap energy of the TiO2 QDs was evaluated using KubelkaMunk (KM) function [13]. Based on this function, F(R)h versus h plot (Figure 4 (b)) give the band gap energy of TiO2 QDs was 3.79 eV and its equivalent wavelength exists in the UV region of the electromagnetic spectrum. Hence, the results of the UV-Vis DRS indicates that the excitation occurs in the synthesized TiO2 QDs on the irradiation of UV light.
Figure 4. (a) Diffuse reflectance spectrum of TiO2 QDs, and, (b) Determination of band gap
of TiO2 QDs based on K-M function.
3.4 Photocatalytic degradation of MO and MB
This work mainly focused on the degradation of model dyes such as methyl orange (MO) and methylene blue (MB) using the synthesized TiO2 QDs. The UV-Vis DRS results confirmed that the photocatalytic activity can be performed using UV light since the band gap of TiO2 QDs is 3.79 eV. The time dependent (MO or MB) degradation percentage is shown by Figure 5. Figure 5 shows that with increasing irradiation time, the MO/MB degradation percentage is increasing which suggests the complete degradation of the dyes. After 80 mins 8
of irradiation, the model dyes were almost completely degraded with the use of synthesized TiO2 QDs and its degradation efficiency are ~97.1% and 97.9% which was calculated using equation (1). It was found that the synthesized TiO2 QDs exhibited superior photocatalytic degradation than that of commercial Degussa P-25 TiO2 catalyst. Few researchers synthesized TiO2 for photo catalytic applications [17-24]. However, the result of the present work depicts higher degradation when compared to these reports [17-24]. The difference in degradation time depends on the preparation methods, size and morphology of the samples [6].
Figure 5. The time dependent percentage degradation of (a) MO, and (b) MB using as
synthesized TiO2 QDs and commercial Degussa P 25.
The proposed mechanism of photocatalytic activity is presented in the schematic diagram as shown by Figure 6.
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e-
e-
Reduction process
e-
Conduction band
UV light
E x c i t a t i o n
TiO2 Quantum dots
B a n d
3.79 eV
G a p
Valence band h+
h+
Oxidation process
h+
Figure 6. The mechanism of photocatalytic activity of TiO2 QDs.
The photocatalytic activity can be initiated by the irradiation of the UV light on the mixed solution (aqueous dye + TiO2 QDs). The valence band electrons moves to the conduction band due to the irradiation of the UV light. This creates holes in the valence band and electrons in the conduction band. The holes react with water molecules present in the dye (oxidation) and simultaneously the electrons in the conduction band react with oxygen molecules (reduction). In both the processes, the •OH radical are produced. The •OH radicals effectively degrades the organic pollutants (dye) under UV light irradiation. Based on the earlier reports, the photocatalytic reaction mechanism is proposed as follows [6]:
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Figure 7. The recycling ability of TiO2 QDs for MO degradation.
For industrial and environmental applications, the recycling ability of catalysts is essential to find the stability and the reusability of the catalyst. To achieve the real time applications, the photocatalytic activity is repeated five times. Figure 7 shows the recycling process and the ability to degrade the MO. After recycling, there were no remarkable changes in the degradation. This clearly indicates that the synthesized TiO2 QDs are reusable and enough stable. 4. Conclusions
In summary, the TiO2 QDs having anantase phase and tetragonal structure was successfully synthesized by sol-gel method. The average size of the TiO2 QDs was about 5 nm which was confirmed by TEM. The K-M function exhibits the optical band gap value of synthesized TiO2 QDs was ~3.79 eV which lies in the UV region. The photocatalytic results indicate that the synthesized TiO2 QDs effectively degraded the model dyes MO and MB under UV light. The degradation efficiency was calculated and found ~97.1% and 97.9% for MO and MB respectively in 80 minutes of irradiation. Further, the recycling results imply 11
that the catalyst is enough stable and could be easily reused which is favorable for potential environmental and industrial applications.
Acknowledgements We acknowledge SAIF-IIT Chennai, India and IIC-IIT Roorkee, India for FE-SEM and TEM characterizations.
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